Question 2A
C contains two cations and one anion.
- Dissolve all of C in about 10 cm3 of distilled water. Carry out the following exercises on portions of the resulting solution:
- To about 2 cm3 of the solution, add NaOH(aq) in drops then in excess. Warm gently the resulting mixture.
- To about 2 cm3 of the solution, add BaCl2(aq) followed by excess dilute HCl.
- 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 |
(i) |
C(aq)+ NaOH(aq) in drops mixture warmed
|
Green (gelatinous) precipitate formed Colourless gas with choking smell evolved which turned moist |
Fe2+ present
NH3(1) gas |
(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-