Chemistry Paper 1 (Practical) ,May/June 2011

Questions:				1	2	3	4	5	6		Main

General Comments
Weakness/Remedies
Strength

Question 5

F is a mixture of two compounds. Carry out the following exercises on F. Record your observations and identify any gas(es) evolved. State the conclusion drawn from the results of each test.

Put all of F into a beaker or a boiling tube and add about 10 cm3 of distilled water. Stir the mixture thoroughly and filter. Keep both the filtrate and the residue.
(i) Divide the filtrate into two portions.
I.To the first portion, add dilute HNO3 followed byAgNO3(aq).
II. Add excess NH3(aq) to the resulting mixture.
(ii) To the second portion, add NH3(aq) in drops and then in excess.
Add about 5 cm3 of distilled water to the residue in a test tube. Boil and allow to cool. Add few drops of iodine solution.

OBSERVATION

Majority of the responding candidates carried out the required tests, made their observations and drew inferences in the prescribed format. However, some candidates lost marks for not reporting that no visible reaction was observed on adding HNO3 to the filtrate and for including pb2+ as ions present when the filtrate was treated with NH3(aq)

Observation of jelly-like paste was also not recorded by many candidates when water was added to residyue and then heated.

On addition of iodine to the cooled mixture, most candidates recorded black ppt instead of blue-black colouration. General performance was fair.

The expected answers are:

	TEST	OBSERVATION	INFERENCE
a)	F + water, stirred mixture and filtered	Partly dissoves/soluble Colourless filtrate White residue	F contains soluble and insoluble salts
b) (i) I II	Filtrate + dil HNO3 + Ag NO3(aq) Mixture in (b)(i) + excess NH3 solution	No visible reaction White precipitate formed Precipitate dissovles	Cl- present Cl- confirmed
b)(ii)	Filtrate + HN3(aq) in drops then in excess	White gelatinous precipitate Precipitate dissovles	Zn+2 or A13+ may be present Zn2+ confirmed
c)	Residue + water + heat allow to cool + iodine solution	Jelly-like paste is formed Dark/deep blue/blue black colouration is obtained	Starch is confirmed