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.