Answer is: concentration of hydrogenium ions is 9,54·10⁻⁵ M.
c(HNO₂) = 0,075 M.
c(NaNO₂) = 0,035 M.
Ka(HNO₂) = 4,5·10⁻⁵.
This is buffer solution, so use Henderson–Hasselbalch equation:
pH = pKa + log(c(NaNO₂) ÷ c(HNO₂)).
pH = -log(4,5·10⁻⁵) + log(0,035 M ÷ 0,075 M).
pH = 4,35 - 0,33.
pH = 4,02.
[H₃O⁺] = 10∧(-4,02).
[H₃O⁺] = 0,0000954 M = 9,54·10⁻⁵ M.
