Chemistry Paper 3 WASSCE (PC), 2021

Question 1

 

  1. A is a solution containing 6.00 g of impure sodium trioxocarbonate (IV) in 1.00 dm3.
    B is 0.100 moldm-3 hydrochloric acid. Titration of 25.0 cm3 portions of A with B using methyl orange as indicator gave the following results:

    Burette readings / cm3

    1

    2

    3

    Final reading

    24.80

    36.80

    43.40

    Initial reading

    0.00

     

    18.80

    Volume of Bused

     

    24.30

     

    (a)        (i)         Complete the table.
    (ii)        Calculate the average volume of B used.
    The equation of the reaction is:
    Na2CO3(aq) + 2HCl(aq) 2NaCl(aq) + H2O(l) + CO2(g)        
    (b)        From the information provided, calculate the:
    (i)         concentration, in moldm-3 of the sodium trioxocarbonate (IV) in A;
    (ii)        mass of sodium trioxocarbonate (IV) in 1.00 dm-3 of solution A;
    (iii)       percentage by mass of sodium trioxocarbonate (IV) in the impure sample.
    [C = 12.0, O = 16.0, Na = 23.0]

                                                                                                                                        [14 marks]      

Observation

This was a popular question among the candidates as majority of them responded to it.
In part (a), majority of the candidates mistook A for acid and B for base and this error affected their performance.
In part (b), few candidates were able to calculate the concentration, in moldm-3 of the sodium trioxocarbonate (IV) in A. in addition to this, they could not calculate the percentage by mass of sodium trioxocarbonate (IV) in the impure sample.

The expected answers include:

(a)      (i)


Burette readings / cm3

1

2

3

Final reading

24.80

36.80

43.40

Initial reading

0.00

12.50      

18.80

Volume of B used

24.80    

24.30

24.60      

            (ii)        Average volume of B used =24.80 + 24.60               
2
=  24.70 cm3     

(b)