waecE-LEARNING
Chemistry Paper 3,Nov/Dec 2011  
Questions:   1 2 3   Main
General Comments
Weakness/Remedies
Strength

























































Question 2

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

 

Test

Observation

Inference

(i)

Compound + NaHCO3(aq)

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

Alkanoic acid

(ii)

Compound + K2Cr2O7/H+((aq)  + heat

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

Alkanol

(iii)

Compound +
………………………………….

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

Alkyne

(iv)

Compound +
…………………………………..

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

Reducing sugar

(b) Aqueous solutions of aluminum chloride, calcium chloride, iron (III) chloride and zinc trioxonitrate (V) are contained in four separate reagent bottles whose labels have been accidentally removed.  Using only aqueous ammonia solution, show how to identify the content of each bottle.  Tabulate your results.

 

 

_____________________________________________________________________________________________________
OBSERVATION

Candidates were expected to copy and complete gaps in the table as demanded by the question.
Many of the candidates gave a good account of themselves.  However, some of them lost marks as a result of their inability to march organic test with the appropriate observation despite the fact that the inference was provided in each of the four cases.  Majority of the candidates could not differentiate between ordinary white precipitate and white gelateneous precipitate.
A few candidates did not tabultate their responses hence they lost marks.

The expected answers are:
(a)

 

Test

Observation

Inference

(i)

Compound + NaHCO3(aq)

Colourless gas/effervescence
Gas turns lime water milky

Alkanoic acid

(ii)

Compound + K2Cr2O7/H+((aq)  + heat

Orange solution turns green

Alkanol

(iii)

Compound + Ammoniacal soln of Cu2Cl2 (accept CuCl) OR Ammoniacal soln of AgNO3 [Accept + Br2/CCl4
OR  + KMnO4/H+(aq)] 

Reddish-brown ppt  OR White ppt. [Reddish-brown colour of Br2, discharged or decolourized OR Purple colour decolourized].

Alkyne

(iv)

Compound + Benedict/Fehling’s solution + heat

Brick-red precipitate

Reducing sugar

(b)

 

Test

Observation

Inference

(i)

Sample solution + NH3(aq) in drops;
in excess

White gelatinuous precipitate

Precipitate insoluble

A13+ or Zn2+ present

A13+ present
A1Cl3  indicated

(ii)

Sample solution + NH3(aq) in drops; then in excess

No precipitate formed/No visible reaction

Ca2+  present
CaCl2 indicated

(iii)

Sample solution + NH3(aq) in drops; then in excess

Reddish-brown (gelatinuous) precipitate
Precipitate insoluble

Fe3+  present
FeCl3 indicated

(iv)

Sample solution + NH3(aq) in drops;
then in excess

White gelatinous precipitate

Precipitate dissolves/soluble

Zn2+ or A13+ present
Zn2+ present
Zn(NO3)2 indicated

 

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