Nitrous acid has a Ka of 4.7x10-4. What is the pH of a buffer solution containing 0.15 M HNO2 and 0.20 M NO2?

Use the Henderson-Hasselbalch equation:

[H+] = Ka * [HA] / [A-]

where

• [HA] = concentration HNO2 (acid)

  and

• [A-] = concentration NO2- (base)

  and

• Ka = 4.7*10-4

Filling in and calculating:

[H+] = Ka * [HA] / [A-] = (4.7*10-4) * (0.15) / (0.20) = 3.5*10-4 ,

so :

pH = -log[H+] = -log(3.5*10-4) = 3.45