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Chemistry Paper 3,Nov/Dec 2010  
Questions:         1 2 3       Main
General Comments
Weakness/Remedies
Strength

























































Question 2

C was a mixture of an inorganic salt and organic compound. Tests were carried out on C.
Copy and complete the table and hence state what C could be.

 

 

Test

Observation

Inference

 

(a) (i)

C + 10cm3 of distilled

C partially dissolved

 

 

 

water, stirred and filtered

to give colourless

 

 

 

 

filtrate and a white

 

 

 

 

residue

 

(in

Filtrate + Litmus paper

 

Neutral filtrate

(b)

(i)

First portion of filtrate +

 

 

 

 

AgN03(aq)

 

 

 

 

+HN03(aq)

 

 

 

 

+NH3(aq)

 

Cl- confirmed

 

(ii)

Second portion of filtrate

White gelatinous

 

 

 

+ NaOH(aq) in drops

precipitate

 

 

 

then in excess

Precipitate dissolved

 

 

(iii)

Third portion of filtrate

White gelatinous

 

 

 

+ NH3(aq) in drops

precipitate

 

 

 

then in excess

Precipitate dissolved

 

(c)

(i)

Portion of residue +

 

Starch present

 

 

 

Iodine solution

 

 

(ii)

Portion of residue +

 

Starch

 

 

HCl(aq) and boil

 

hydrolysed

 

 

cool and add iodine

 

 

 

 

solution

 

 


                                                                                                                   

_____________________________________________________________________________________________________
OBSERVATION

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

Candidates copied and completed the table however, a good number of them lost marks because of wrong observations and inferences and inability to state the composition of salt C. The correct table from candidates was as follows:

 

Test

Observation

Inference

(a) (i)

C + l0cm3 distilled water,

C partially dissolved to give

C contains soluble

 

and filtered after stirring

colourless filtrate and white

and insoluble

 

 

residue

Compounds/salts

(ii)

Filtrate + litmus paper

No visible reaction/Litmus

Neutral filtrate

 

 

paper remains unchanged

 

(b) (i)

First portions of filtrate +

White ppt

 

 

AgN03(aq) then + HN03(aq)

Ppt undissolved/remained

 

 

+ NH3(aq)

Ppt insoluble

 

 

 

Ppt dissolved

CI- confirmed

(ii)

Second portion of filtrate +

 

 

 

NaOH(aq) in drops

White gelatinous precipitate

Zn2+ or A}3+

 

Then in excess

Precipitate

Zn2+ or A}3+

 

 

soluble/ dissolved

 

(iii)

Third portion of filtrate +

White gelatinous

Zn2+ or A}3+

 

NH3(aq) in drops

Precipitate

Zn2+ confirmed

 

Then in excess

Precipitate dissolved

 

(c) (i) .

Portion of residue + iodine

Blue-black colouration

 

 

solution

 

Starch present

(ii)

Portion of residue +

No blue - black colouration

Strach hydrolysed

 

HCI(aq) and boil, cool and

or brown colour of iodine

 

 

add iodine solution

remains

 

C contains Zinc chloride/ZnCl2 and starch.





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