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Chemistry Paper 2 (Essay) ,May/June 2009  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments

Question 7

a (i)     Define each of the following terms:

               I.     deliquescence;
               II.     efflorescence.

 (ii)   Give one example of a substance that undergoes each of the processes in 7(a)(i).  [6 marks]

b      Outline how three colourless gases suspected to be ethane, ethene and ethyne could be distinguished in the laboratory.                                                    [4 marks]

c (i)  What is water of crystallization?                                                         

(ii)  Hydrated calcium chloride (CaCl2  xH2O)contains 49.32% water of crystallization. Calculate the value of x.

                             [CaCl2    =  111,  H2O   =  18]
                                                                                                                     [6 marks]
   Name one calcium compound used

  1. in the manufacture of cement;
  2. as a dessicant;
  3. in the production of plaster of Paris (POP);
  4. to neutralise acidic soils.                                                                [4 marks]

d   A concentrated solution of sodium chloride was electrolysed using graphite electrodes.

i. State the ions present in the solution,

ii.Name the products at the

  1. anode,
  2. cathode.

iii. Give the by-product of the electrolysis.      [5 marks]


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

In part (a)(i), candidates correctly defined deliquescence and efflorescence.  However, some of them lost marks because they confused process with substance or compound.

The expected responses were as follows:

  I.      Deliquescence is a process by which a compound absorbs moisture/water from the atmosphere and dissolves in it to form a solution.

  II.     Efflorescence is a process whereby a hydrated salt loses all or part of its  water of crystallization when exposed to the air.

In part (a)(ii), candidates correctly gave one example of a substance that undergoes each of the processes from among the list thus:
                            Deliquescence:    NaOH/FeCl3/CaCl2/PCl5 etc.  .
                            Efflorescence:      Na2CO3.10H2O/ Na2SO4 .10H2O/MgSO4 .7H2O etc.

In part (b), candidates correctly gave an outline of how the three colourless solutions suspected to be ethane, ethene and ethyne could be distinguished in the laboratory thus:

  •  To a sample of each of the gases, add acidified KMnO4 /Br2  water (vapour)
  •  No visible reaction shows ethane OR decolourization of  KMnO4 /Br2 water (vapour) shows ethene and ethyne.
  •  To each of the two remaining samples, add ammoniacal copper chloride/ammoniacal AgNO3 /Ag2O    
  •  No visible reaction shows ethene/formation of reddish-brown precipitate/white precipitate indicate the presence of ethyne.

In part (c)(i), most candidates did not know that water of crystallization was the definite amount of water some substances chemically combine with when they form salt from their aqueous solution. However, they correctly calculated the value of x in the hydrated calcium chloride as follows:
                      % of anhydrous CaCl2   is 100 – 49.32  =  50.68%

                Number of moles of   CaCl2                          H2O

                                                          50.68                 49.32
                                                            111                     18

                                                          0.457                 2.74
                                                          0.457                 0.457     
                                                           1: 6
                                                           x  =   6
In part (d), most candidates used formula instead of the name as required by the question and hence, lost the marks.  The expected answers from candidates were:

  • Calcium trioxocarbonate (IV)
  • Calcium chloride
  • Calcium tetraoxosulphate (VI)
  • Calcium oxide/calcium hydroxide

In part (e), most candidates stated the ions present in the solution as Na+ & Cl- and  H+ & OH-, named the products at the anode and cathode as chloride and hydrogen respectively and gave the by – product  of the electrolysis as NaOH(aq).

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