Answer is: pH of solution is 3.05.
c(HNO₂) =
0.20 M.
c(NaNO₂) =
0.10 M.
Ka(HNO₂) =
4.5·10⁻⁴.
This is buffer solution, so use Henderson–Hasselbalch
equation:
pH = pKa + log(c(NaNO₂) ÷ c(HNO₂)).
pH = -log(4.5·10⁻⁴) + log(0.10 M ÷ 0.20 M).
pH = 3.35 - 0.30.
pH = 3.05.
