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Chemistry Paper 3,Nov/Dec 2008  
Questions:         1 2 3       Main
General Comments

Question 2

(a)        Consider the following reagents:
I.     Bromine water;
II.    Benedict’s solution;
III.   NaHCO3 (aqueous).

(i)    Name the class of organic compounds that could be identified with each of
                    the reagents.
           (ii)    State what would be observed in each case.
                                                                                                                                [6 marks]

(b)        Name two gases that should be prepared in a fume chamber.  Give a reason for
            your answer.
                                                                                                                                [3 marks]
(c)        The following table shows the pH ranges of indicators P, Q and R.





    pH Working range

3.1  -  4.4

  6.0  -  7.6

  8.0  -  9.6

Select the indicator(s) that would be suitable for titrating:
(i)         strong acid with strong base;
(ii)        strong acid with weak base;
(iii)       weak acid with strong base.
                                                                                                                                [5 marks]


This question was attempted by majority of the candidates and the performance was fair
In (a)(i) I – III, most candidates displayed ignorance of the class of organic compound that could be identified with each of the given reagents and hence, could not state what would be observed in each case in (a)(ii) I – III.

The expected answers were

(a) (i)  I         alkenes/alkynes (unsaturated bydrocarbons)
          II         reducing sugars
          III        alkanoic acids

     (ii)   I.      Bromine water – decolourization
 II.      Benedict’s solution – brick red precipitate
 III.     NaHCO3    -  Effervescence/evolution of a gas.

In (b), candidates were able to name two gases that should be prepared in a fume chamber however, some of the candidates lost marks because they could not give a reason for their answer.  The expected response was any two gases out of chlorine/ammonia/hydrogen-
chloride gas/carbon (II) oxide/sulphur(IV)oxide/nitrogen(IV) oxide because they are poisonous/have offensive smell/are dangerous to health.

In (c), candidates lost marks because they did not adhere to instructions by using names of indicators instead of P, Q and R.

The expected answers were

            (i)        P,  Q  and   R
           (ii)        P
           (iii)        R


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