Question 1B
1. D is 0.050 moldm-3 solution of an unknown mineral acid HxY.
E is a solution of KOH containing 0.025 mole in 250 cm3 of solution.
(a) Put D into the burette and titrate it against 20.0cm3or 25.0 cm3 portions
Of E using phenolphthalein as indicator.
Repeat the titration to obtain consistent titre values.
Tabulate your results and calculate the average volume of D used.
(b) From your results and the information provided, calculate the:
(i) number of moles of acid in the average titre;
(ii) number of moles of base in the volume pipetted:
(iii) mole ratio of acid to base.
(c) Write a balanced equation for the reaction between the acid HxY and base KOH.
(d) (i) State the basicity of acid HxY.
(ii) Suggest what HxY could be.
[21 marks]
Observation
The question was acid-base titration. The question was direct, and self explanatory. The candidates that responded to it were familiar with the demands of the question.
In part (a), majority of the candidates tabulated their burette readings correctly. However, some committed arithmetic error when they were subtracting the initial burette readings from the final. Majority of the candidates used concordant values to determine the average volume of acid used.
In part (b), majority of the candidates got the number of mole of acid to base correctly, but could not calculate the mole ratio of acid to base, due to the approximation error made by the candidates.
In part (c), majority of the candidates could not write a balanced equation for the reaction between the acid HxY and base KOH.
In part (d), majority of the candidates could not state the basicity of the acid HxY.
Expected answers include:
Question 1
(a) Two consistent/ concordant litres
Averaging
(b) Concentration of E:
