The question was popular with the candidates and the performance was poor.
In (a)(i), candidates knew that the law illustrated was Charles' law.
n (a)(ii), they correctly stated that the volume of a given mass of gas is directly proportional to its temperature in (Kelvin) provided that the pressure remains constant was the statement of
In(a)(iii), only few candidates correctly sketched a graph to illustrate the law as follows:
In(a)(iv), most candidates applied the law to calculate the new volume of oxygen thus:
V1 = 28.0cm3
T1 = 270
C = 27 x 273 = 300k
T2 = 00
C + 273 = 273k
V2 = ?
V1/T1 = V2/T2
V2 = V1T2/T2 = 28 x 273/300
In part (b), candidates could not give the required explanation in (i)-iv). The expected answer for
each of (i) - Iv) was as follows:
(i) Chlorine bleaches item, (example garment) as dilute acid
During rinsing the water removes acid that can damage the material
(ii) Dilute HN03 oxidizes the hydrogen formed immediately to water
(iii) CO2 is heavier than air and does not support combustion
(iv) To remove the traces of acid fumes/HCl.
In (c)(i), candidates named oxygen and hydrochloric acid as the two products formed when chlorine
water is exposed to sunlight. However, some of them lost the marks because they wrote O2 and
HCl which were not names but formula.
In (c)(ii), only very few candidates knew that NaOCI/NaCI was the main product of chlorine with
cold dilute NaOH.
In(c)(iii), candidates could not state the variation of boiling point and oxidizing ability among the
halogen nor give reason for the variation.
The expected answer from candidates were as follow:
I. Boiling point increases down the halogen group due to increase in molecular mass/van
der Waals forces
II. Oxidizing ability of the halogen decreases down the group due to decrease in effective
nuclear charge as a result of increase in atomic size/decrease in electronegativity.