waecE-LEARNING
Chemistry Paper 3,Nov/Dec 2012  
Questions:   1 2 3 Main
General Comments
Weakness/Remedies
Strength

























































Question 3

  (Question 3

    1. Name the substance that would be used in the laboratory for:
  1. drying ammonia;
  2. converting copper (II) oxide to copper;
  3. testing for the presence of water.

 

(b)        Describe briefly a test to distinguish between hydrochloric acid and ethanoic acid.
(c)        Explain briefly why a solution of Na2CO3 is basic while that of A12(SO4)3 is acidic.
(d)       State what would be observed when:
(i)        freshly prepared AgC1 is left for sometime in the presence of sunlight;

    • hydrogen sulphide is bubbled into a jar of acidified potassium tetraoxomanate (VII) solution.                                                                                        
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OBSERVATION

Majority of the candidates attempted this question, but the performance was poor on the average.
In part (a),  some candidates gave the chemical formulae instead of name thereby loosing marks.
In part (b), most of the candidates could not correctly distinguished between HCl and CH3COOH chemically.
In (c) majority of the candidates did poorly.  the question required knowledge of hydrolysis of simple salts.  Candidates rather used litmus test and lost valuable marks.
In (d) (i) some candidates reported the colour of AgCl when exposed as black instead of grey.
In d(ii) majority of the candidates that attempted the question answer it theoretically.  Some gave the required result but mentioned sulphur as being deposited instead of just mentioning a yellow deposit.
The expected answers include:

3.

(a)(i)  Calcium oxide                                      
           
   (ii)     (Dry) hydrogen gas/methane/butane  

   (iii)    Anhydrous copper (II) tetraoxosulphate (VI) / Anhydrous cobalt (II) chloride
                                                                                                                                      
(b)        Pour silver trioxonitrate (V) solution/Lead (II) trioxonitrate (V) solution
            into each of the solutions.
            White precipitate is formed with HCl, no visible reaction
            with ethanoic acid.     
                                                                                                                       
OR
To each sample of the solution add ethanol followed by few drops of tetraoxosulphate (VI) acid and heat.
No sweet smell with HCl.                                                                              
Sweet smell with ethanoic acid            
 
(c)        Na2CO3 is a salt of a weak acid and a strong base.
            It undergoes hydrolysis to give alkaline solution.
            Al2(SO4)3 is salt of a strong acid and weak base.
            It undergoes hydrolysis to give acidic solution.        
                                                                                                                       
(d)       (i)         White colour of AgCl turns grey

            (ii)        Purple colour of acidified potassium
                        tetraoxomanganate (VII) solution turns colourless
                        with yellow deposit/precipitate/solid.                                                            
                                                                                   

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