Calculate the pH of a 2.6M solution of hydrocyanic acid.

Question

The acid dissociation constant K_a of hydrocyanic acid (HCN) is 6.2 x 10^-10. Calculate the pH of a 2.6M solution of hydrocyanic acid. Round your answer to 

1 decimal place. 

pH =

Answer 1

HCN ==> H⁺ + CN^- ... Ka = 6.2x10^-10

Ka = [H⁺][CN^-] / [HCN]

Ka = 6.2x10^-10

[H⁺] = [CN^-] = ?

[HCN] = 2.6 M

Solving for [H⁺] ...

6.2x10^-10 = (x)(x) / 2.6 - x ... since Ka is so small, we can ignore the x in the denominator

6.2x10^-10 = x² / 2.6

x² = 1.61x10^-9

x = [H⁺] = 4.0x10^-5 M

pH = -log [H⁺]

pH = -log 4.0x10^-5

pH = 4.4