Question 2
C is a mixture of two inorganic compounds. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusions you draw from the results of each test.
- Put all of C in a boiling tube and add about 10 cm3 of distilled water. Shake thoroughly and filter. Keep both the residue and the filtrate.
-
- To about 2 cm3 of the filtrate, add a few drops of silver trioxonitrate (V) followed by dilute HNO3.
- Add excess NH3 solution to the resulting mixture in 2(b)(i).
-
- Put the residue in a test tube, add about 2 cm3 of dilute HCl and shake.
- Add NH3 solution in drops to the mixture from 2(c)(i) and then in excess.
Observation
This question specifically tested skill on qualitative analysis, and it was attempted by majority of the candidates. The performance of the candidates was fair.
In part (a), majority of the candidates obtained the filtrate and residue after filtration.
In part (b), majority of the candidates could not write correct observations and inferences.
In part (c), some of the candidates lost marks to wrong charge on ions and wrong inferences.
The expected answers include:
| TEST | OBSERVATION | INFERENCE | |
|---|---|---|---|
| (a) | C + distilled water and filter | C partially dissolved green residue Colourless filtrate | Fe2+ or Cu2+ salt |
|
(b)(i) (ii) |
Filtrate + AgNO3(aq)
+ HNO3(aq) resulting mixture in (b)(i) + excess NH3(aq) |
White precipitate Precipitate insoluble/No visible change Precipitate soluble |
Cl-, CO32-, or CO32-, S2-
(Any 2 for 2 marks)
(-1 for any wrong ion) Cl (Cl- must be mentioned above to score) Cl present (Cl- must be mentioned in b(i) to score) |
| (c)(i) | Residue + dil HCl | Residue dissolved to form a blue solution (effervescence) / colourless, odourless gas evolved The gas turned lime water milky |
Cu2+ present CO2(g) from CO32- |
| (ii) | Portion of mixture
from c(i) + NH3(aq) in drops then in excess |
(Light) blue precipitate (Do not accept deep blue precipitate) Precipitate dissolved in excess to form deep blue solution |
Cu2+ confirmed |