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Chemistry Paper 3,Nov/Dec 2010  
Questions:         1 2 3       Main
General Comments

Question 3

(a) Outline how a mixture of ammonium chloride, potassium chloride and lead (II)
          chloride could be separated in the laboratory.                                            [ 7 marks]

(b) Name one drying agent that is
(i) solid,
(ii) liquid [2 marks]

(c) Mention one laboratory apparatus used for:

(i) preparing a standard solution;
(ii) converting vapour to liquid;
(iii) keeping solid samples dry;
(iv) generating gases intermittently;
(v) determining the pH of a solution. [5 marks]

(d)(i) Describe a chemical test that could be used to distinguish between pure water and sodium tetraoxosulphate(VI)solution
(ii) Zn(OH)2 and Ca(OH)2 are white substances What reagent would be suitable to distinguish between the two substances?    [ 4 mark s ]



The question was attempted by most candidates and the performance was poor.

In part (a), candidates were unable to outline how a mixture of ammonium chloride, potassium chloride and lead (II) chloride could be separated in the laboratory. The expected answers from candidates were as follows:

- Pour the mixture into an evaporating dish and cover with a glass funnel.
- Heat the mixture
- Ammonium chloride sublimes and collects in the funnel
- Add distilled water to the mixture
- Potassium chloride dissolves
- Filter and collect the lead (II) chloride as residue
- Evaporate the filtrate to dryness to obtain potassium chloride.

In part (b), candidates correctly named calcium chloride/calcium oxide/Silica gel as solid drying agent in (i) and concentrated tetraoxosulphate (VI) acid as liquid drying agent in (II).

In part (c), candidates correctly mention one laboratory apparatus used in(i)-(v) as follows:
(i)       Volumentric/standard flask
(ii)      Liebig condenser
(iii) desiccators
(iv) kipps apparatus
(v)      pH meter.

In part (d) (i), candidates could not describe a chemical test to distinguish between pure water and sodium tetraoxosulphate (VI) solution. The required description from candidates was as follows:

- To a sample of each liquid, add BaCl2(aq)
- The sample that forms white precipitate is sodium tetraoxosulphate(V1)
- No precipitate shows the presence of pure water.

In(d) (ii), only few candidates stated NaOH(aq)/NH3(aq) as the reagent suitable to distinguish the two white substances.

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