This is a very quick demonstration showing that two solids can react together. White lead nitrate and white potassium iodide react to make yellow lead iodide.
Lesson Organisation
This demonstration is very quick and will take no more than 2 minutes.
Apparatus and Chemicals
For one demonstrationEye protection (goggles)A small screw-top jarBalanceWeighing boats or similar, 2
Lead nitrate (Toxic, Dangerous for the environment), 20gPotassium iodide (Low hazard), 20g
Technical Notes
Lead nitrate (Toxic, Dangerous for the environment) Refer to CLEAPSS Hazcard 57APotassium iodide (Low hazard) Refer to CLEAPSS Hazcard 47B
1 The resulting solid mixture from the demonstration should be retained in a sealed container for professional disposal.
Procedure
HEALTH & SAFETY: Wear goggles
a Weigh out equal masses of both compounds. These are then in approximately the stoichiometric ratio. Between 10 g and 20 g of each is suitable.
b Mix the solids in a screw topped jar and shake for several seconds. The yellow colour of lead iodide will be seen.
c Make a little more of the mixture and place it quickly into a beaker containing a little water. The reaction will be much more rapid.
Teaching notes
The demonstration might have more impact if the jar is opaque and the yellow product can be poured out and shown to the unsuspecting audience. Have a white background available.
Point out that for a reaction ot occur, particles of the reactants must meet. This is much easier in solution (where the particles are free to move) than in the solid state.
The reaction is:
Pb(NO3)2(s) + 2KI(s) → 2KNO3(s) + PbI2(s)
All of these compounds are white except lead iodide, which is yellow.
Lead ethanoate can be substituted for lead nitrate, but the reaction is much slower.
The experiment Diffusion in liquids is a class practical using the same compounds but as solutions.
Sunday, January 11, 2009
Determining the relative molecular mass of butane
A sample of gas from a small pressurised cylinder is collected over water to measure the volume, and the mass found by the decrease in mass of the cylinder. A simple calculation allows the relative molecular mass (RMM) to be found. The most convenient gas for this is butane, but other gases may be available in similar small cylinders.
Lesson organisation
This is most likely to be done as a teacher demonstration. Teachers of advanced students may wish to consider the possibility of a student practical, but would need to carry out very careful risk assessments in the context of the capabilities of their students.
The collection of a gas sample, and the weighing of the gas cylinder before and after this, should take about 5-10 minutes as a demonstration.
Apparatus and chemicals
Eye protectionAccess to a fume cupboard (see note 1)
Measuring cylinder (1 dm3)Stand and clamp (see note 2)Trough (see note 3)Delivery tube, flexible and gas-tight (see note 4)Top-pan balance (see note 5)
Optional:
Thermometer to measure room temperature +/- 0.5 oC, with digital display if available Access to an accurate measurement of atmospheric pressure.
Gases available in small pressurised cylinders, for example:
• butane in lighter refill cans, from a camping gas stove or a gas blowlamp• propellant gas from aerosol cans (this could be turned into an investigation into which gas is being used as the propellant)• in rural areas the laboratory gas supply may be butane or propane.
Technical notes
Butane (Extremely flammable) Refer to CLEAPSS Hazcard 45A and Laboratory Guide L164
1 A fume cupboard should be used if it is difficult to attach the delivery tube or control the valve.
2 The stand and clamp should be able to hold the large measuring cylinder full of water securely.
3 The trough should be sufficiently large to allow the easy immersion of the lower end of the upturned measuring cylinder filled with water, and then allow the displacement of 1 dm3 of water.
4 The flexible delivery tube must be long enough to permit easy manipulation for the gas collection. Make sure the delivery tube will connect securely to the canister of gas. The precise way in which the delivery tube may be attached to the pressurised cylinder of butane will depend on the cylinder involved, and the design of its valve; some ingenuity may be required. Some types do NOT re-seal. The connection must be gas-tight and secure. Lighter refill cans should make a simple connection to tubing of appropriate diameter. Removing the burner from camping gas stoves and blowlamps should enable the tubing to be connected directly above the valve.
5 A top-pan balance accurate to +/- 0.01g is sufficient, but +/-0.001g would be ideal, with (if available) output to a computer to display the reading.
Procedure
HEALTH & SAFETY: Wear eye protection throughout. Remove all possible sources of ignition. Ensure adequate room ventilation.
a Before the lesson, fill the measuring cylinder with water to the brim, close the mouth of the cylinder firmly with the palm of the hand, and invert the cylinder into the trough of water. Clamp the cylinder firmly, allowing sufficient room under the cylinder mouth to insert the end of the delivery tube.
b Weigh the gas canister.
c Connect the delivery tube to the canister. Place and hold the other end of the delivery tube under the mouth of the inverted measuring cylinder.
d Carefully open the valve of the canister and collect exactly 1 dm3 of gas, ensuring the water levels inside and outside the cylinder are the same at the end. Close the valve.
e Disconnect the delivery tube from the gas canister and dry the outside of the canister thoroughly, if necessary, and re-weigh.
f Release the gas from the measuring cylinder with due regard to safety, remembering it is significantly denser than air – preferably in the fume cupboard, or out of a window.
g Record room temperature and pressure if required (see below).
Teaching notes
There are two possible routes for using the results of the experiment to calculate the RMM:
1 One mole of a gas, irrespective of it’s chemical nature, occupies approximately 24 dm3 at standard room temperature and pressure. The experiment has established the mass of 1 dm3, so the mass of 1 mole is simply 24 times that mass.
2 For a more accurate analysis, if the class has already studied the ideal gas equation, they can use the relation: pV = nRT to calculate the number of moles in 1 dm3. This requires the records of room temperature and pressure at the time of the experiment. From this the mass of one mole can be calculated.
Investigating the nature of the propellants in aerosol cans is essentially an extension of this experiment. The composition of most present-day aerosol propellants is a mixture of butane isomers and propane. The varying proportions of propane and butane will lead to a measured RMM between those of propane (46) and butane (58). By measuring the apparent RMM, the proportions of propane and butane can be calculated. However, the other ingredients of the aerosol can may affect the measurements.
Oxygen (Oxidising) is also available in small pressurised canisters (Refer to CLEAPSS Hazcard 69). Teachers adapting this experiment for measuring the RMM of oxygen will need to consultant their employer's risk assessments, but this may be a preferred alternative to butane for use as a class experiment. However, the purity of the oxygen in these cylinders may be significantly less than 100%, which will affect the RMM value obtained.
