Chemistry Paper 3B WASSCE (SC), 2022

Question 2B

 

  1. F is a mixture of two inorganic salts. Carry out the following exercises on F.

Record your observations and identify any gas(es) evolved.
State the conclusions you draw from the results of each test.

  1. Put all of F into a boiling tube and add about 10 cm3 of distilled water. Stir well and filter. Keep both the filtrate and residue. Test the filtrate with litmus paper.
  2. To about 2 cm3 of the filtrate add few drops of BaCl2(aq) followed by dilute HCl.
  3. Put all the residue into a test tube and add 1 cm3 of dil HCl. Divide the resulting mixture into two.
    1. To the first portion of the mixture, add NaOH(aq) in drops and then in excess.
    2.  To the second portion of the mixture, add NH3(aq) in drops and then in excess.

 

Observation

 

This question was popular among the candidates as majority of them responded to it.

 

In part (a), majority of the candidates were able to write the observations and inferences correctly.

 

In part (b), only few candidates were able to deduce the inferences.

 

In part (c), majority of the candidates wrote correct observations and inferences.

 

The expected answers include:

 

F = ZnCO3(s) + Na2SO4(s)

 

 

TEST

OBSERVATION

INFERENCE

(a)

F + water
and filter

 

Filtrate + litmus paper

Partly soluble,
colourless filtrate 
white residue 

No effect on either blue or red litmus paper 

 

 

filtrate is neutral   

(b)

Filtrate + BaCl2(aq)

 

+ dil HCl

White precipitate 

 

Insoluble in HCl(aq)

 

CO32- , SO32- ,  SO42-, S2-
present

SO42- 
Confirmed

(c)

Residue +
dil HCl

Effervescence occurred
Colourless, odourless gas   evolved
turned lime water milky  

CO2   evolved from CO32-

 

 

 

(i)

Solution from (c) + NaOH(aq) in drops

then in excess

White gelatinous precipitate 

 

precipitate soluble

Zn2+ /Al3+ 

 

Zn2+ / Al3+

(ii)

Second portion of solution from  (c)
+ NH3(aq) in drops

then in excess

White gelatinous
precipitate 

 

Soluble in excess NH3(aq)

Zn2+ ,Al3+ (both mentioned to score)

 

Zn2+  confirmed