KEAM 2011 Physics and Chemistry Paper

Ionisation of weak acid at C1=0.1M
HA ⇌  ‌‌H+ (aq)+ A−(aq)
C1     0  0   : Initial concentration
C1(1−α1) ‌C1α1 C1α1  : Initial concentration
∴‌‌Ka=‌

C1α1⋅C1α1

C1(1−α2)

=‌

C1α1

(1−α1)

=C1α12   (∵α1<<<1)
Ionisation of weak acid C2=0.025M
HA ⇌  ‌‌H+ (aq)+ A−(aq)
C2     0  0   : Initial concentration
C2(1−α2) ‌C2α2 C2α2  : Initial concentration
∴‌‌Ka=‌

C2α2⋅C2α2

C2(1−α2)

=‌

C2α22

(1−α2)

=C2α22   (∵α2<<<1)
∵ Ionisation constant of an acid is a constant and does not change with concentration.
∴‌‌C1α12=C2α22
or ‌‌α22=‌

C1α12

C2

=‌

0.1×(1)2

0.025

or α22=4
or ‌‌α2=2
∴ The percentage of ionisation of weak acid in 0.025M solution is 2%