waecE-LEARNING
Chemistry Paper 2,Nov/Dec 2011  
Questions:   1 2 3 4 7 8   Main
General Comments
Weakness/Remedies
Strength







































































Question 2

(a)Consider the following list of elements:

Nitrogen, fluorine, aluminum and potassium.
Which of the elements

(i)forms diatomic molecule with a triple bond?

(ii)is the most reactive electropositive element?

(iii)forms amphoteric oxide?

(iv)is the most reactive non-metal

(b)  (i) State what would be observed if chlorine gas is passed into an aqueous solution of:

(1) iron (II) chloride;

(2)Potassium bromide.

(ii)        Give the names of the products in 2(b)(i)I and 2(b)(i)II.
(iii)       Write an ionic equation to represent each of 2(b)(i)I and 2(b)(i)II.
       (iv)      Suggest the type of reaction in each of 2(b)(i)I and 2(b)(i)II.

(c)State two differences between conductors and electrolytes.

(d)Consider the reaction represented by the following chemical equation:

2NH3(g) + H2SO4(aq)                  (NH4)2SO4(aq)
Determine the mass of ammonium tetraoxosulphate (VI) that would be produced from 85g of ammonia.

     [ H = 1.00, N = 14.0, O = 16.0, S = 32.0

                              

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OBSERVATION

Properties of the elements listed in question 2(a) were correctly interpreted by candidates as well as the products obtained when chlorine is passed into each of the solution of iron(II) chloride and potassium bromide.  However, candidates could not give the correct observation of change of colour in the solutions.

The differences between conductors and electrolytes were poorly stated because candidates failed to use the appropriate technical terms.
Part (d) was a numerical question and this was poorly handled.

The expected answers are:

2(a)      (i)         nitrogen
(ii)        potassium
(iii)       aluminum
(iv)       fluorine

 (b)       (i)         I.          green solution turns yellow/brown
            II.        colourless solution turns orange/brown
            I.           iron (III) chloride

II.          bromide and potassium chloride

I.          2Fe2+(aq)  + Cl2(g)                              2 Fe3+(aq)  + 2Cl-(aq)

                                    II.      2Br-(aq) + Cl2(g)                     2Cl-(aq) + Br2
          Redox reaction and redox/displacement reaction

     Conductors            Electrolytes
Not decomposed by electric current

decomposed by electric current

Are usually solids

  are aqueous solutions or molten salts

Free mobile electrons conduct current ions conduct electric current

 

                                    (differences must correspond to score)

     (d)        Molar mass of NH3 = 14 + 1 x 3  = 17 gmol-1
            Molar mass of (NH4)2 SO4 = [(14 + 1 x 4) x 2 + 32 + 16 x 4)] = 132g mol-1
            (2 x 17)g of NH3 = 132g (NH4)2SO4
34   = 132g
85g = 132 x 85
           34
   =   330g
                        OR

Molar mass of NH3 = 14 + 1 x 3 = 17g mol-1 Mass to mass relationship
Molar mass of (NH4)2 SO4 = [(14 + 1 x 4) x 2  + 32 + 16 x 4) = 132g mol-1

No. of moles of NH3 = 85 = moles
                                                  17
Since 2 moles of NH3 = mole (NH4)2 SO4
               5moles= 1x5

 

= 2.5moles (NH4)2 SO4                                                                                                                                           
Mass of (NH4)2 SO4 = 2.5 x 132
                                              =   330g

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