Chemistry Paper 3A WASSCE (SC), 2022

Question 2A

 

C contains two cations and one anion.

 

  1. Dissolve all of C in about 10 cm3 of distilled water. Carry out the following exercises on portions of the resulting solution:
  1. To about 2 cm3 of the solution, add NaOH(aq) in drops then in excess. Warm gently the resulting mixture.
  2. To about 2 cm3 of the solution, add BaCl2(aq) followed by excess dilute HCl.
  1. From your results, identify the cations and anions in the sample.

[18 marks]

 

 

Observation

 

This question was based on qualitative analysis of a mixture of two salts. This question was attempted by majority of the candidates and their performance was above average.

 

In part (a), majority of the candidates were able to state the observations and inferences.

 

In part (b), majority of the candidates were able to identify the cations and anions in the sample.

 

The expected answers include:

 

Fe(NH4)2 (SO4)2

 

 

TEST

OBSERVATION

INFERENCE

(a)

C + distilled water

Dissolved to give a pale green solution  

Fe2+, Cu2+ may be present  
(Both ions mentioned to score)

(i)

C(aq)+ NaOH(aq) in drops
then in excess

mixture warmed

 

Green (gelatinous) precipitate formed  
Precipitate insoluble 

Colourless gas with choking smell evolved which turned moist
red litmus paper blue or forms white fumes
with HCl gas

 

Fe2+  present

 

 

NH3(1) gas
from NH4+ 

(ii)

C(aq) + BaCl2(aq)

 

+ excess dil HCl

White precipitate formed  

 

Precipitate insoluble

SO32- , CO32- , SO42-, S2-present

 

SO42-   present (SO42- should be mentioned above to score)

 

(b)        Cations present are Fe2+ and NH4+   
            Anion present is SO42-