Chemistry Paper 3A WASSCE (SC), 2020

Question 3A

 

  1. zinc salt, E when heated strongly, produced a brown gas with pungent smell, a colourless gas which rekindled a glowing splint and a residue which was allowed to cool.
    1. Identify the salt E.
    2. Write an equation for the decomposition of E.
    3. State what would be observed when the residue was allowed to cool.

    [5 marks]

  2. Describe how 250 cm-3 of 0.2 mol dm-3 H2SO4 could be prepared from 150 cm3 of a 1.0 mol dm-3 stock solution of the acid.
  3. [7 marks]

  4. State the effect of aqueous solution of Al2 (SO4 )3 on litmus paper.

[1 mark]

 

 

Observation

 

This involved a general knowledge in Chemistry. Majority of the candidates performed below    average.

In part (a), majority of the candidates could not identify salt E and write an equation for the decomposition of E.

In part (b), majority of the candidates could not describe how 250 cm3 of 0.2 mol dm-3 H2SO4 could be prepared from 150 cm3 of a 1.0 mol dm-3 stock solution of the acid.

In part (c), majority of the candidates could not state the effect of aqueous solution of Al2(SO4)3 on litmus paper.

The expected answers include:

(a)        White precipitate of BaSO4 barium tetraoxosulphate(VI) is formed.
(b)        Effervescence / Evolution of gas producing a gas with a smell of rotten eggs and a green solution is formed.
(c)        Effervescence / Evolution of colourless, odourless gas (Light green) solution turned brown after sometime.