A thermometric titration-AS PRACTICAL CHEMISTRY
A thermometric titration
Sodium hydroxide solution is titrated with hydrochloric acid. The temperature change is measured each time a portion of acid is added. The highest temperature indicates the end-point of the titration, and this is used to calculate the concentration of the hydrochloric acid.
Sodium hydroxide solution is titrated with hydrochloric acid. The temperature change is measured each time a portion of acid is added. The highest temperature indicates the end-point of the titration, and this is used to calculate the concentration of the hydrochloric acid.
Lesson organisation
This is best carried out individually or in pairs. The experiment takes about one hour.
Apparatus and chemicals
Each group will need:
Eye protection: goggles
Thermometer (0 – 100 °C) (see note 1)Two insulated (polystyrene) cupsBeaker (250 cm3)Burette (50 cm3)Burette standClamp and stand (optional)Cork, one-holed (optional - to fit thermometer)Pipette (20 or 25 cm3)Pipette safety filler
Hydrochloric acid, 2.00 mol dm–3 (Irritant at this concentration), about 75 cm3 (see note 2)Sodium hydroxide solution, 1.50 mol dm–3 (Corrosive at this concentration), about 30 cm3 (see note 3)
This is best carried out individually or in pairs. The experiment takes about one hour.
Apparatus and chemicals
Each group will need:
Eye protection: goggles
Thermometer (0 – 100 °C) (see note 1)Two insulated (polystyrene) cupsBeaker (250 cm3)Burette (50 cm3)Burette standClamp and stand (optional)Cork, one-holed (optional - to fit thermometer)Pipette (20 or 25 cm3)Pipette safety filler
Hydrochloric acid, 2.00 mol dm–3 (Irritant at this concentration), about 75 cm3 (see note 2)Sodium hydroxide solution, 1.50 mol dm–3 (Corrosive at this concentration), about 30 cm3 (see note 3)
Technical notes
Hydrochloric acid (Irritant at concentration used) Refer to CLEAPSS Hazcard 47A and Recipe Card 31Sodium hydroxide solution (Corrosive at concentration used). Refer to CLEAPSS Hazcard 91 and Recipe Card 65
1 Instead of using the thermometer to stir the titration mixture, it could be clamped in position in a cork, as shown in the diagram, and the mixture swirled after each addition of acid.
Alternatively, a temperature sensor attached to a computer can be used in place of a thermometer. Data logging software could then be used to provide a detailed plot of the readings.
2 The concentration of the hydrochloric acid should not be indicated on bottle.
3 The concentration of the sodium hydroxide should be indicated on bottle.
4 The solutions need to be as concentrated as they are in order to achieve reasonable changes in temperature.
Procedure
HEALTH & SAFETY: wear goggles
a Stand an insulated cup in a beaker for support.
b Using a pipette and safety filler, transfer 20 cm3 (or 25 cm3 ) of the sodium hydroxide solution into the cup, and measure the steady temperature.
c Using the burette, add a small portion (3 – 5 cm3) of dilute hydrochloric acid to the solution in the cup, noting down the actual volume reading. Stir by swirling the cup and measure the highest temperature reached.
d Immediately add a second small portion of the dilute hydrochloric acid, stir, and again measure the highest temperature and note down the volume reading.
e Continue in this way until there are enough readings to decide the maximum temperature reached during this experiment. You will need to add at least 30 cm3 of the acid.
f Plot a graph of temperature against the volume of acid added, and use extrapolation of the two sections of the graph to deduce the maximum temperature reached without heat loss.
g Use your results to calculate the concentration of the hydrochloric acid.
Teaching notes
The main concern in this experiment is the heat loss. If possible a lid should be used. More reliable results can be achieved using two polystyrene cups (one inside the other).
With abler or older students, it is possible to discuss the extrapolation of the cooling curve to estimate the maximum temperature reached without heat loss. The link below gives an example of how extrapolation is used to determine the maximum temperature reached.
To reinforce the theory involved here, an indicator could also be used to show that the end-point really did occur at the highest temperature.
AS and A Level Practical
Volumetric Analysis 3
To determine the relative molecular mass of a soluble base
Introduction
In Volumetric Analysis 1 & 2 you prepared a standard solution of sodium carbonate and used it to standardise an unknown concentration of dilute hydrochloric acid. In this practical you will use your new-found skills to find out the relative molecular mass of an unknown group 1 carbonate – the mysterious "Substance Z". Group 1 carbonates are soluble in water (although Li2CO3 is only sparingly soluble) and will react with dilute hydrochloric acid according to the overall equation below:
X2CO3(aq) + 2HCl(aq) → 2XCl(aq) + CO2(g) + H2O(l)
(X represents a group 1 element)
If you know the amount of hydrochloric acid that will react with a known amount of Substance Z, you should be able to determine the Mr of Substance Z and so identify the group 1 element in it.
You will need to make careful notes about your experiment as you go along today.
Apparatus
Consult your notes from Volumetric Analysis 1 & 2 to decide upon the apparatus you need.
Make sure that your practical write-up includes the apparatus you use today.
Method
Consult your notes from Volumetric Analysis 1 & 2 and The Burette to remind yourself of the procedures needed for safe and accurate working.
Make sure that your practical write-up includes the methods you use today.
1. Weigh out accurately between 1.3g and 1.7g of Substance Z.
Record your weighings in a suitable form. Dissolve your weighed Substance Z in de-ionised water, and make up the solution to 250cm3 in a volumetric flask.
2. Clean your burette with de-ionised water and then with the standard 0.100M hydrochloric acid to be used for the titration.
3. Pipette 25cm3 of the Substance Z solution into a clean conical flask.
Using methyl orange indicator, titrate with the standard hydrochloric acid.
4. Repeat step 3 until concordant results are obtained.
Record your results as in Volumetric Analysis 2.
After cleaning and clearing away, determine the identity of Substance Z as described overleaf.
