Chemistry Paper 3A May/June 2015

Question 2A

C and D are inorganic salts.  E is a solution of an organic compound.  Carry out the following exercises on C, D and E.  Record your observations and identify any gas(es) evolved.  State the conclusion you draw from the result of each test.

(a)

To all of C, add about 2cm3 of distilled water in a boiling tube.
Shake to dissolve and test the solution with litmus paper.

(b)  

(i)         Test E with litmus paper.
(ii)        Add all of E to the solution obtained in 2(a)

(c)

(i)         Add about 5cm3 of distilled water to D in a test tube and
shake thoroughly.  Divide the resulting solution into two portions.

(ii)        To the first portion, add few drops of BaCl2(aq) followed by excess dil. HCl.

(iii)       To the second portion, add NH3(aq) in drops and then in excess.
                                                                                                                                    [21 marks]

Observation

 

The question was attempted by most candidates, and the performance was fairly good. 

 However, some of the candidates lost marks as result of non-adherence to rubrics, wrong  observations and inferences.

            The expected answers include:

 

 

Test

 

Observation

 

Inference

(a)

C(s) + H2O + litmus paper

Red litmus paper turned blue

Alkaline/Basicsolution

   (b)(i)

E(aq) + litmus paper

Blue litmus paper turned red

Acidic solution

(ii)

E(aq) + C(aq)

 

Effervescence occurred
Colourless and odourless gas which turned lime water  milky was evolved

CO2 from HCO3-/ CO32-

(c) (i)

D(s) + H2O

D dissolved to give a colourless solution

D is a soluble salt

(ii)

D(aq) + BaCl2(aq)

+ dil.HCl (excess)

White precipitate

Precipitate insoluble

S2-SO32-, CO32- or SO42-

SO42- present

(iii)

D(aq) + NH3(aq) in drops

White gelatinous precipitate

Zn2+ Al3+

 

then in excess

Precipitate dissolved

Zn2+ confirmed