waecE-LEARNING
Chemistry Paper 3,Nov/Dec 2013  
Questions:   1 2 3 Main
General Comments
Weakness/Remedies
Strength
Question 2

D is an inorganic salt. E is a solution of an inorganic salt. The tests recorded in the table below were performed on D and E.

Copy and complete the table.

 

Test

Observation

Inference

(a)

D + distilled water

Did not dissolve

……………………..

(b)

D + dil. HCI + heat

Dissolved to form blue or greenish-blue solution

 

…………………….

(c)

Solution of D
+ NaOH(aq) in drops then in excess

Blue gelatinous precipitate
…………………………

 

……………………..

(d)

Solution of D
+ NH3(aq) in drops then in excess

 

…………………………..
……………………………

Cu2+ present
Cu2+ confirmed

(e)

(i)  E(aq) + NaOH(aq) in    drops and in excess

No precipitate

………………….

(ii)Mixture in 2(e)(i) warmed

……………………………
…………………………..
…………………………..

NH3 gas evolved, therefore NH4+ present

(f)

E(aq)  +BaCl2(aq)

White chalky precipitate.  No gas evolved, precipitate did not dissolve

…………………….
…………………….
……………………

In a titration experiment, 22.50 cm3 of an acid solution A containing 10.6 g of NaHSO4 per dm3 reacted with 25.0 cm3 of solution B containing X g of NaOH per dm3. The equation for the reaction is: NaHSO4(aq) + NaOH(aq) Na2SO4(aq) + H2(l)

(a) From the information given above, calculate the:

(i) concentrate of A in moldm– 3;
(ii) concentrate of B in moldm– 3;
(iii) value of X;
(iv) mass of Na2SO4 formed during the reaction. [ H = 1.00; 0 = 16.0; Na = 23.0; S = 32.0]    (16 marks)

(b)

(i) Name a suitable indicator for the titration experiment.
(ii) State the apparatus used to measure the volume of solution: I. A; II B;   (3 marks)

_____________________________________________________________________________________________________
OBSERVATION

The candidate made a fair attempt on this question. Majority of the candidates obeyed the instruction “copy and complete”, thus was commendable. Some candidates still cannot differentiate between gelatinous and chalky precipitate. Results of test in drops and excess still seem confusing to some candidates as some of them gave the result together (i.e. only in excess).

The expected answers include:

 

 

Test

 

Observation

 

Inference

(a)

 

D + distilled water

 

Did not dissolve

 

D is an insoluble salt

(b)

D + dil. HCl + heat

Dissolved to form blue or greenish blue solution

 

Cu2+  may be present

(c)

Solution of D
+ NaOH(aq) in drops
then in excess

 

Blue gelatinous precipitate.
Precipitate insoluble

 

Cu2+ present

(d)

Solution of D
+ NH3(aq) in drops
then in excess

 

Blue gelatinous Precipitate
Precipitate dissolves forming deep blue solution

 

Cu2+ present
Cu2+ present confirmed

(e)(i)

E(aq) + NaOH(aq) in drops and in excess

No precipitate

NH4+may be present

  (ii)

Mixture in 2(e)(i) warmed

Colourless gas evolved
Pungent/choking/irritating smell turns red litmus to blue/forms dense white fume with conc. HCl

NH3 gas evolved
 
NH4+  present

(f)

E(aq) + BaCl2(aq)
+ excess HCl(aq)

White chalky precipitate.
No gas evolved
Precipitate did not dissolve

CO32-SO32- SO42-
SO42-   Present
(SO42- must have been mentioned above)

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