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

Question 2

  • (a)        (i)    State Boyle’s law.  
    (ii)   Give the mathematical expression of  Boyle’s law.  
    (iii)  Sketch the graphical representation of Boyle’s law.                                      [5 marks]

    (b)       A given mass of gas occupies 500 cm3 at 30oC and 6.5 x 104 Nm-2 .
                Calculate the volume of the gas at s.t.p.

                [Standard pressure = 1.01 x 105  Nm-2]                                                   [3 marks]
    (c )      Explain why an inflated balloon expands in warm water.                           [3 marks]

    (d)      (i)    What is meant by the term vapour density?
              (ii)    Determine the vapour density of SO2. 
                                                                                                                    [4 marks]   
                                         [S  =   3,  O  =  16 ]

    (e)    (i)    Which of the following gases, SO2 and NH3 can be collected by
                     I.    upward delivery;
                    II.   downward delivery?  

           (ii)    What would be the nature of the solution formed by dissolving each of the following
                    substances in water separately?

    1. SO2
    2. NH3


          (iii)    State the property exhibited by SO2 and NH3 in the following equations:

    1. 2H2S(aq)   +   SO2(g)    →     3S(s)   +   2H2O(l)


    1. 3CuO(s)   +   2NH3(g)    →     3Cu(s)  +    3H2O(l)    +   N2(g)

                                                                                                                                    [6 marks] 
    (f)    (i)    State two alloys of tin.
            (ii)  Give one use of each of the alloys in 2(f)(i)                                                        [4 marks] 



    This part is the most popular question among the candidates and the performance was good.

    In (a), candidates correctly stated Boyle’s law, gave the mathematical expression of the law and sketched the graphical representation of it.  However, some of them lost some marks because they failed to label the axes of the graph rightly. 

    The expected response from candidates were as follows:

    (i.)Boyle’s law states that the volume of a given mass of gas is inversely proportional to its pressure if the temperature remains constant.

       (ii)        P   ⍺    1       or PV  =  K  or V    ⍺    1 
                     V                                             P



    In part (b), candidates correctly determined the volume of the gas at s.t.p. thus:

                                    P1  V1    =   P2  or V2          
                                       T1                  T2                                         

                  V2    =     P1 V1 T1
                                    P2   T1           

                  V2   =    6.5  x 104  x  500 x  273
                                (273 + 30) x  1.01 x 105

                  V2    =   290 cm3  

    In (c), candidates could not explain why an inflated balloon expands in hot water.  They did not know that the air particles in the balloon acquire more kinetic energy thus increasing the pressure inside the balloon and in order to keep the pressure constant, the balloon expands.

    In (d), candidates correctly defined vapour density as the number of times a given volume of a gas or vapour is as heavy as an equal volume of hydrogen gas at the same temperature and pressure.  Also, they calculated the vapour density of SO2 as follows:

                                      molar mass of   SO2  =   32  +  16  x  2     =  64g mol-1 
                                      vapour density x 2  =  relative molar mass
                                      vapour density    =     2    =    32 
    In part (e)(i) and (ii), candidates knew that NH3 could be collected by upward delivery while SO2 could be collected by downward delivery and that NH3 in water is basic while SO2 in water is acidic.

    In (e)(iii), most candidates could not correctly state the property exhibited by SO2 and NH3  in the given equations.  The expected answers from candidates were: 

    1. SO2   -    oxidizing agent;
    2. NH3    -    reducing agent.


    In part (f), candidates stated two alloys of tin from among bronze, type metal and soft solder; also they gave one use of each of the alloys as follows:

    bronze             -           for making coins, metals/other ornaments;
    type metal       -           for printing;

    soft solder       -           for plumbing/welding.                          


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