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

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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