Chemistry Paper 3 WASSCE (PC 2ND), 2022

Question 1

 

In a volumetric analysis, 24.80 cm3 of 0.100 mol dm-3 of a mineral acid, Y, neutralized 25.0 cm3 of an alkaline solution containing 5.80 g of Na2CO3 per dm3.

(a)        From the information provided, calculate the:                  

(i)         number of moles of Y;            

(ii)        number of moles of Na2CO3;

(iii)       mole ratio of the acid to base in the analysis.

 

(b)        (i)         What is the basicity of Y;

            (ii)        Suggest what Y is likely to be;

            (iii)       Give a reason for the answer suggested in 1(b)(ii).

                        [Na = 23, O = 16.0, C = 12.0]

 

(c)        In a titration, 23.40 cm3 of A containing 15g of CH3COOH per dm3 reacted with
25.0 cm3 of solution B, containing x g of Na2CO3 per dm3.

      The equation for the reaction is:

                        From the information provided, calculate the:

(i)         concentration of A in mol dm-3;

(ii)        concentration of B in mol dm-3;

(iii)       value of x.

                        [ H = 1.0, C = 12.0, O = 16.0, Na = 23.0]
[22 marks]

 

Observation

 

This question was compulsory for candidates. Their performance was below average


In part (a), majority of the candidates could not calculate the number of moles of Y and number of moles of Na2CO3.


In part (b), majority of the candidates could not state the basicity and suggest what Y could be.


In part (c), majority of the candidates were able to calculate the concentration of A and B in moldm-3. They also determined the value of x in the compound.


The expected answers include:

    (b)(i)    Its basicity is one (to score mole ratio must be 2 : 1 in a (iii))
(ii)   Y could be HCl/HNO3 (Do not accept any monobasic organic acid) (to score basicity 
must be)
     (iii)  Y contains one ionizable hydrogen atom per molecule (to score, basicity must be
correct)

                                                                                                                                    (12 marks)