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

  • (a)       State Gay Lussac’s law.                                                                                          [2 marks]

     (b)      Carbon (II) oxide reacted with oxygen to form carbon (IV) oxide in a sealed tube
                i. Write a balanced equation for the reaction.

    ii. If 40 cm3 of the carbon (II) oxide were mixed with 30 cm3 of oxygen,

    • calculate the volume of carbon (IV) oxide produced.
    • Which reactant is in excess and by how much?
    • What was the total volume of the gaseous mixture at the end of the reaction?              [8 marks]

    (c)        Consider the following oxides:
                 CaO, SiO2, CO, NO2  and ZnO.

    Which of the oxide(s)

    • is an acidic oxide that is insoluble in water?
    • reacts with water to give alkaline solution?
    • is amphoteric?
    • is neutral?
    • is/are gaseous at room temperature?                                                           [6 marks]
    (d)      Explain why
    1. colourless concentrated trioxonitrate (V) acid turns yellow,
    2. dilute trioxonitrate(V)acid does not liberate hydrogen when it reacts with magnesium.                                                                                                 [5 marks]
    (e)       Write a chemical equation for the thermal decomposition of
      (i) Cu(NO3) 2(s);  
    (ii)  NH4NO3(s).         [4 marks]


  • This question was not so popular with the  candidates and the performance was fair.

    In part (a), candidates correctly stated that when gases react, they do so in volumes which are simple ratios to one another and to the volume of the products if gaseous, provided that the temperature and pressure remain constant was the statement of Gay Lussac’s Law.

    In part (b), candidates wrote balanced equation for the reaction of carbon (II) oxide and oxygen to form carbon (IV) oxide as follows:

                               2CO(g), + O2(g)    →     2CO2(g) .     

    They also tackled the calculation arising therefrom correctly thus:

    1.  from the given equation,


    2 volumes of CO(g)  and 1 volume of O2(g)   →    2 volumes CO2  2

    ratio of reacting volumes

    40cm3   +   20cm3  →  40cm3

    40cm3  of  CO  requires 20cm3  of oxygen to form 40cm3  of CO2 .

    ∴  40cm3  of  CO2  is produced

    1. excess reactant is oxygen


    volume is 30cm3 – 20cm3  = 10cm3

    1. total volume of gaseous mixture


    10cm3 O2   +  40cm3CO2   =   50cm3
    Candidates correctly answered (c), but could not explain why colourless concentrated trioxonitrate (V) acid turns yellow nor dilute trioxonitrate (V) acid does not liberate hydrogen when it reacts with magnesium in (d).

    Also in (e), candidates failed to correctly write a chemical equation for the thermal decomposition of the nitrates given.

    The expected responses were

    (c)   (i)       SiO2
           (ii)      CaO

    1. ZnO

    (iv)      CO
     (v)      CO and NO2

    (d)     (i)           conc HNO3 slowly decomposes releasing brown NO2 which dissolves in the acid
                             to give the yellow colour.

              (ii)          HNO3 is an oxidizing acid.  Any hydrogen produced is oxidized to water.

     (e)     (i)           2Cu (NO3) 2(s)    (heat)    2CuO(s)  +   4NO2(g)    +  O2(g)
        (ii)   NH4 NO3(s)     (heat)   N2O(g)      +   2H2O(g)


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