Question 2
2. (a)An aqueous solution of an inorganic salt is suspected to contain two or more of the following ions:
Al3+, pb2+, Ca2+, Zn2+, CO32-, Cl- and SO32-.
Test on separate portions of the solution with the following reagents, NaOH(aq), NH3(aq), BaCl2(aq) and AgNO3(aq) gave the following observations.
(i) White gelatinous precipitate, soluble in excess NaOH(aq).
(ii) White gelatinous precipitate, soluble in excess NH3(aq).
(iii) White precipitate with BaCl2(aq) which dissolves in excess HCl (aq).
(iv) White precipitate with AgNO3(aq).
Observation
From the above observations, deduce the ions present in test solution. State the inference for each observation in a tabular form under the headings test, observation and inference.
(b) (i) State the reagents that could be used to identify the functional group(s) present in an unsaturated alkanoic acid.
(ii) Tabulate how the test may be carried out, stating clearly what observations would be made in each test.
In part (a), majority of the candidates could not deduce the ions present in the test solution and state the inference for each observation.
In part (b), majority of the candidates could not state the reagents that could be used to identify the functional group(s) present in an unsaturated alkanoic acid.
The expected answers include:
|
TEST |
OBSERVATION |
INFERENCE |
(i) |
Sample + NaOH(aq) |
White gelatinous precipitate, |
Al3+(1) |
(ii) |
Sample + NH3(aq) |
White gelatinous |
Zn2+ present (1) |
(iii) |
Sample + BaCl2(aq) |
White precipitate, |
SO32-(1) |
(iv) |
Sample + AgNO3(aq) |
White precipitate |
Cl- /CO32-(1) |
(b) (i) Na2CO3(aq)/NaHCO3(aq) for the alkanoic functional group (-COOH) (1)
and then KMnO4(aq) / HOBr / (Br2 / CCl4)/K2Cr2O7(aq) for the alkene or alkyne Functional Group
(b) (ii)
|
TEST |
OBSERVATION |
INFERENCE |
(i) |
Sample + NaHCO3(aq) |
Effervescence occurs / gas evolves |
Alkanoic acid |
(ii) |
Sample + KMnO4(aq) or Sample + HOBr or Sample + Br2/ CCl4 Sample + K2Cr2O7(aq) |
Purple colour yellowish-brown Reddish-brown orange colour |
Sample is unsaturated/Alkene / Alkyne |