As in Volumetric Analysis 2, 1 mole of X2CO3 will react with 2 moles of HCl (see equation below):
X2CO3(aq) + 2HCl(aq) → 2XCl(aq) + CO2(g) + H2O(l)
(X represents a group 1 element)
1. Calculate the number of moles of HCl there were in your mean titre.
2. Calculate the number of moles of HCl that would react with the entire 250cm3 of Substance Z solution.
3. Work out the number of moles of X2CO3 were there in the 250cm3 of Substance Z solution.
You now know:
•the mass of X2CO3 in your Substance Z solution; and
•the number of moles of X2CO3 in your Substance Z solution.
4. Calculate the mass of one mole of X2CO3.
5. What is Substance Z, and why? Copyright © 2003 Nigel Saunders N-ch1-37
Volumetric Analysis 3
To determine the relative molecular mass of a soluble base
Technician's Notes Prior to practical
Sodium carbonate*
Heat required amount of sodium carbonate (Na2CO3) to drive off water of crystallisation.
Either: heat in an evaporating dish over a Bunsen burner for 30 minutes approx., or
heat in a drying oven at about 110oC for 1 hour.
Agitate the solid periodically with a clean glass rod.
Transfer to a desiccator after heating, and label it "Substance Z - Harmful".
Care: Use tongs and eye protection.
Beware of hot solid and apparatus.
Sodium carbonate forms caustic alkaline solutions with water; if spilt on skin wash with plenty of water.
Analytical balances
Please check cleanliness and correct functioning of analytical balances.
De-ionised water
Please check 6th Form wash bottles are clean and filled with de-ionised water.
Make sure that additional de-ionised water is available in the aspirator.
Burettes
Please check the cleanliness and correct functioning of the burettes.
Per class
Sodium carbonate solid. Allow approx. 2.5g per student.
Analytical balances .
Top pan digital balances (minimum of two if possible).
De-ionised water.
0.100M hydrochloric acid* ( a good home-made solution should suffice for this practical).
Allow 200cm3 per student.
Methyl orange indicator solution (the more bottles the better).
Per student
(Normally found in lab anyway)
1 x pair of safety goggles
1 x bench mat
2 x 100cm3 beaker
2 x 250cm3 beaker
1 x 250cm3 conical flask
1 x glass funnel (check that it will enter the neck of the volumetric flask easily) Copyright © 2003 Nigel Saunders N-ch1-37
Per student
(Additional apparatus to put out)
1 x glass rod (long)
1 x 250cm3 volumetric flask with stopper to fit
1 x 25cm3 bulb pipette
1 x pipette filler (check correct functioning)
1 x burette (see overleaf)
1 x burette stand
1 x weighing bottle with lid
1 x 6th Form wash bottle containing de-ionised water
1 x small spatula
1 x white tile
1 x small plastic filter funnel
1 x copy of N-ch1-37 (student guide to practical)
*Health and Safety Notes
Hydrochloric acid
Corrosive.
Use pre-prepared standard solution, or refer to Hazcards for correct method to prepare an accurate 0.100M solution.
Sodium carbonate (solutions and solid)
Sodium carbonate solutions are alkaline and therefore caustic.
Exercise care in handling - wear eye protection and, if spilt, wash with a lot of water.
To determine the relative molecular mass of a soluble base
Introduction
In Volumetric Analysis 1 & 2 you prepared a standard solution of sodium carbonate and used it to standardise an unknown concentration of dilute hydrochloric acid. In this practical you will use your new-found skills to find out the relative molecular mass of an unknown group 1 carbonate – the mysterious "Substance Z". Group 1 carbonates are soluble in water (although Li2CO3 is only sparingly soluble) and will react with dilute hydrochloric acid according to the overall equation below:
X2CO3(aq) + 2HCl(aq) → 2XCl(aq) + CO2(g) + H2O(l)
(X represents a group 1 element)
If you know the amount of hydrochloric acid that will react with a known amount of Substance Z, you should be able to determine the Mr of Substance Z and so identify the group 1 element in it.
You will need to make careful notes about your experiment as you go along today.
Apparatus
Consult your notes from Volumetric Analysis 1 & 2 to decide upon the apparatus you need.
Make sure that your practical write-up includes the apparatus you use today.
Method
Consult your notes from Volumetric Analysis 1 & 2 and The Burette to remind yourself of the procedures needed for safe and accurate working.
Make sure that your practical write-up includes the methods you use today.
1. Weigh out accurately between 1.3g and 1.7g of Substance Z.
Record your weighings in a suitable form. Dissolve your weighed Substance Z in de-ionised water, and make up the solution to 250cm3 in a volumetric flask.
2. Clean your burette with de-ionised water and then with the standard 0.100M hydrochloric acid to be used for the titration.
3. Pipette 25cm3 of the Substance Z solution into a clean conical flask.
Using methyl orange indicator, titrate with the standard hydrochloric acid.
4. Repeat step 3 until concordant results are obtained.
Record your results as in Volumetric Analysis 2.
After cleaning and clearing away, determine the identity of Substance Z as described overleaf.
As in Volumetric Analysis 2, 1 mole of X2CO3 will react with 2 moles of HCl (see equation below):
X2CO3(aq) + 2HCl(aq) → 2XCl(aq) + CO2(g) + H2O(l)
(X represents a group 1 element)
1. Calculate the number of moles of HCl there were in your mean titre.
2. Calculate the number of moles of HCl that would react with the entire 250cm3 of Substance Z solution.
3. Work out the number of moles of X2CO3 were there in the 250cm3 of Substance Z solution.
You now know:
•the mass of X2CO3 in your Substance Z solution; and
•the number of moles of X2CO3 in your Substance Z solution.
4. Calculate the mass of one mole of X2CO3.
5. What is Substance Z, and why? Copyright © 2003 Nigel Saunders N-ch1-37
Volumetric Analysis 3
To determine the relative molecular mass of a soluble base
Technician's Notes Prior to practical
Sodium carbonate*
Heat required amount of sodium carbonate (Na2CO3) to drive off water of crystallisation.
Either: heat in an evaporating dish over a Bunsen burner for 30 minutes approx., or
heat in a drying oven at about 110oC for 1 hour.
Agitate the solid periodically with a clean glass rod.
Transfer to a desiccator after heating, and label it "Substance Z - Harmful".
Care: Use tongs and eye protection.
Beware of hot solid and apparatus.
Sodium carbonate forms caustic alkaline solutions with water; if spilt on skin wash with plenty of water.
Analytical balances
Please check cleanliness and correct functioning of analytical balances.
De-ionised water
Please check 6th Form wash bottles are clean and filled with de-ionised water.
