Phenol CHOH also called carbolic acid has a pK of 989 It is

Phenol (C&HOH;, also called carbolic acid) has a pK of 9.89. It is used to preserve body tissues and is quite toxic. Calculate the percent ionized in 0.0553 M phenol--a dilute solution. 6 5 \'

Solution

Let a be the dissociation of the weak acid
HA <---> H + + A-
initial conc. c 0 0
change -ca +ca +ca
Equb. conc. c(1-a) ca ca

Dissociation constant , Ka = ca x ca / ( c(1-a)
= c a2 / (1-a)
In the case of weak acids a is very small so 1-a is taken as 1
So Ka = ca2
==> a = ( Ka / c )
Given pKa= -log Ka = 9.89.

Ka= 10^-pKa= 1.29x10^-10
c = concentration = 0.0553 M
Plug the values we get a = 4.83x10^-5
% dissociation = a x 100 = 4.83x10^-3