Chemistry 3 May/June 2014

Chemistry Paper 3,May/June 2014

Questions:		1	2	3		Main

General Comments
Weakness/Remedies
Strength

Question 2

C contains two cations and two anions. Perform the following exercises
on C. Record your observations and identify any gas(es) evolved. State the
conclusion you draw from the result of each test.

(a) Dissolve all of C in about 10 cm3 of distilled water:
Stir the resulting solution thoroughly.

(i) To about 2cm3 of the solution, add few drops of
AgNO3 solution, followed by HNO3(aq).

(ii) To another 2cm3 portion of the solution, add dil. HC1
followed by BaC12 solution.

(iii) To another 2 cm3 portion of the solution, add NaOH(aq)
dropwise and then in excess. Warm the mixture.

                        (iv)       To another 2cm3 portion of the solution, add NH3(aq)
                                    dropwise and then in excess.
                                                                                                                                    [20 marks]

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OBSERVATION

This question is based on qualitative analysis. It involved the identification of cations and onions.

The question was attempted by majority of the candidates and their performance was fair.

Majority of the candidates carried out the required text, made their observation and inferences logically.

Some candidates lost marks due to wrong test, observation, inference or the used of wrong charges on ions.

The expected answers include:

	Test	Observation	Inference
(a)(i)	C(aq) + AgNO3(aq) + HNO3(aq) Mixture + NH3(aq) exces	White precipitate formed Precipitate insoluble. Precipitate soluble / dissolved	Cl-, S2-, CO32- indicated Cl- present
(ii)	C(aq) + HCl(aq) + BaCl2(aq)	No visible reaction / no gas evolved White (chalky) precipitate formed	SO42- present
(iii)	C(aq) + NaOH(aq) in drops in excess Mixture warmed	White gelatinous precipitate White precipitate dissolved / soluble Colourless gas evolved pungent / choking / irritating smell evolved Gas turns red litmus paper blue/forms dense white fumes with concentrated HCl.	Zn2+ , Al3+ indicated Zn2+, Al3+ indicated NH3 gas evolved NH4+ present
(iv)	C(aq) + NH3(aq) in drops then excess	White gelatinous precipitate Precipitate dissolved / soluble	Zn2+ , Al3+ indicated Zn2+