Question 3A
- 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.
- Identify the salt E.
- Write an equation for the decomposition of E.
- State what would be observed when the residue was allowed to cool.
- 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.
- State the effect of aqueous solution of Al2 (SO4 )3 on litmus paper.
[5 marks]
[7 marks]
[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.