Observation

Majority of the candidates responded to this question but their performance was below average.

In part (a), majority of the candidates could not solve this question.

In part (b), majority of the candidates could not describe a chemical test to distinguish between HCl and dilute HNO3.

The expected answers include:

(a) C1VI = C2V2
0.5 x 100 = 0.3 x V2
V2 = 0.5 x 100  
           0.3
= 166.67 cm3  
(accept 166.7 / 167)
Volume of water = 166.67 – 100  
= 66.67 cm3

(b)  Add AgNO3(aq) to each of the solutions in a test tube. Formation of a white precipitate   
indicates that the solution is HCl, if no precipitate is formed then the solution is HNO3
AgNO3 + HCl → AgCl + HNO3

            OR
          Heat each sample in a test tube
Brown gas/ fumes indicates HNO3, no brown gas / fumes indicate HCl
4HNO3 → 2H2O + 4NO2 + O2
           OR
         To each sample in a test tube, add fresh FeSO4(aq), followed by conc. H2SO4 along the   
side of the test tube: brown ring indicates HNO3, no brown ring indicates HCl
FeSO4 + NO → FeSO4.NO