Chemistry Paper 3B WASSCE (SC), 2018

Question 2B

 

F is a double salt. Carry out the following exercises on F.
Record your observations and identify any gas(es) evolves.
State the conclusions you draw from the result of each test.

  1. Put all of F in a beaker and add about 10 cm3 of distilled water. Stir the resulting solution thoroughly. Test the solution with litmus paper.

 

  1. (i)         To about 2 cm3 of the solution, BaCl2 solution followed by HCl(aq).

(ii)        To another 2 cm3 portion of the solution, add NaOH(aq) in drops and then in excess. Heat the mixture.

(iii)        To another 2 cm3 portion of the solution, add K2Fe(CN)6 solution.
[20 marks]

 

Observation

 

In part (a), majority of the candidates added distilled water to F and wrote correct observations.

In part (b), majority of the candidates wrote wrong observations and inferences.

The expected answers include: 

 

TEST

OBSERVATION

INFERENCE

(a)

F+ distilled water + stirring

F(aq) + litmus paper

F dissolved to form a light green solution

Turns blue litmus paper red 

 

 

Solution of F is acidic 

(b)(i)

F(aq) + BaCl2(aq)

 

+ HCl(aq)

White precipitate formed

 

Precipitate insoluble

SO42-, SO32-, CO32-,

 

SO42-  present

(ii)

F(aq) + NaOH(aq)in drops

then in excess
+ heat

 

 

 

 

Green (gelatinuous) precipitate formed.

Precipitate insoluble in excess
Colourless gas with choking/pungent/irritating smell  given off.
Gas turned damp red litmus paper blue / forms white dense fumes with HCl.
Precipitate turned reddish brown

Fe2+

 

Fe2+

 

NH3(g) from NH4+

Fe3+ from Fe2+

(iii)

F(aq) + K3Fe(CN)6(aq)

Deep blue /colouration formed
Dark blue/Prussian blue
(Do not accept light blue)

Fe2+ confirmed