What is the pH?

Question

What is the pH of a solution of 0.300 M HNO₂ containing 0.220 M NaNO₂? (Ka of HNO₂ is 4.5 × 10⁻⁴)

Answer 1

The solution described in this problem is a buffer since it contains a weak acid (HNO2) and the conjugate base (NaNO2). To determine the pH, we can use the Henderson Hasselbalch equation...

pH = pKa + log [conj.base]/[acid]

pH = ?

pKa = -log Ka = -log 4.5x10^-4 = 3.35

[conj.base] = 0.220 M

[acid] = 0.300

Solving for pH...

pH = 3.35 + log (0.220/0.300) = 3.35 + 1.824

pH = 5.17