Chemistry 3 Nov/Dec 2013

Chemistry Paper 3,Nov/Dec 2013

Questions:		1	2	3	Main

General Comments
Weakness/Remedies
Strength

Question 2

D is an inorganic salt. E is a solution of an inorganic salt. The tests recorded in the table below were performed on D and E.

Copy and complete the table.

	Test	Observation	Inference
(a)	D + distilled water	Did not dissolve	……………………..
(b)	D + dil. HCI + heat	Dissolved to form blue or greenish-blue solution	…………………….
(c)	Solution of D + NaOH(aq) in drops then in excess	Blue gelatinous precipitate …………………………	……………………..
(d)	Solution of D + NH3(aq) in drops then in excess	………………………….. ……………………………	Cu2+ present Cu2+ confirmed
(e)	(i) E(aq) + NaOH(aq) in drops and in excess	No precipitate	………………….
(e)	(ii)Mixture in 2(e)(i) warmed	…………………………… ………………………….. …………………………..	NH3 gas evolved, therefore NH4+ present
(f)	E(aq) +BaCl2(aq)	White chalky precipitate. No gas evolved, precipitate did not dissolve	……………………. ……………………. ……………………

In a titration experiment, 22.50 cm3 of an acid solution A containing 10.6 g of NaHSO4 per dm3 reacted with 25.0 cm3 of solution B containing X g of NaOH per dm3. The equation for the reaction is: NaHSO4(aq) + NaOH(aq) Na2SO4(aq) + H2(l)

(a) From the information given above, calculate the:

(i) concentrate of A in moldm– 3;
(ii) concentrate of B in moldm– 3;
(iii) value of X;
(iv) mass of Na2SO4 formed during the reaction. [ H = 1.00; 0 = 16.0; Na = 23.0; S = 32.0] (16 marks)

(b)

(i) Name a suitable indicator for the titration experiment.
(ii) State the apparatus used to measure the volume of solution: I. A; II B; (3 marks)

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OBSERVATION

The candidate made a fair attempt on this question. Majority of the candidates obeyed the instruction “copy and complete”, thus was commendable. Some candidates still cannot differentiate between gelatinous and chalky precipitate. Results of test in drops and excess still seem confusing to some candidates as some of them gave the result together (i.e. only in excess).

The expected answers include:

	Test	Observation	Inference
(a)	D + distilled water	Did not dissolve	D is an insoluble salt
(b)	D + dil. HCl + heat	Dissolved to form blue or greenish blue solution	Cu2+ may be present
(c)	Solution of D + NaOH(aq) in drops then in excess	Blue gelatinous precipitate. Precipitate insoluble	Cu2+ present
(d)	Solution of D + NH3(aq) in drops then in excess	Blue gelatinous Precipitate Precipitate dissolves forming deep blue solution	Cu2+ present Cu2+ present confirmed
(e)(i)	E(aq) + NaOH(aq) in drops and in excess	No precipitate	NH4+may be present
(ii)	Mixture in 2(e)(i) warmed	Colourless gas evolved Pungent/choking/irritating smell turns red litmus to blue/forms dense white fume with conc. HCl	NH3 gas evolved NH4+ present
(f)	E(aq) + BaCl2(aq) + excess HCl(aq)	White chalky precipitate. No gas evolved Precipitate did not dissolve	CO32-SO32- SO42- SO42- Present (SO42- must have been mentioned above)