The question was attempted by most candidates and the performance was good.
In part (a) (i) and (ii), candidates correctly defined atomicity as the number of atoms present in a given molecule (of an element or a compound) and mentioned one element each for question (ii) I – III respectively, thus.
I - helium/neon/argon etc
II - hydrogen/oxygen/nitrogen/chlorine/fluorine etc
III - phosphorus.
In part (b)(i), candidates wrote correct orbital electron configuration of 20Ca and 9F as follows
- 1s2 2s2 2p6 3s2 3p6 4s2
- 1s2 2s2 2p5
In part (b)(ii) – (iv), candidates knew that 20Ca and 9F belong to group 2 and 7, 9F has one unpaired electron and that 20Ca 24 has 18 electrons.
In (c)(i), candidates correctly wrote a balanced equation for the thermal decomposition of KCƖO3 thus:
2KCƖO3(s) → 2 KCl(s) + 3O2)g).
In part (c) (ii), candidates correctly mentioned (Mn O2) manganese (IV) oxide as the catalyst that could be used to increase the rate of reaction.
In part (c) (iii), candidates correctly determined the volume of oxygen produced at s.t.p. as follows:
molar mass of KCƖO3 = 39 + 35.5 + 16X3 / 122.5
number of moles of KCƖO3 = 5.0 = 0.0408 mole
mole ratio of KCƖO3: O2 = 2
hence number of moles of O2 evolved = 0.0408 x 3
∴ volume of O2 evolved =22.4 x 0.0408 x 3
= 1.37 dm3
Alternative methods were also accepted.
In part (d)(i) and (ii) candidates correctly mentioned the products formed when I and II were heated and stated the colour changed observed when the residue in I and II are allowed to cool, respectively, as follows:
(i) I. ZnO / CO2
II. CuSO4 / CuSO4.H2O
- I. Yellow to white/grey
- White / grey colour remained unchanged.