Make sure that additional de-ionised water is available in the aspirator.
Burettes
Please check the cleanliness and correct functioning of the burettes.
Per class
Sodium carbonate solid. Allow approx. 2.5g per student.
Analytical balances .
Top pan digital balances (minimum of two if possible).
De-ionised water.
0.100M hydrochloric acid* ( a good home-made solution should suffice for this practical).
Allow 200cm3 per student.
Methyl orange indicator solution (the more bottles the better).
Per student
(Normally found in lab anyway)
1 x pair of safety goggles
1 x bench mat
2 x 100cm3 beaker
2 x 250cm3 beaker
1 x 250cm3 conical flask
1 x glass funnel (check that it will enter the neck of the volumetric flask easily) Copyright © 2003 Nigel Saunders N-ch1-37
Per student
(Additional apparatus to put out)
1 x glass rod (long)
1 x 250cm3 volumetric flask with stopper to fit
1 x 25cm3 bulb pipette
1 x pipette filler (check correct functioning)
1 x burette (see overleaf)
1 x burette stand
1 x weighing bottle with lid
1 x 6th Form wash bottle containing de-ionised water
1 x small spatula
1 x white tile
1 x small plastic filter funnel
1 x copy of N-ch1-37 (student guide to practical)
*Health and Safety Notes
Hydrochloric acid
Corrosive.
Use pre-prepared standard solution, or refer to Hazcards for correct method to prepare an accurate 0.100M solution.
Sodium carbonate (solutions and solid)
Sodium carbonate solutions are alkaline and therefore caustic.
Exercise care in handling - wear eye protection and, if spilt, wash with a lot of water.
Friday, January 9, 2009
To standardise hydrochloric acid- AS chemistry practicals
Volumetric Analysis 2
To standardise hydrochloric acid
Introduction
In the last practical you prepared a standard solution of sodium carbonate.
Today, you will use it to find the concentration of dilute hydrochloric acid by titration.
This process is known as standardising the hydrochloric acid.
The reaction between sodium carbonate and hydrochloric acid takes place in two stages:
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq) (1)
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
Two indicators are needed to cover both stages:
•in stage 1, phenolphthalein is most suitable, and will respond to the pH change associated with the formation of sodium hydrogencarbonate, NaHCO3.
•in stage 2, methyl orange is most suitable, and will respond to the pH change associated with the final formation of sodium chloride, NaCl.
As a result, this practical gives you experience of titration using two different indicators (the phenolphthalein colour change is easy to spot, whereas the methyl orange colour change is quite difficult to judge).
Apparatus
Goggles
Bench mat
100cm3 beaker
250cm3 beaker
250cm3 conical flask
25cm3 bulb pipette
Pipette filler
Burette
Burette stand and holder
Plastic filter funnel
White tile
Teat pipette
Access to:
your standard sodium carbonate solution
dilute hydrochloric acid to standardise
phenolphthalein indicator solution
methyl orange indicator solution
Methods
1. Transfer a 25cm3 aliquot (portion) of your sodium carbonate solution to a 250cm3 capacity conical flask. Add a few drops of phenolphthalein indicator solution.
2. Titrate with the hydrochloric acid. The end-point of the titration is when the solution just changes from pink to colourless. Note the titre, then add a few drops of methyl orange.
3. Titrate with the hydrochloric acid. The end-point of the titration is when the solution just changes from yellow to red. Note the second titre.
4. Repeat steps 1 - 3 until concordance (i.e. until the readings are the same or within 0.1cm3).
Tabulate your titrations as described in The Burette sheet. You will need two sets of tables.
5. After tidying away, do the calculations described overleaf.
Calculations
1. Calculate the Mr of Na2CO3.
Ar (Na) = 23 Ar (C) = 12 Ar (O) = 16
2. Look back at the accurate mass of sodium carbonate you used in the last practical.
Using your answer to step 1, calculate the number of moles of Na2CO3 that you dissolved in 250cm3 of water during Volumetric Analysis 1.
3. Use your answer to step 2 to calculate the number of moles of Na2CO3 in the 25cm3 transferred to the conical flask.
Stage 1 Phenolphthalein results
4. The equation for the first stage of the reaction between sodium carbonate and hydrochloric acid is shown below again:
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq)
From the equation, you can see that 1 mole of Na2CO3 will react with 1 mole of HCl.
How many moles of HCl will react with the number of moles of Na2CO3 calculated in step 3?
5. The answer to step 4 tells you how many moles of HCl were in your first mean titre.
Divide this number by the volume of the first mean titre: this is the concentration of HCl in mol dm−3.
Stage 2 Methyl orange results
6. The equation for the second stage of the reaction between sodium carbonate and hydrochloric acid is shown below again:
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
From the equation, you can see that 1 mole of NaHCO3 will react with 1 mole of HCl.
The number of moles of NaHCO3 is equal to the number of moles of Na2CO3.
How many moles of HCl will react with the number of moles of NaHCO3 calculated in step 3?
7. The answer to step 6 tells you how many moles of HCl were in your second mean titre.
Divide this number by the volume of the second mean titre: this is also the concentration of HCl.
8. Your answers to steps 5 and 7 should be identical. Comment on your findings. N-ch1-36
Volumetric Analysis 2
To standardise hydrochloric acid
Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Results
1. Phenolphthalein indicator (first part of each run)
Burette reagent
Approx. 0.075M hydrochloric acid
Conical flask reagent
Standard sodium carbonate solution
Indicator
Phenolphthalein
To standardise hydrochloric acid
Introduction
In the last practical you prepared a standard solution of sodium carbonate.
Today, you will use it to find the concentration of dilute hydrochloric acid by titration.
This process is known as standardising the hydrochloric acid.
The reaction between sodium carbonate and hydrochloric acid takes place in two stages:
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq) (1)
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
Two indicators are needed to cover both stages:
•in stage 1, phenolphthalein is most suitable, and will respond to the pH change associated with the formation of sodium hydrogencarbonate, NaHCO3.
•in stage 2, methyl orange is most suitable, and will respond to the pH change associated with the final formation of sodium chloride, NaCl.
As a result, this practical gives you experience of titration using two different indicators (the phenolphthalein colour change is easy to spot, whereas the methyl orange colour change is quite difficult to judge).
