Chemistry Paper 3A WASSCE (SC), 2022

Question 1A

 

A is an aqueous solution of Iodine. B is 0.100 mol dm-3 sodium trioxothiosulphate (II).

  1. Put B into the burette. Pipette 20.0 cm3 or 25. 0 cm3 of A into a conical flask. Add B from the burette until the reddish-brown colour fades to pale yellow, then add a few drops of starch indicator to obtain a dark blue solution. Continue adding B slowly from the burette until one drop of B causes the blue colour to disappear, leaving a colourless solution.

Repeat the titration to obtain concordant titre values.
Tabulate your results and calculate the average volume of B used.
The equation for the reaction is:


 

  1. From your results and the information provided, calculate the:
    1.  Concentration in mol dm-3 of iodine in A;
    2. mass in grammes of iodine in 1.00 dm3 of A.
    [ I=127.0]


[17 marks]

 

 

Observation

 

Majority of the candidates responded to this question and their performance was above average.

In part (a), majority of the candidates carried out the titration experiment correctly;

In part (b), majority of the candidates calculated the concentration in mol dm-3 of iodine in A and mass of iodine in 1.00 dm3 of A.

            The expected answers include:

(a)        Two concordant titres