Question 1
1. A contains 0.79 g of KMnO4 per 250 cm3 of solution.
B is a solution of (NH4)2SO4. FeSO4. 6H2O of unknown concentration. 25.0 cm3 portions of B were acidified and titrated against A.
The results are recorded in the table below.
Burette readings / cm3 |
1 |
2 |
3 |
Final reading |
24.80 |
34.50 |
- |
Initial reading |
0.00 |
- |
6.55 |
Volume of A used/cm3 |
- |
24.00 |
24.10 |
- (i) Complete the table.
- From your results and the information provided, calculate the:
- concentration of A in moldm-3;
- concentration of B in moldm-3;
- concentration of SO42- in moldm-3 in B.
(ii) Calculate the average volume of A used.
The equation of the reaction is:
(iv) mass of SO42- ions in 250 cm3 of B.
[O = 16.0; S = 32.0; K = 39.0; Mn = 55.0; Fe = 56.0]
[18 marks]
Observation
In part (a), majority of the candidates calculated the volume of A used correctly.
In part (b), majority of the candidates could not calculate the concentrations of A and B in moldm-3.
Some of them could not determine the mole ratio from the reaction equation.
The expected answers include:
(a) (i)
Burette Reading / cm3 |
1 |
2 |
3 |
Final Reading |
24.80 |
34.50 |
30.65 |
Initial Reading |
0.00 |
10.50 |
6.55 |
Volume of A used / cm3 |
24.80 |
24.00 |
24.10 |
(ii) From the equation of reaction:
(iii)
(iv)