Apparatus
Goggles
Bench mat
100cm3 beaker
250cm3 beaker
250cm3 conical flask
25cm3 bulb pipette
Pipette filler
Burette
Burette stand and holder
Plastic filter funnel
White tile
Teat pipette
Access to:
your standard sodium carbonate solution
dilute hydrochloric acid to standardise
phenolphthalein indicator solution
methyl orange indicator solution
Methods
1. Transfer a 25cm3 aliquot (portion) of your sodium carbonate solution to a 250cm3 capacity conical flask. Add a few drops of phenolphthalein indicator solution.
2. Titrate with the hydrochloric acid. The end-point of the titration is when the solution just changes from pink to colourless. Note the titre, then add a few drops of methyl orange.
3. Titrate with the hydrochloric acid. The end-point of the titration is when the solution just changes from yellow to red. Note the second titre.
4. Repeat steps 1 - 3 until concordance (i.e. until the readings are the same or within 0.1cm3).
Tabulate your titrations as described in The Burette sheet. You will need two sets of tables.
5. After tidying away, do the calculations described overleaf.
Calculations
1. Calculate the Mr of Na2CO3.
Ar (Na) = 23 Ar (C) = 12 Ar (O) = 16
2. Look back at the accurate mass of sodium carbonate you used in the last practical.
Using your answer to step 1, calculate the number of moles of Na2CO3 that you dissolved in 250cm3 of water during Volumetric Analysis 1.
3. Use your answer to step 2 to calculate the number of moles of Na2CO3 in the 25cm3 transferred to the conical flask.
Stage 1 Phenolphthalein results
4. The equation for the first stage of the reaction between sodium carbonate and hydrochloric acid is shown below again:
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq)
From the equation, you can see that 1 mole of Na2CO3 will react with 1 mole of HCl.
How many moles of HCl will react with the number of moles of Na2CO3 calculated in step 3?
5. The answer to step 4 tells you how many moles of HCl were in your first mean titre.
Divide this number by the volume of the first mean titre: this is the concentration of HCl in mol dm−3.
Stage 2 Methyl orange results
6. The equation for the second stage of the reaction between sodium carbonate and hydrochloric acid is shown below again:
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
From the equation, you can see that 1 mole of NaHCO3 will react with 1 mole of HCl.
The number of moles of NaHCO3 is equal to the number of moles of Na2CO3.
How many moles of HCl will react with the number of moles of NaHCO3 calculated in step 3?
7. The answer to step 6 tells you how many moles of HCl were in your second mean titre.
Divide this number by the volume of the second mean titre: this is also the concentration of HCl.
8. Your answers to steps 5 and 7 should be identical. Comment on your findings. N-ch1-36
Volumetric Analysis 2
To standardise hydrochloric acid
Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Results
1. Phenolphthalein indicator (first part of each run)
Burette reagent
Approx. 0.075M hydrochloric acid
Conical flask reagent
Standard sodium carbonate solution
Indicator
Phenolphthalein
Volumetric Analysis 1- As practical chemistry
Volumetric Analysis 1
To make a standard solution of sodium carbonate
Introduction
A standard solution is one whose concentration is known exactly. Standard solutions of liquids, for example acids, are easy to prepare and are usually supplied. Standard solutions of solids can be prepared by weighing a mass of solid, and dissolving it in a known volume of solution in a volumetric flask. Today, you are going to prepare a standard solution of sodium carbonate to use later in another practical.
Apparatus
Goggles
Bench mat
100 cm3 beaker
250 cm3 beaker
250 cm3 volumetric flask with stopper
Filter funnel
Glass rod
Teat pipette
Spatula
Label
De-ionised water
Anhydrous sodium carbonate, Na2CO3(s)
Methods
Read through the Methods. Make a suitable blank results table, complete with units in the headings.
1. Using the ± 0.1g balance, weigh approximately between 1.2g and 1.4g of sodium carbonate into the small beaker. Do not record the mass.
2. Using the ± 0.01g balance, weigh the small beaker and its contents accurately.
Record this mass.
3. Transfer the contents of the small beaker into the large beaker.
Weigh the small beaker again using the ± 0.01g balance.
Record this mass.
The difference between the two accurate masses is the mass of sodium carbonate in your beaker.
4. Add de-ionised water cautiously down the side of the large beaker.
Use about 150cm3 of water, and swirl the beaker to mix the contents.
5. Stir using a glass rod to dissolve the solid completely.
6. Transfer the solution into the volumetric flask using the funnel.
Remember: pour down the glass rod;
remove the last drop of solution from the glass rod onto the funnel.
Wash the beaker, rod and funnel several times using de-ionised water from the wash bottle, letting the washings go into the flask.
7. Make up to the mark on the volumetric flask with de-ionised water.
Stopper firmly, and shake the flask thoroughly to mix the contents.
8. Label the flask clearly with your name, the date, and the contents of the flask.
Volumetric Analysis 1
To make a standard solution of sodium carbonate:
Technician's Notes
Prior to practical
Sodium carbonate
Heat required amount of sodium carbonate (Na2CO3) to drive off water of crystallisation.
Either: heat in an evaporating dish over a Bunsen burner for 30 minutes approx., or
heat in a drying oven at about 110°C for 1 hour.
Agitate the solid periodically with a clean glass rod.
Transfer to a desiccator after heating, and label it "sodium carbonate".
Care: Use tongs and eye protection.
Beware of hot solid and apparatus.
Sodium carbonate forms caustic alkaline solutions with water; if spilt on skin wash with plenty of water.
Analytical balances
Please check cleanliness and correct functioning of analytical balances.
De-ionised water
Please check 6th Form wash bottles are clean and filled with de-ionised water.
Make sure that additional de-ionised water is available in the aspirator.
Requirements per class
Sodium carbonate solid (see above). Minimum of 3g per student approx.
Analytical balances (see above).
Top pan digital balances (minimum of two if possible).
De-ionised water (see above).
Requirements per student
1 x 250cm3 beaker (dry)
1 x 250cm3 volumetric flask with stopper to fit
1 x glass funnel (check that it will enter the neck of the volumetric flask easily)
1 x glass rod
1 x weighing bottle with lid
1 x 6th Form wash bottle containing de-ionised water
1 x teat pipette
1 x small spatula
1 x self-adhesive label
To make a standard solution of sodium carbonate
Introduction
A standard solution is one whose concentration is known exactly. Standard solutions of liquids, for example acids, are easy to prepare and are usually supplied. Standard solutions of solids can be prepared by weighing a mass of solid, and dissolving it in a known volume of solution in a volumetric flask. Today, you are going to prepare a standard solution of sodium carbonate to use later in another practical.
