Chemistry Paper 2 (Essay) ,May/June 2012

Chemistry Paper 2 (Essay) ,May/June 2013

Questions:		1	2	3	4	5	6	7	8		Main

General Comments
Weakness/Remedies
Strength

Question 2

(i)   Define each of the following terms:
            I.          normal salt;
            II.        acid salt.

        (ii)   Tetraoxosulphate (VI) acid and sodium hydroxide react to produce salt and water.
            Write a balanced chemical equation for the formation of:
I.          normal salt;
            II.        acid salt.
                                                                                                                              [8 marks]

(b) (i) Explain briefly the term acid-base indicator.
(ii) Copy and complete the following table.

Indicator	Colour in acidic medium	Colour in basic medium
Methyl orange
Phenolphthalein

For each of the following titrations, state the most suitable indicator:
strong acid against strong base;
strong acid against weak base;
weak acid against strong base.

[9 marks]

(c) Baking soda and hydrochloric acid react according to the following equation:
NaHCO3(aq) + HCl(aq) NaCl(aq)+ CO2(g) + H2O(l)

Calculate the mass of baking soda that would produce 10g of carbon (IV) oxide.
[H = 1.00, C = 12.0, O = 16.0, Na = 23.0] [7 marks]

Give a reason why a given mass of sodium hydroxide pellets cannot be used to prepare a standard solution. [3 marks]

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OBSERVATION

This question was attempted by majority of the candidates and the performance was fair.
In (a), majority of the candidates correctly define normal and acid salt. However, quite a number of candidates could not write a balance chemical equation for the formation of an acid salt.
In b(i), some candidates were unable to explain the term acid-base indicator. In b(ii), most of the candidates could not give the colour of phenolphthalein in basic medium.
In (c), some of the candidates calculated the relative molecular mass of baking soda correctly. however, some candidates could not answer the question correctly due to poor mathematic skills. In (d), majority of the candidates could not give the reason why sodium hydroxide pellet could not be used to prepare a standard solution.

The expected answers include:

(a) (i) I. Normal salt – salt formed when all the replaceable hydrogen ions/replaceable hydrogen atoms of an acid are replaced by a metal/ammonium ion.
OR
A salt formed when all the OH- of an alkali are replaced by anionic group of an acid

II. Acid salt – salt formed when only some/part of the replaceable hydrogen
ions/atom of an acid are replaced by a metal.

      (ii) I. 2NaOH + H2SO4   →   Na2SO4 + 2H2O
           II. NaOH + H2SO4 →   NaHSO4 +H2O

(b) (i)   Acid-base indicator – is either a weak acid or weak base which gives different
              colour in acid or alkaline medium/depending on pH of the solution.

(ii)

Indicator	Colour in acidic medium	Colour in base medium
Methyl orange	Red/Pink	Yellow
Phenolphthalein	Colourless	Pink/Red

(iii)

methyl orange/methyl red/ phenolphthalein – any named indicator e.g. litmus,

       bromocresol blue or red
      II.   methyl orange
     III. phenolphthalein
(c) Relative Molar Mass (CO2) = 12 + (2 x 16) = 44
       Relative Molar Mass (NaHCO3) = 23 + 1 + 12 + (3 x 16) = 84
       NaHCO3 + HCl → NaCl + CO2 + H2O
       From the equation of the reaction
       44 g CO2 ≡ 84g baking soda
   10 g CO2≡
                             = 19.09 g
                             = 19.1 g
       OR
Number of moles of CO2     = 10
                                                44
                                                  = 0.227
         Mass of baking powder = 0.227 x 84
                                                    = 19.068
                                                     = 19.1 g

(d)   Sodium hydroxide absorbs water /is deliquescent and absorbs carbon (IV) oxide from air/
         and this would make mass taken unreliable/add to its mass