Page 398 - SUBSEC October 2017_Neat

02112020/CAPE/SPEC/MS/2017
- 5 –

CHEMISTRY
UNIT 01 — PAPER 02
MARK SCHEME

KC UK XS
(d) Pure water is a poor conductor of electricity due to the
following equilibrium which lies to the far left
H2O(l) H (aq) + OH (aq).
+
-

Since the degree of ionization is very small, it is a poor
conductor. [2 marks]
3
In the presence of H+, pure water is a good conductor as the
concentration of free ions present increases. [1 mark]

(e) (i) The Kc expression for water becomes

Kc = [H (aq)][OH (aq)]
-
+
[H2O(l)]

Since the H2O is effectively constant, this is incorporated
into the Kc constant giving a new constant
2
Kw = [H (aq)][OH (aq)] [2 marks]
-
+

1
(ii) Kw increases with temperature.

As T increases, more energy is available to ionize
+
H2O. [H ] increases, hence Kw increases. [1 mark]

(f) (i) pH of soln A = -log (2 x 10 ) = 4.7 [1 mark]
-5
3
pH of soln B = -log (1 x 10 ) = 2 [1 mark]
-2

pH of soln C = -log (1 x 10 -14 ) = 14 [1 mark]

(ii) pH scale relative to water: solution pH
C 14
Pure Water 7.0 1
A 4.7
B 2.0
[1 mark]

(g) (i) A buffer solution is one that resists changes in pH when
small quantities of acid or base are added to it.
[1 mark] 1