This question was attempted by most candidates and the performance was good.
In (a)(i) I and II, majority of the candidates were able to correctly define mole as the amount containing as many elementary entities as the number of atoms in 12g of
carbon– 12 and concentration as the amount of solute/substance in moles per dm3 of solution.
In(a)(ii), candidates correctly determined the mass of magnesium oxide that would be produced by 1 mole of magnesium from the given equation thus:
2 moles of Mg ≡ 2moles of MgO
2 moles of Mg ≡ 2 (24 + 16)g of MgO
1 mole of Mg ≡ 80 = 40g
2
In (b)(i), only a few candidates could correctly write the balanced chemical equation for the reaction between chlorine gas and iron (II) chloride solution as follows:
Cl2(g) + 2FeCl2(aq) → 2FeCl3(aq)_
In (b)(ii), majority of the candidates stated that the reaction is oxidation instead of redox because of change in oxidation number of both Fe and Cl2 in FeCl3.
In (b)(iii), candidates correctly wrote that the colour of the acidified KMnO4 would change from purple to colourless.
In (c)(i)-(iii), most candidates wrote the general application of the separation methods instead of the required industrial application. The expected answers from candidates were:
Crystallization – sugar production/pharmaceutical production
Filtration – water treatment plants
Fractional distillation – petroleum refineries.
In(d)(i) – (iii), only a few candidates correctly wrote the names of the precipitates formed while a good number of them wrote the formulae. The required answer were
Silver chloride
Barium tetraoxosulphate (VI)
Lead trioxocarbonate (IV)
In (e)(i) and (ii), most candidates correctly listed three characteristic properties of transition metals from among the following:
- formation of coloured compounds/ions
- exhibition of variable oxidation states
- formation of complex ions
- exhibit paramagnetism
- show catalytic properties
and that only chromium and Manganese belong to the first transition series.
