Question

What is the H3O+ ion concentration in a 0.10 M solution of phenol? (PhOH: Ka=1.0 x 10^-10)

Answer 1

for simplicity lets write weak acid as HA

HA          ----->     H+   +   A-
0.100                 0         0
0.100-x               x         x

Ka = [H+][A-]/[HA]
Ka = x*x/(c-x)
since ka is small, x will be small and it can be ignored as compared to c
So, above expression becomes

Ka = x*x/(c)
so, x = sqrt (Ka*c)
x = sqrt ((1.0E-10)*0.100) = 3.16E-6
so,
[H+] = 3.16*10^-6 M

Answer: 3.16*10^-6 M