Calculate the pH of a 0.100 M KCN solution

Question

Chemistry

Answer 1

KCN is the salt of a strong base (KOH) and a weak acid (HCN), and thus the salt in aqueous solution will have a basic pH.

One needs to then look at the hydrolysis of the cyanide anion, CN^-, which is as follows:

 

CN^- + H2O ==> HCN + OH ^-   (note: CN^- acts as a base, and so one need to know the Kb for CN^-)

 

Looking up the Ka for HCN, I find it to be 6.7x10^-10, and recall that Kb = Kw/Ka = 1x10^-14/6.7x10^-10 = 1.5x10^-5

 

Kb = 1.5x10^-5 = [HCN][OH^-]/[HCN]

1.5x10^-5 = (x)(x)/0.100

x^2 = 1.5x10^-6

x = [OH-] = 1.2x10^-3 M

pOH = 2.9

pH = 14 - pOH = 11.1 (basic as predicted)