Chemistry Paper 1 (Practical) ,May/June 2012

Questions:				1	2	3	4	5	6		Main

General Comments
Weakness/Remedies
Strength

Question 2

C and D are two aqueous solutions. Carry out the following exercises on C and D. record your observations and identify any gas(es) evolved. State the conclusion you draw from the result of each test.
(a) (i) To about 2 cm3 portion of C, add NaOH(aq) in drops until in excess.
                        Warm the mixture.
(ii)To another 2 cm3 portion of C, add HCl(aq) followed by BaCl2(aq).                                                                                                                                                                                                                                   [9 marks]
(b) (i)         To about 2 cm3 portion of D, add NH3(aq) in drops and then in excess.

To another 2 cm3 portion of D, add AgNO3(aq) followed by HNO3(aq).

[8 marks]

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OBSERVATION

This question was attempted by majority of the candidates and the performance was good.
Candidates correctly carried out the expected tests, made their observations and inferences in the prescribed format.
However some candidates lost marks due to the following:
-           not adhering to instruction e.g adding HNO3 before adding AgNO3 to a portion of D;
-           incorrect observation and inferences;
-           use of wrong charges on ions infered.
The expected answers were:

Test	Observation	Inference
(a) (i) C + NaOH(aq) + warm	No visible reaction/no ppt. A colourless gas with pungent/chocking/irritating smell evolved. Gas turned damp red litmus blue/formed white fumes with conc. HCl vapour.	Gas is NH3 from NH4+
(ii) C + HCl(aq) + BaCI2(aq)	No visible reaction. White ppt formed.	SO42- present.
(b) (i) D + NH3(aq) + excess	Reddish-brown/brown ppt. Ppt insoluble.	Fe3+ present.
(ii) D + AgNO3(aq) + HNO3 (aq)	White ppt. Ppt insoluble.	C- or CO32- present. C- present.