Apparatus
Goggles
Bench mat
100 cm3 beaker
250 cm3 beaker
250 cm3 volumetric flask with stopper
Filter funnel
Glass rod
Teat pipette
Spatula
Label
De-ionised water
Anhydrous sodium carbonate, Na2CO3(s)
Methods
Read through the Methods. Make a suitable blank results table, complete with units in the headings.
1. Using the ± 0.1g balance, weigh approximately between 1.2g and 1.4g of sodium carbonate into the small beaker. Do not record the mass.
2. Using the ± 0.01g balance, weigh the small beaker and its contents accurately.
Record this mass.
3. Transfer the contents of the small beaker into the large beaker.
Weigh the small beaker again using the ± 0.01g balance.
Record this mass.
The difference between the two accurate masses is the mass of sodium carbonate in your beaker.
4. Add de-ionised water cautiously down the side of the large beaker.
Use about 150cm3 of water, and swirl the beaker to mix the contents.
5. Stir using a glass rod to dissolve the solid completely.
6. Transfer the solution into the volumetric flask using the funnel.
Remember: pour down the glass rod;
remove the last drop of solution from the glass rod onto the funnel.
Wash the beaker, rod and funnel several times using de-ionised water from the wash bottle, letting the washings go into the flask.
7. Make up to the mark on the volumetric flask with de-ionised water.
Stopper firmly, and shake the flask thoroughly to mix the contents.
8. Label the flask clearly with your name, the date, and the contents of the flask.
Volumetric Analysis 1
To make a standard solution of sodium carbonate:
Technician's Notes
Prior to practical
Sodium carbonate
Heat required amount of sodium carbonate (Na2CO3) to drive off water of crystallisation.
Either: heat in an evaporating dish over a Bunsen burner for 30 minutes approx., or
heat in a drying oven at about 110°C for 1 hour.
Agitate the solid periodically with a clean glass rod.
Transfer to a desiccator after heating, and label it "sodium carbonate".
Care: Use tongs and eye protection.
Beware of hot solid and apparatus.
Sodium carbonate forms caustic alkaline solutions with water; if spilt on skin wash with plenty of water.
Analytical balances
Please check cleanliness and correct functioning of analytical balances.
De-ionised water
Please check 6th Form wash bottles are clean and filled with de-ionised water.
Make sure that additional de-ionised water is available in the aspirator.
Requirements per class
Sodium carbonate solid (see above). Minimum of 3g per student approx.
Analytical balances (see above).
Top pan digital balances (minimum of two if possible).
De-ionised water (see above).
Requirements per student
1 x 250cm3 beaker (dry)
1 x 250cm3 volumetric flask with stopper to fit
1 x glass funnel (check that it will enter the neck of the volumetric flask easily)
1 x glass rod
1 x weighing bottle with lid
1 x 6th Form wash bottle containing de-ionised water
1 x teat pipette
1 x small spatula
1 x self-adhesive label
Finding the formula of magnesium oxide- As practical chemistry
Finding the formula of magnesium oxide
Aims (Eye protectionmust be worn )
When magnesium is heated in air, it reacts with oxygen. During this oxidation reaction, magnesium oxide is produced. This increases the mass. If we know the mass of magnesium at the start, and the mass of magnesium oxide produced at the end, we can work out the mass of oxygen which has been combined with the magnesium. We can use these masses to work out the formula of magnesium oxide.
Apparatus
Goggles
Bench mat
Tripod
Bunsen burner
Pipe-clay triangle
Crucible and lid
Tongs
10cm length of magnesium ribbon
Small piece of sandpaper
Digital balance
Methods
1. Weigh the empty crucible with its lid, and write down the result in a table.
2. Clean the piece of magnesium ribbon with sandpaper, then coil it loosely around a pencil.
Put the magnesium ribbon into the crucible and put the lid on.
Weigh the crucible, lid, and magnesium together, and write down the result in your table.
3. Put the crucible onto the pipe-clay triangle. Leave the lid slightly ajar to to allow air into the crucible. Heat gently for a minute, then strongly. Continue heating until the reaction has finished (the magnesium will glow at first, then look a bit like a furry grey-black caterpillar – it really will!)
4. Turn the Bunsen burner off, and allow the crucible to cool for a few minutes.
Reweigh the crucible with its lid and contents, and write down the result in your table. cruciblepipe-claytriangletripodHEAT
Results (suggested table only – do not write on this sheet)
crucible + lid =
crucible + lid + magnesium =
crucible + lid + contents after reaction =
Conclusions
1. Work out the mass of magnesium used in the experiment. Apparatus to oxidise magnesium
2. Work out the mass of magnesium oxide formed.
3. Use your answers to (1) and (2) to work out the mass of oxygen gained.
4. Look up Ar(Mg) and Ar(O). Use these numbers, and the masses worked out in parts (1) and (3), to work out the number of moles of magnesium and oxygen involved.
5. Finally, work out your formula for magnesium oxide. You will probably find that the formula has a complicated number in it – round it off to 1 decimal place.
6. The accepted formula is MgO. How close did you get?
Explain why your formula might be different from the accepted one.
Aims (Eye protectionmust be worn )
When magnesium is heated in air, it reacts with oxygen. During this oxidation reaction, magnesium oxide is produced. This increases the mass. If we know the mass of magnesium at the start, and the mass of magnesium oxide produced at the end, we can work out the mass of oxygen which has been combined with the magnesium. We can use these masses to work out the formula of magnesium oxide.
Apparatus
Goggles
Bench mat
Tripod
Bunsen burner
Pipe-clay triangle
Crucible and lid
Tongs
10cm length of magnesium ribbon
Small piece of sandpaper
Digital balance
Methods
1. Weigh the empty crucible with its lid, and write down the result in a table.
2. Clean the piece of magnesium ribbon with sandpaper, then coil it loosely around a pencil.
Put the magnesium ribbon into the crucible and put the lid on.
Weigh the crucible, lid, and magnesium together, and write down the result in your table.
