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Chemistry Paper 2 (Essay) ,May/June 2010  
Questions:   1 2 3 4 5 6 7 8   Main
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Question 2

(a) (i) Define saturated solution.
(ii)The solubility of KN03 at 20°C was 3.00 mol dm-3 If 67.0g of KN03 was added to 250cm3 of water and stirred at 20°C, determine whether the solution formed was saturated or not at that temperature.

                           [7 marks]

(b) (i) Distinguish between dative bond and covalent bond.
(ii) Explain why sugar and common salt do not conduct electricity in the solid state.
(iii) State the type of intermolecular forces present in:
      1.            hydrogen fluoride;
        II.           argon.
(iv) Consider the compounds with the following structures:
S - H ----N and 0 - H -----N
In which of the compounds is the hydrogen bond stronger? Give reason for your answer. [ 12 marks]

(c) (i) State Dalton's Law of Partial Pressure.

(ii)    If 200cm3 of carbon(IV) oxide were collected over water at 18°C and 700 mmHg, determine the volume of the dry gas at s.t.p.

[ standard vapour pressure of water at 18°C = 15 mmHg] [6 marks]



_____________________________________________________________________________________________________
OBSERVATION

The question was attempted by few candidates and the performance was fair.

In part(a)(i), candidates were able to correctly define saturated solution as that which contains the maximum amount of solute it can dissolve at a given temperature (in the presence of undissolved solute).

In part(a)(ii), only very few of the candidates could determine whether the solution formed was saturated or not as follpws:

Solubility of KN03 in in g dm-3 = 3.00 x 101 = 303
.. 1000cm3 of saturated solution = 303g
250cm3 of the solution = 303 x 250
                                                1000
                                           = 75.8 g

Since the quantity of KN03 added (67.0) to 250 cnr' of water is less than maximum amount required to form a saturated solution, then the solution is unsaturatedj Any other correct method was accepted]

In part(b )(i), only few candidates knew that in dative bond, only one of the participating atoms/species donated electrons to be shared by both atoms while in covalent bond both participating atoms/species contribute equally the electrons being shared.

In (b )(ii), candidates could not explain why sugar and common salt do not conduct electricity in the solid state. The expected answer from candidate was that sugar is covalent while common salt (NaCl) is electrovalent/ ionic. Electrical conductivity (in compounds) depends on presence of mobile ions. Sugar does not conduct electricity because it does not contain ions (they are molecules) while solid common salt does not conduct electricity because its ions are not mobile.

In part (b )(iii), candidates knew that the intermolecular forces present in hydrogen fluoride and argon were hydrogen bond and van der Waal's forces respectively.

In part (b)(iv), candidates knew that 0 - H ---- N has the stronger hydrogen bond because oxygen is more electronegative and smaller in size than sulphur.

In part (c)(i) and (ii), most candidates could state Dalton's Law of Partial Pressure but only very few of them could correctly determine the volume of the dry gas at s.t.p

The expected answers from candidates in(i) and (ii) respectively were:

(I)     the total pressure exerted by a mixture of gases which do not react chemically is equal to the sum of the individual partial pressures of the gases in the mixture
(II)    pressure of the dry gas (P 1) = 700 - 15 = 685 mmHg
VI = 200cm3, TI = 18°C = 273 + 18 = 291K, P2 = 760 mmHg,
T2 = 273
P1V1 = P2V2
  T1         T2
V2 = P1V1T2   =       
          P2T1

= 685 x 200 x 273
   760 x 291

= 169.1cm3 (169cm3 was accepted)

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