Chemistry 3 Nov/Dec 2008

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Chemistry Paper 3,Nov/Dec 2008

Questions:					1	2	3				Main

General Comments
Weakness/Remedies
Strength

Question 3

            X is a solution of a simple salt.
            Y is a trioxonitrate (V) salt.

The tests recorded in the table below were performed as indicated. Copy and complete the table

	TEST	OBSERVATION	INFERENCE
(a)(i)	X + NaOH(aq) in drops, then in excess	_____________ _____________	Cu2+ present
(ii)	X + NH3(aaq) in drops, then in excess	_____________ _____________	_____________
(iii)	X + BaCl2(aq) then followed by _____________	White precipitate precipitate insoluble	_____________ _____________
(b)(i)	Y + H2O	_____________	Y is a soluble salt
(ii)	Y(aq) + NaOH(aq) in drop then in excess	White gelatinous ppt _____________	_____________
(iii)	Y(aq) + NH3(aq) in drops, then in excess	_____________ _____________	Zn2+ present
(iv)	Y(aq) + FeSO4(aq) followed by _____________	_____________	NO3- present

[18 marks]

_____________________________________________________________________________________________________

OBSERVATION

This question was attempted by most candidates and the performance was fair.

Candidates showed good knowledge of qualitative analysis by copying and completing the table as required by the question. However, some of them lost marks because they could not correctly assign charges on ions.

The expected duly completed table was as follows:

	TEST	OBSERVATION	INFERENCE
(a)(i)	X + NaOH(aq) in drops, then in excess	blue precipitate formed precipitate insoluble	Cu2+ present
(ii)	X + NH3(aaq) in drops, then in excess	light blue precipitate precipitate soluble/dissolves in excess to form a deep form a deep blue solution	Cu2+ confirmed
(iii)	X + BaCl2(aq) drops, then in excess	White precipitate precipitate insoluble	SO42-,SO32- or CO32- present SO42-, present
(b)(i)	Y + H2O	Y dissolves	Y is a soluble salt
(ii)	Y(aq) + NaOH(aq) in drop then in excess	White gelatinous ppt precipitate soluble/dissolves	Zn2+ or Al3+ present
(iii)	Y(aq) + NH3(aq) in drops, then in excess	White gelatinous precipitate precipitate soluble/dissolves	Zn2+ present
(iv)	Y(aq) + FeSO4(aq) followed by Conc. H2SO4/ H2SO4	Brown ring at the junction of the two solutions	NO3- present