3. Put the crucible onto the pipe-clay triangle. Leave the lid slightly ajar to to allow air into the crucible. Heat gently for a minute, then strongly. Continue heating until the reaction has finished (the magnesium will glow at first, then look a bit like a furry grey-black caterpillar – it really will!)
4. Turn the Bunsen burner off, and allow the crucible to cool for a few minutes.
Reweigh the crucible with its lid and contents, and write down the result in your table. cruciblepipe-claytriangletripodHEAT
Results (suggested table only – do not write on this sheet)
crucible + lid =
crucible + lid + magnesium =
crucible + lid + contents after reaction =
Conclusions
1. Work out the mass of magnesium used in the experiment. Apparatus to oxidise magnesium
2. Work out the mass of magnesium oxide formed.
3. Use your answers to (1) and (2) to work out the mass of oxygen gained.
4. Look up Ar(Mg) and Ar(O). Use these numbers, and the masses worked out in parts (1) and (3), to work out the number of moles of magnesium and oxygen involved.
5. Finally, work out your formula for magnesium oxide. You will probably find that the formula has a complicated number in it – round it off to 1 decimal place.
6. The accepted formula is MgO. How close did you get?
Explain why your formula might be different from the accepted one.
Making Standard Solutions- As chemistry practicals
Making Standard Solutions
What is a standard solution?
A standard solution is a solution whose concentration is known accurately. Its concentration is usually given in mol dm–3. When making up a standard solution it is important that the correct mass of substance is accurately measured. It is also important that all of this is successfully transferred to the volumetric flask used to make up the solution. The following procedure will make sure that this happens.
Background calculations
1. Work out the number of moles needed to make up a solution with the required volume and concentration.
moles = concentration x volume
concentration is in mol dm–3 … and …
volume is in dm3, so if the volume is given in cm3, divide it by 1000 to get dm3
Show your working in the space below.
2. Now work out the relative formula mass, Mr, of the chosen substance. Show your working in the space below.
3. Work out the mass of substance needed using your answers from steps 1 and 2. Show your working in the space below.
To find the Mr of carbon dioxide, CO2:
CO2 has … 1 carbon atom … 1 x 12 = 12
2 oxygen atoms … 2 x 16 = 32
add together … = 44
It helps to remember that:
“mass is mister mole”, or
mass = Mr x mole
Making up the solution
• Take a watch glass and place it on the balance. Tare the balance (set it to zero).
Carefully weigh out the required mass of substance.
• Transfer this amount to a beaker. Add water from a wash bottle to dissolve it.
Use some of the water to rinse all the substance off the watch glass. Do this at least twice.
• Stir with a glass rod until all the solid is dissolved, then transfer the solution to the volumetric flask.
Use more water from the wash bottle to rinse out the beaker and the glass rod. Do this at least twice.
• Add water to just below the line on the volumetric flask.
Add the final drops with a teat pipette to ensure that the bottom of the meniscus is on the line.
• Put the lid on the flask and turn the flask over a couple of times to mix the solution.
• Label your solution with your name, the date, and the contents, e.g. 2.0M NaCl. Then tidy up!
What is a standard solution?
A standard solution is a solution whose concentration is known accurately. Its concentration is usually given in mol dm–3. When making up a standard solution it is important that the correct mass of substance is accurately measured. It is also important that all of this is successfully transferred to the volumetric flask used to make up the solution. The following procedure will make sure that this happens.
Background calculations
1. Work out the number of moles needed to make up a solution with the required volume and concentration.
moles = concentration x volume
concentration is in mol dm–3 … and …
volume is in dm3, so if the volume is given in cm3, divide it by 1000 to get dm3
Show your working in the space below.
2. Now work out the relative formula mass, Mr, of the chosen substance. Show your working in the space below.
3. Work out the mass of substance needed using your answers from steps 1 and 2. Show your working in the space below.
To find the Mr of carbon dioxide, CO2:
CO2 has … 1 carbon atom … 1 x 12 = 12
2 oxygen atoms … 2 x 16 = 32
add together … = 44
It helps to remember that:
“mass is mister mole”, or
mass = Mr x mole
Making up the solution
• Take a watch glass and place it on the balance. Tare the balance (set it to zero).
Carefully weigh out the required mass of substance.
• Transfer this amount to a beaker. Add water from a wash bottle to dissolve it.
Use some of the water to rinse all the substance off the watch glass. Do this at least twice.
• Stir with a glass rod until all the solid is dissolved, then transfer the solution to the volumetric flask.
Use more water from the wash bottle to rinse out the beaker and the glass rod. Do this at least twice.
• Add water to just below the line on the volumetric flask.
Add the final drops with a teat pipette to ensure that the bottom of the meniscus is on the line.
• Put the lid on the flask and turn the flask over a couple of times to mix the solution.
• Label your solution with your name, the date, and the contents, e.g. 2.0M NaCl. Then tidy up!
Estimating Errors in Chemistry-As level chemistry practicals
You must be able to estimate the size and importance of errors in practical work to gain full marks in your practical assessments. The table summarises what you need to do about errors in each Skill:
1.Organise the procedure to be followed, selecting appropriate techniques, reagents and apparatus, with due regard to precision of measurement and scale of working.
2.Make and record measurements to a degree of precision allowed by the apparatus used.
Identify sources of error, and recognise the limitations of experimental measurements.
3.Assess the reliability of your data and the conclusions drawn from them, taking into account the errors in the data obtained.
The detail required is much less than that required for Physics A Level, but you must say more than "I might have forgotten to wash out my beaker" – this is operator incompetence rather than an error in the sense required by the examination board. This guide is intended to show you how you can estimate the size and importance of errors in Chemistry to help you to gain full marks in assessments.
Mass
1. Consider weighing 1g of solid. If you use a two decimal place balance, the mass recorded will be to the nearest 0.01g. In this example, the % error will be:
0.01 x 100 = 1%
2. Consider weighing the same 1g of solid on a three decimal place balance. The mass recorded will be to the nearest 0.001g, and so the % error will be:
0.001 x 100 = 0.1% There is much less error involved in this procedure, but other
1 procedures like volume measurements might swamp this improvement.
3. Consider weighing 10g of solid on the two decimal place balance. In this case the % error will be:
0.01 x 100 = 0.1% This is less than weighing 1g on this balance, so choose the right
10 balance for the amount of material to be weighed.
