WAEC Standard for Titration in Alternative to practical Chemisty
1. E was prepared by dissolving 33.60 g of a mixture of FeSO4. 7H2O and NaCl to make 1.0 dm3 solution..
F was 0.0200 moldm-3 KMnO4.
25 cm3 of E was pipetted into a conical flask acidified with 20.0 cm3 of dil. H2SO4 and titrated with F.
The results are recorded in the table below.
- (i) Copy and complete the table.
(ii) Calculate the average volume of F used.
Equation of reaction:
- ) From your results and the information provided, calculate the:
(i) concentration of FeSO4. 7H2O in E in moldm-3;
(ii) concentration of FeSO4. 7H2O in E in g dm-3;
(iii) percentage by mass of FeSO4. 7H2O in the mixture.
[O = 16.0; S = 32.0; H = 1.0; Fe = 56.0] [21 marks]
- (i) Explain briefly why hydrochloric acid cannot be used to provide an acidic medium in titrations involving KMnO4.
(ii) Explain why the upper rim of the meniscus is read in KMnO4 titrations.
(iii) Why was no indicator used in the titration? [21 marks]
IThis was a compulsory question. Candidates performed averagely in the question.
In part (a), majority of the titration table by supplying the missing values.
In part (b), few of the candidates calculated the concentrations of FeSO4.7H2O in E in moldm-3 and g dm-3. Majority of the candidates were used to the popular formula CaVa/CbVb = na/nb, and could not apply the alternative method of calculating the concentration.
In part (c), majority of the candidates could not explain why hydrochloric acid cannot be used to provide an acidic medium in titrations involving KMnO4. Majority of them could not state why indicator was not used in the titration.
The expected answers include:
(i) Hydrochloric acid cannot be used because KMnO4 will react with it to produce chlorine gas / KMnO4 will oxidise HCl(aq) to produce chlorine gas OR HCl (aq) will reduce KMnO4 to produce Mn2+
(ii) KMnO4 is a coloured solution hence, the lower meniscus cannot be seen while the upper meniscus can be seen and read.
(iii) No indicator was used because KMnO4 is a self-indicator / self – indicating/ acts or serves as an indicator.