waecE-LEARNING
Chemistry Paper 3,Nov/Dec 2012  
Questions:   1 2 3   Main
General Comments
Weakness/Remedies
Strength

























































Question 2

 Question 2

  1. Q was a mixture of two inorganic salts.  The mixture was dissolved in water and filtered.

Tests were carried out on both the filtrate and residue.
Copy and complete the table below and state the ions present in Q.

 

 

Test

Observation

Inference

(i)

Portion of filtrate + dil. HCl + BaCl2(aq)

No visible reaction

----------------------
----------------------

(ii)

Portion of filtrate + dil. HNO3 + AgNO3(aq).

No visible reaction

----------------------
----------------------

(iii)

Portion of filtrate + dil. NaOH(aq) in drops and then in excess.

White gelatinous precipitate soluble in excess dilute NaOH(aq)

----------------------
----------------------
----------------------

(iv)

Portion of residue + dil. HNO3.

Effervescene of a colourless, odourless gas

----------------------
----------------------

(v)

Portion of residue + dil. HCl. Resulting mixture heated and later cooled.

----------------------------- 
-----------------------------
-----------------------------

Pb2+

present

(vi)

Portion of residue + KI(aq).

-----------------------------
-----------------------------

----------------------
----------------------

  1. Copy and complete the following table showing the characteristic test for each class of organic compound indicated.

 

 

Test

Observation

Inference

(i)

Compound + NaHCO3(aq)

----------------------
----------------------

Alkanoic acid present

(ii)

Compound + Htr/K2Cr2O7((aq)  + heat

----------------------
----------------------

Alkanol present

 

_____________________________________________________________________________________________________
OBSERVATION

This question was based on both qualitative inorganic and organic analysis.  Majority or the candidates answered this question but the performance was poor.
In (a), few candidates gave the correct inferences to the test/observations provided.  Some candidates lost mark in a(i) for not inferring that SO32-, CO32- or SO42-1 was absent.

In (b) only a few candidates gave the required observations for the test and inferences provided.

The expected answers include:
2.
(a)

 

Test

 

Observation

 

Inference

  1. Portion of filtrate +

dil.HCl + BaCl2 (aq)

 

No visible reaction.

SO32-, CO32-, SO42- absent.

(ii)     Portion of filtrate +
dil.HNO3 + AgNO3(aq) + excess  NH3(aq)

 

No visible reaction

Cl-, Br- , I- absent.

(iii)  Portion of filtrate +
        NaOH(aq)   in drops and
        then in excess

White gelatinous precipitate soluble in excess NaOH(aq)

Al3+,  Zn2+  may be present
 

(v)    Portion of residue +
        dil.HNO3

Effervescence of a colourless, odourless gas

CO2 gas  from
CO32- 
-

(vi)  Portion of residue +
        dil.HCl. Resulting
        mixture heated and later
        cooled.

White precipitate dissolves on heating and re-appears on cooling.

Pb2+ present

(vii) Portion of residue + KI (aq)

Yellow precipitate

Pb2+ confirmed/present

                                                                                                                       
(b)

 

Test

 

Observation

 

Inference

(i) Compound + NaHCO3(aq)

Effervescence/colourless gas
Turns limewater milky

Alkanoic acid present

(ii)  Compound + H+/K2Cr2O7(aq) + heat

Orange  solution turns green

Alkanol present

 

Powered by Sidmach Technologies(Nigeria) Limited .
Copyright © 2015 The West African Examinations Council. All rights reserved.