Volume
4. Consider measuring 25cm3 in a 25cm3 measuring cylinder. The measurement will normally be to the nearest 0.5cm3, so the % error will be:
0.5 x 100 = 2.0% Rather upsetting if you have just weighed to ± 0.1%, so what can you
25 do? Turn over to find out …
5. If you use a transfer pipette, the accuracy is quoted on the apparatus itself, but is normally 0.25%, so using this is usually more appropriate than a measuring cylinder. It is important to remember when designing your own experiments that pipettes come in a limited range of volumes with 25cm3 being most commonly used, so try to avoid peculiar volumes which would need measuring cylinders. You could, however, consider graduated pipettes.
6. If you use a graduated pipette, as with all measuring equipment, specify a capacity closest to the target volume. For example, if you want 25cm3 of liquid using a 25cm3 graduated pipette, the reading will usually be to the nearest 0.2cm3, so the % error will be:
0.2 x 100 = 0.8% Much better than using the measuring cylinder, but still worse than
25 choosing to measure the same volume with a transfer pipette.
7. If you use a burette, you can obtain readings to the nearest 0.05cm3. If you aim for a titre of 25cm3, the % error would be:
0.05 x 100 = 0.2% Used well, the burette is very accurate (note that many Skill 2
25 assessments – Implementing – concentrate on burette skills).
Why aim for a titre of 25cm3? If you obtain a titre of 5cm3, the error involved is 1% (very much more than 0.2%); if you obtain a titre of 50cm3, the error is 0.1% (not much of an improvement for using twice as much titrant).
8. Finally, consider the standard flask. Like the transfer pipette, the accuracy is quoted on the apparatus itself, but is normally 0.125%. For large volumes it is likely to be the most accurate piece of apparatus, but again remember to avoid peculiar volumes in your experimental designs.
Time
9. Consider a reaction time of 100s. Measured on a common laboratory stopclock, the % error is:
1 x 100 = 1.0% This is quite large, and gets worse if reaction rates are faster. For
100 example, the error involved in a 10s reaction is ± 10%. Doing a
preliminary experiment may help avoid this when designing.
10. Finally, consider a reaction time of 100s measured on a digital stopwatch. These often show times to the nearest 0.01s. The % error would be:
0.01 x 100 = 0.01%
This is wonderful, but how quickly can you stop and start it?
100 Lots of people practice this when they should be doing something else –
my best times are around 0.10s. In this case, the error is really more
like 0.1%, and operator incompetence becomes important.
Do some preliminary calculations to establish the likely quantities. Alter these, if appropriate, to avoid non-standard volumes. Specify the most suitable apparatus.
Record your results to the precision allowed by the apparatus. For example, if the balance reads to 0.01g, make sure you don’t round up 6.78g to 6.8g or 7g.
Estimate the % error involved in each measurement. Work out the total apparatus error. Identify the apparatus or step that produces the biggest error.
Work out whether the apparatus error is enough to account for any difference between your result and your teacher’s result. Suggest more precise apparatus.
AS and A Level Practical Work- burette
How to use it
1. Fix the burette into the burette holder, taking care that it is vertical and stable.
Place a beaker underneath the burette.
2. Close the tap, and run some de-ionised water into the top of the burette.
Let the water clean the inside of the burette.
Open the tap, and allow the water to drain out. Repeat.
3. Close the tap, and (using the funnel) run some of the required reagent, e.g. acid, into the top of the burette. Open the tap, and allow the reagent to drain through into the beaker. Repeat.
4. Close the tap, and fill the burette to just above the 0.00 cm3 mark with the required reagent.
Remove the funnel. Make sure that there are no air bubbles inside the burette.
Slowly open the tap, and allow the reagent to run down to (or just past) the 0.00 cm3 mark.
Close the tap.
5. Remove the beaker, and place a white tile under the burette. Put a conical flask under the burette, and adjust the height of the burette so that the tip is just above the lip of the conical flask.
The burette is now ready for use.
1.Construct Results tables like the ones below. Before you start, record the reagents used.
2. In the first run, you should overshoot the end-point a little.
Indicate that you have done this by drawing a pencil line through the Run 1 column and writing "overshoot" against it.
3. In subsequent runs, allow the burette reagent to run through more slowly as you reach the end-point to get a more accurate result. Record the start and end volumes as you go.
Record volumes to the nearest 0.05cm3, i.e. all volumes should end in .x0cm3 or .x5cm3.
4. If your three accurate runs are very different from each other, repeat until a more consistent result is obtained, i.e. to concordance (± 0.10cm3). Put a pencil tick against the titres you use in your calculation of the mean titre (see below).
5. Calculate the mean volume delivered (the titre) and record it in the table
1. Fix the burette into the burette holder, taking care that it is vertical and stable.
Place a beaker underneath the burette.
2. Close the tap, and run some de-ionised water into the top of the burette.
Let the water clean the inside of the burette.
Open the tap, and allow the water to drain out. Repeat.
3. Close the tap, and (using the funnel) run some of the required reagent, e.g. acid, into the top of the burette. Open the tap, and allow the reagent to drain through into the beaker. Repeat.
4. Close the tap, and fill the burette to just above the 0.00 cm3 mark with the required reagent.
Remove the funnel. Make sure that there are no air bubbles inside the burette.
Slowly open the tap, and allow the reagent to run down to (or just past) the 0.00 cm3 mark.
Close the tap.
5. Remove the beaker, and place a white tile under the burette. Put a conical flask under the burette, and adjust the height of the burette so that the tip is just above the lip of the conical flask.
The burette is now ready for use.
1.Construct Results tables like the ones below. Before you start, record the reagents used.
2. In the first run, you should overshoot the end-point a little.
Indicate that you have done this by drawing a pencil line through the Run 1 column and writing "overshoot" against it.
3. In subsequent runs, allow the burette reagent to run through more slowly as you reach the end-point to get a more accurate result. Record the start and end volumes as you go.
Record volumes to the nearest 0.05cm3, i.e. all volumes should end in .x0cm3 or .x5cm3.
4. If your three accurate runs are very different from each other, repeat until a more consistent result is obtained, i.e. to concordance (± 0.10cm3). Put a pencil tick against the titres you use in your calculation of the mean titre (see below).
5. Calculate the mean volume delivered (the titre) and record it in the table
Subscribe to:
Posts (Atom)