Chemistry 3 Nov/Dec 2008

Chemistry Paper 3,Nov/Dec 2008

Questions:					1	2	3				Main

General Comments
Weakness/Remedies
Strength

Question 2

D is a mixture of a soluble lead salt and an insoluble white powder. The mixture was added to 15 cm3 of distilled water, stirred and filtered.

The tests indicated in the table below were carried out on the filtrate and the residue.

Copy and complete the table as appropriate.
Do not complete the inference column in 2(a)(i) and 2 (a)(ii).

*Test*	P	Q	R
Portion of filtrate + NaOH(aq) in drops ;then in excess			8.0 - 9.6
Portion of filtrate + NH3(aq) in drops, then in excess
Portion of the filtrate+FeSO2(aq) NOpresent + Conc. H2SO4		NO^-3present
Residue + dil.HNO3		CO^2-₃present
Portion of solution from White precipitate (b)(i) + NaOH(aq) in insoluble in excess drops, then in excess	White precipitate (b)(i) + NaOH(aq) in insoluble in excess
Portion of solution from (b)(i) + NH3(aq) in drops, then in excess

Select the indicator(s) that would be suitable for titrating:
(i) strong acid with strong base;
(ii) strong acid with weak base;
(iii) weak acid with strong base.

_____________________________________________________________________________________________________

OBSERVATION

This question was attempted by majority of the candidates and the performance was fair. Candidates correctly copied and completed the table as required but some of them lost marks for wrong observations and inferences. The expected table from candidates was as follows:

*Test*	P	Q	R
Portion of filtrate + NaOH(aq) in drops ;then in excess	White precipitate Soluble in exces then in excess		8.0 - 9.6
Portion of filtrate + NH3(aq) in drops, then in excess	White precipitate insoluble in excess
Portion of the filtrate+FeSO2(aq) NOpresent + Conc. H2SO4	Brown ring	NO^-3present
Residue + dil.HNO3 ;	Effervescence Gas turned lime water milky. Colourless solution/ ppt dissolves	CO^2-₃present
Portion of solution from White precipitate (b)(i) + NaOH(aq) in insoluble in excess drops, then in excess	White precipitate (b)(i) + NaOH(aq) in insoluble in excess
Portion of solution from (b)(i) + NH3(aq) in drops, then in excess	No ppt/no visible