Chemistry Paper 3 WASSCE (PC 2ND), 2018

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

  1. (i) Complete the table.
  2.  

    (ii)  Calculate the average volume of A used.
    The equation of the reaction is:

     

  3. From your results and the information provided, calculate the:
    1. concentration of A in moldm-3;
    2. concentration of B in moldm-3;
    3. concentration of SO42- in moldm-3 in B.

 

(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)