Chemistry Paper 3 WASSCE (PC), 2018

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,
Soluble in excess

Al3+(1)
Zn2+(1)

(ii)

Sample + NH3(aq)

White gelatinous
Precipitate, Soluble
in excess

 

Zn2+ present  (1)

(iii)

Sample + BaCl2(aq)
+ excess HCl(aq)

White precipitate,
Soluble in excess acid

SO32-(1)
CO32-  (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
Present (-COOH)  

(ii)

Sample + KMnO4(aq)

or

Sample + HOBr

or

Sample + Br2/ CCl4
or

Sample + K2Cr2O7(aq)

Purple colour  
decolorized   

yellowish-brown
colour decolorized

Reddish-brown
Colour of Br2
decolorised

orange colour
turns green

 

Sample is unsaturated/Alkene / Alkyne