Question

Calculate the pH of a 0.315 M Phenylmethanamine solution (pKb = 4.7).

Answer 1

Lets write the base as B+

use:
pKb = -log Kb
4.7= -log Kb
Kb = 1.995*10^-5
B dissociates as:

B +H2O     ----->     BH+   +   OH-
0.315                   0         0
0.315-x                 x         x


Kb = [BH+][OH-]/[B]
Kb = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes

Kb = x*x/(c)
so, x = sqrt (Kb*c)
x = sqrt ((1.995*10^-5)*0.315) = 2.507*10^-3

since c is much greater than x, our assumption is correct
so, x = 2.507*10^-3 M



So, [OH-] = x = 2.507*10^-3 M


use:
pOH = -log [OH-]
= -log (2.507*10^-3)
= 2.60


use:
PH = 14 - pOH
= 14 - 2.60
= 11.40
Answer: 11.40