Part (a)(i): Few candidates correctly answered this part. Most candidates could not explain that insoluble CaCO3 formation is responsible for the milkness produce when CO2 is bubbled through lime water while the disappearance of milkness is due to the formation of soluble Ca(HCO3)2.
In question (a)(ii) few candidates correctly attempted this question. Majority of the candidates could not explain that calcium hydroxide is not amphoteric and does not react with NaOH whereas lead (II) hydroxide is amphoteric and so reacts with excess NaOH.
Part (b): In question b(i) and (ii) few candidates correctly attempted this question. Majority of the candidates could not calculate the mass of sodium trioxocarbonate(IV) required to prepare 250cm3 of 0.15 moldm-3 solution.
Part (C): In question (i) and (ii) few candidates attempted the questions. Majority of candidates could not give examples of gases that are collected through upward displacement of air and those that are collected through downward displacement of air.
The expected answers were:
Q3. (a) (i) Lime water turns milky with CO2 because CaCO3/ CaCO3(s) is formed.
Milkiness disappear when excess CO2 reacts with CaCO3 in water medium forming the soluble Ca(HCO3)2/ Ca(HCO3)2 (aq).
(ii) Calcium hydroxide is not amphoteric. Does not react with an alkali NaOH whereas lead (II) hydroxide is amphoteric so reacts with excess NaOH.
(b) (i) Primary standard solution is one whose concentration is known and can be used to standardize another solution.
OR
Primary standard solution is a solution of known concentration prepared from pure/non-deliquescent/non-hygroscopic substance.
(ii) M (Na2CO3) = 106 gmol-1
m(Na2CO3) = C x M x V
= 0. 15 x 106 x 0. 25
= 3. 98 g
(c) (i) Carbon(IV) oxide, sulphur (IV) oxide, hydrogen chloride, oxygen,
nitrogen (IV) oxide, chlorine, hydrogen sulphide.
- Ammonia, oxygen, hydrogen, methane.
