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Chemistry Paper 2,Nov/Dec 2010  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments

Question 2

(a) (i) Mention the various states of matter
(ii) Arrange the states of matter in decreasing order of:
               I.           kinetic energy of particles;
               II.          disorderliness between particles;
               III.         forces of cohesion between particles.
(iii) A pure sample of a substance melts at 35°C and boils at 100°C.
                 I.            Sketch a graph to show the heating curve
                  II.          What is the state of the substance at normal room temperature? [9 marks]
(b) Consider the following equation:


What volume of CO2(g) measured at s.t.p. would be produced when 25.0 g of KHC03(s) was completely decomposed?
[K = 39.0 = 16.0; C = 12.0; H = 1.00; Molar volume of a gas ats.t.p. = 22.4 drn" mol"] [ 5 marks]

(c) When ammonium chloride was heated in a dry test tube, it was observed that the damp red litmus paper placed at the tip of the test tube immediately turned blue and after some seconds turned red.

(i) State the gas that was responsible for the:
I.           red litmus paper turning blue;
II.           blue colour of the litmus paper changing back to red.

(ii) Give reason for your answer in 2 (c)(i)
(iii) Write an equation for the decomposition [6 marks]
         (d)   (i) Name two salts that could react with dilute acids to produce sulphur (IV) oxide
(ii) Mention
                 I. two uses of sulphur (IV) oxide;
                 II. one chemical property of sulphur (IV) oxide. [5 marks]



The question was attempted by most candidates and the performance was fair.
In(a)(i) and (ii), Candidates correctly mentioned solid, liquid and gas as the states of
matter and gave the arrangement of the states of matter as follows:

(ii) I Gas > liquid > solid
     II       Gas > liquid > solid
     III       Solid > liquid > Gas

In (a)(iii) I and II, most candidates could not sketch the required heating curve graph and hence, did not know the substance. The expected answer from candidates were as follow:

II. Solid.

In part (b), candidates correctly determined the volume of CO2(g) at S.T.P thus:

Molar mass of KHCO3 = 39 + 1 + 12 + ( 3 x 16) = 100gmorl-1
No. of mole of KHCO3 = 25.0 = 0.25 mol
From the equation
     2 moles of KHC03 = Imole CO2
     No. of mole of CO2 = ½ x 0.25 mol
                                       = 0.125 mol.
Volume of CO2 at s.t.p
     = 0.125 mol x 22.4 dm3 mol-1
     = 2.8 dm3


Alternate Method

Molar mass of KHCO3 = 39 + 1 + 12 + (3 x 16) = 100gmol-1
2 x 100g KHCO3 = 22.4 dm3
., 25g = 22.4 x 25
           = 2.8dm3

In part (c), most candidates correctly inferred ammonia and HCL in (i) I and II. However, most of them could not explain their answer in (ii). In (c)(iii), most candidates correctly wrote an eqation for the decomposition

The expected responses from candidates in each of (ii) and (iii) were as follows:

(ii) NH3 being lighter than HCI/NH3 travels faster than HCL
(iii) NH4CI(s) ~ NH3(g) + HCI(g)

In part (d) (i), candidates named potassium trioxosulphate(IV) and sodium trioxosulphate(IV) as the two salts that would react with dilute acids to produce sulphur(IV)oxide. In(d)(ii), candidates mentioned two uses and one chemical property of sulphur(IV)oxide from among the following:

  1. - as germicide

- manufacture of tetraoxosulphate(VI) acid
- as preservative
- as refrigerant
- as bleaching agent.
II.               Sulphur (IV) oxide combines with water to form solution of trioxosulphate (IV) acid
reacts with oxygen in the presence of catalyst to form sulphur(VI)oxide
reacts with alkali to form salt and water
reacts with H2S to form water
bleaching action.



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