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 Chemistry Paper 1 (Practical) ,May/June 2008
 Questions: 1 2 3 4 5 6 Main
Weakness/Remedies
Strength

Question 1

A  is 0.050 moldm-3 of  acid  HyX.
B  is a solution of NaOH containing 0.025 moles per 250 cm3 of solution.
(a)Put A into the burette and titrate it against 20.00 cm3 or 25.00 cm3 portions of B using phenolphthalein as indicator.  Tabulate your readings and calculate the average volume of A used.[10 marks]
(b)From your results and the information provided above, calculate the
(i)amount of acid in the average titre;
(ii)amount of base in 20.00 cm3 or 25.00 cm3;
(iii)mole ratio of acid to base. [8 marks]
(c)Write a balanced chemical equation for the reaction between the acid HyX and the base NaOH.            [2 marks]
(d)State the basicity of the acid HyX . [1 mark]

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OBSERVATION

This question was attempted by most candidates and the performance was poor.

In (a), most candidates correctly tabulated their readings and calculated the average volume of A used.  However, some of them lost marks because they manipulated their readings to agree with those of the supervisor and used non concordant titres to calculate the average volume of A used.

In (b)(i)-(iii), most candidates could not calculate the amount of acid in the average titre, amount of base in 20.00 cm3 or 25.00 cm3 and the mole ratio.  The expected answers to part (b) were as follows:

(i)  Amount of acid in average titre
0.05 x Va   =   a moles
1000
OR
1000 cm3 contains 0.05 moles
∴  Va will contain 0.05 x Va
1000       =   a moles

(ii)  concentration of NaOH in B

0.025 moles        =    0.100 moldm-3
0.25dm3
OR
0.025 x 1000     =   0.100 moldm-3
250

Amount of base (NaOH) in 20/25 cm3

20/25 cm3    x 0.100 moldm3  = b moles say
1000               OR

250 cm3 of B contains 0.025moles of NaOH
∴  20/25 cm3 of the solution will contain 0.025 x 20/25
250
=  b moles say

(iii) moles ratio of acid (HyX) to base  (NaOH)
a   =   z  say
b        1
OR

a  :  b  =  a  :  b
b     b
=  z  say  :  1

In (c), only few candidates who got (b)(i)-(iii) were able to correctly write a balanced chemical equation for the reaction as follows:

zNaOH  +  HyX    →     NazX   +   yH2
In (d), candidates could not correctly deduce the basicity of the acid which was z.