What is the pH of 0.214 M KCN?

Question

What is the pH of 0.214 M KCN?

When search the internet, often the equation is changed to HCN, what happens to the Potassium?

How to solve?

Thank you!

Answer 1

KCN is made from a weak acid (HCN) + a strong base (KOH), so the pH of a solution of KCN will have a basic pH, i.e. pH >7.

To find the pH of this solution, we look at the hydrolysis of CN^-, as follows:

CN^- + H²O ==> HCN + OH^-

We don't look at the K⁺ (potassium) because it is neutral (K⁺ + H₂O => KOH + H⁺)

From the hydrolysis of CN-, we can now write the Kb expression since CN- is acting as a base...

Kb = [HCN][OH^-] / [CN^-]

Looking up the Kb for CN- we find it to be 1.6x10^-5

1.6x10^-5 = (x)(x) / 0.214

x² = 3.4x10^-6

x = 1.85x10^-3 = [OH^-]

pOH = -log 1.85x10^-3 = 2.73

pH = 14 - 2.73

pH = 11.27 (basic as predicted)