Chemistry Paper 1 (Practical) ,May/June 2011

Questions:				1	2	3	4	5	6		Main

General Comments
Weakness/Remedies
Strength

Question 2

C is a mixture of two salts. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusion drawn from the result of each test.

Put all of C into a boiling tube and add about 5cm3 of distilled water. Stir thoroughly and filter. Keep both the residue and the filtrate.
To about 2 cm3 of the filtrate, add few drops of Pb(NO3)2(aq)‑Boil the mixture and then allow to cool.
(i) Put the residue in a test tube and add dilute HNO3. Shake the mixture and divide the solution into two portions.
(ii) To the first portion from (c)(i), add NaOH(aq) in drops and then in excess.
(iii) To the second portion from (c)(i), add aqueous ammonia in drops and thein in excess.

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OBSERVATION

Adding distilled water to the specimen in a boiling tube and making accurate observations were successfully done by most candidates.

Few candidates however were not able to give logical deductions from their observation while some could neither carry out the tests as prescribed nor were able to record their observations.

It was observed that many candidates still confused residue with precipitate and or solid. Other mistakes include: white ppt after filteration instead of white residue; clear filtrate instead of colourless filtrates; ppt disappears instead of ppt dissolves; white chalky ppt instead of white gelationous ppt and carrying out the test on the dry sample instead of solution as demanded by the question.

TEST	OBSERVATION	INFERENCE
a) C + water, mixture stirred and filtered	Partly dissoves/soluble Colourless filtrate White residue (do not accept ppt/solid)	C contains soluble and insoluble salts
b) Filtrate + Pb (NO3)2(aq) Boiled then cooled	White precipitate formed Precipitate dissovles Precipitate reappeares	Cl-, SO42- or CO32-any for (2 marks) (SO32- do not score but do not penalize) Cl- present Cl- confirmed
c) (i) Residue + dil HNO3	Effervescence/bubles/gas evolved colourless odourless gas. Gas turns lime water milky.	CO2 evolved from CO32-
(ii) Solution from (c)(i) + NaOH(aq) in drops then in excess	White gelatinous ppt formed Precipitate dissovles	Zn2+ or A13+ Zn2+ or A13+
(iii) Solution from (c)(i) + HN3(aq) in drops then in excess	White gelatinous Precipatate form Precipitate dissovles	Zn2+ or A13+ Zn2+ confirmed