Question 3A
(a) Describe one chemical test to distinguish between carbon (IV) oxide and sulphur (IV) oxide.
[3 marks]
(b) (i) State the laboratory method of collection of each of the following gases:
(I) H2;
(II) NH3;
(III) HCl.
(ii) Give a reason for each of the answers stated in 3(b)(i). [6 marks]
(c) State the method used in separating each of the following mixtures:
- two miscible liquids;
- soluble salt and insoluble salt.
[3 marks]
(d) Explain briefly why a solution of KCl does not give off a gas when mixed with NaHCO3 solution, but a solution of AlCL3 does.
[3 marks]
Observation
This question was based on the knowledge of Test of Practical. It was attempted by majority of the candidates and their performance was above average.
In part (a), majority of the candidates described one chemical test to distinguish between carbon (IV) oxide and sulphur (IV) oxide.
In part (b), majority of the candidates stated the laboratory method of collection of the gases.
In part (c), majority of the candidates stated the method used in separating the mixtures.
In par (d), majority of the candidates could not explain why a solution of KCl does not give off a gas when mixed with NaHCO3, but a solution of AlCl3 does.
The expected answers include:
(a) Pass the gas through acidified KMnO4 solution if the pink colour does not change then the gas is CO2 if the pink colour changes to colourless then the gas is SO2
OR
SO2 turns acidified K2Cr2O7 solution green while CO2 does not
(b) (i) I. -: H2 - downward displacement of air / upward delivery
II. -: NH3 - upward delivery / downward displacement of air
III. -: HCl - downward delivery / upward displacement of air
(ii) I - lighter / less dense than air
II - lighter / less dense than air
III. - denser / heavier than air
(c) (i) Use of simple / fractional distillation
(ii) Dissolution in water followed by filtration to obtain insoluble salt. Then
crystallization / evaporation to dryness of filtrate to obtain soluble salt.