Page 399 - SUBSEC October 2017_Neat
P. 399
02112020/CAPE/SPEC/MS/2017
- 6 –
CHEMISTRY
UNIT 01 — PAPER 02
MARK SCHEME
KC UK XS
(ii) From the dissociation constants, solution X is a solution
of weak acid while solution Y is a solution of a weak
base.
Sodium hydroxide will therefore react with X to form the
sodium salt. The mixture will consist of the weak acid
and the salt of that acid, which contains the conjugate
base of the acid. This will act as a buffer solution.
Weak acid dissociating: HX = H + X
-
+
-
Salt dissociating: NaX = Na+ + X
[2 marks]
+
(iii) On addition of small amounts of H , the excess X ions
-
from the salt will react with the added H and maintain
+
the pH almost constant, hence buffering action.
[1 mark]
On addition of small amounts of base, e.g. OH ions H +
-
from the acid will react and the acid equilibrium will
then shift to the right in this way almost nullifying the
+
effect of the added H and the pH remains effectively
constant, hence buffering action. [1 mark]
Total 30 marks 10 15 5
