This question was attempted by majority of the candidates and the performance was fair. In (a), candidates answered correctly; however, some candidates did poorly in the use of kinetic theory to explain Charles’ law. They only stated the law and hence lost the marks allotted to that question. In (b), candidates were able to arrange CS2, CO2 andNaH in increasing order of boiling point but majority of the candidates could not give reasons for their answers.
In (c), some candidates found it difficult to calculate the number of moles of Cl- produced.
The expected answers include:
(a) (i) Hygroscopic substance – is defined as a substances which absorbs moisture on
exposure to the atmosphere but does not form solution or change state.
OR
Hygroscopy is a phenomenon where a substance absorbs moisture on exposure to the
atmosphere but does form solution or change state
(ii)
Physical Change |
Chemical change |
- no new substance formed |
- new substance formed |
- change is easily reversible |
- change is not easily reversible |
- heat change is not usually involved |
- Heat is usually given out or absorbed/ it
involves heat |
-no change in mass of substance involved |
- change in mass of substance involved |
(iii) At constant pressure, when the temperature of a gas is increased, the molecules gain
more kinetic energy and move faster. The distance between the molecule increases
resulting in increase in the volume of the gas.
OR
At constant pressure, when the temperature of a gas is decreased, the molecules lose
more kinetic energy and move slowly. The distance between the molecule decreases
resulting in decrease in the volume of the gas.
OR
Kinetic energy is directly proportional to temperature. Increase/decrease in temperature at constant pressure results in increase or decrease in velocity leading to increase or decrease in distance between the molecules resulting in increase/decrease in volume of gas.
(b) (i) CO2 , CS2 , NaH
increasing boiling point
CO2 – weak vander Waal’s forces
CS2 – stronger vander Waal’s forces
NaH – with ionic forces stronger than van der Waal’s forces
(ii) Cl2(g) + 2NaOH(aq) → NaOCl(aq) + NaCl(aq) + H2O(l)
(c) (i) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
(ii) I. From the reaction equation
mole of H2 = mole of Mg
n(H2) = = 0.3839 moles
Hence, number of moles of Mg = 0.3839
mass of Mg = mole x Molar Mass
= 0.3839 x 24
= 9.21g
II % purity =mass of pure Mgx 100
Mass of impure
= x 100
= 61.4%
III. From the equation
moles of Cl- = 2 x moles of Mg
= 2 x 0.3839
= 0.7678 moles
No of Cl- produced x NA
= 0.7678 x 6.02 x 1023
= 4.62 x 1023Cl-
ALTERNATIVE SOLUTION
22.4 dm3 of H2 ≡ 24 g Mg
8.6 dm3 of H2 ≡ 24 x 8.6
22.4
≡ 9.21 g
II % purity =mass of pure Mgx 100
Mass of impure
= x 100
= 61.4%
Moles of Mg = 9.21
24
= 0.3839 mol
Mole of Cl- = 2 x 0.3839
= 0.7678
Number of Cl- = 0.7678 x 6.02 x 1023
= 4.62 x 1023
