Question

Un-ionized hydrogen cyanide (HCN, K_a = 4.8 x 10^-10) is toxic to fish. If the toxic level for un-ionized hydrogen cyanide (HCN) for a certain fish species of concern is 10^-6 mol/L (i.e, the equilibrium concentration for HCN), at what pH does the HCN reach toxic levels?

Answer 1

HCN <-> H+ + CN-

C 0 0 (initial)

C-x x x (at equilibrium)

At equilibrium,

Ka = [H+][CN-]/[HCN]

4.8*10^-10 = [H+][CN-]/(10^-6)

[H+][CN-] = 4.8*10^-16

x*x = 4.8*10^-16

x^2 = 4.8*10^-16

x = 2.2*10^-8 M

So,

[H+] = 2.2*10^-8 M

Given:

[H+] = 2.2*10^-8 M

use:

pH = -log [H+]

= -log (2.2*10^-8)

= 7.6576

Answer: 7.66