Engineering Chemistry S Datta

IONIC EQUILIBRIUM 201
Q. 19. H 3
PO 4
has three K a
values–explain.
Ans. There are three dissociations for H 3
PO 4
and accordingly k a
values.
(i) H 3
PO 4
H + + H 2
PO 4 – ; K
a1
=
+ −
[H ] [H2PO 4]
[H PO ]
3 4
+ −
2 4 2
(ii) H 2
PO
–
4
H + + HPO –2 [H ] [H PO ]
4
; K
a2
=
−
[H PO ]
3 4
+ −3
(iii) HPO
–2
4
H + + PO –3 4
; K
[H ] [PO 4 ]
a3
=
−2
[HPO ]
K a1
> K
a2
> K a3
.
Q. 20. Relate K a
, K b
and K w
.
Ans. (i) HA+HO 2
HO 3
+A –
acid
base
Conjugate pair
+ −
[H O ] [A ]
K a
=
3
[HA]
(ii) A – +HO
2
HA+OH –
Conjugate pair
4
K b
=
[HA] [OH ]
−
[A ]
−
+ −
[H3
O ] [A ]
Now, K a
× K b
=
[HA]
= K w
= 10 –14 .
×
[HA] [OH ]
−
[A ]
−
= [H 3
O + ] [OH – ]
∴ K a
× K b
= K w
or – log K a
– log K b
= – log K w
∴ pK a
+ pK b
= pK w
= 14.
Q. 21. Mention the importance of pH.
Ans. (i) The pH value of soil helps to decide the type of fertiliser to be used in agriculture.
(ii) A small variation of pH of human blood (6.7) causes bodyache.
(iii) pH value helps to determine the condition of reactions in laboratory and in industry.
Q. 22. What is common ion effect?
Ans. The change in equilibrium position due to addition of an ionic compound that
contains one of the ions involved in the equilibrium is known as common ion effect.
(i) The common ion effect can change the ionisation of weak acids and weak bases.
(ii) The common ion effect influences the behaviour of buffer solution.
(iii) The common ion effect alters the solubility of a sparingly soluble salt.
Q. 23. What is a buffer solution?
Ans. Buffer solutions are mixtures of a weak acid and its salt or a weak base and its
salt, which help to keep pH of a solution more or less constant.