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 <span>Henderson–Hasselbalch equation: pH = pKa + log(c(</span>NaNO₂) ÷ c(HNO₂)). pH = -log(4,5·10⁻⁵) + log(0,035 M ÷ 0,075 M). pH = 4,35 - 0,33. pH = 4,02. <span>[H</span>₃O⁺] = 10∧(-4,02). <span>[H</span>₃O⁺] = 0,0000954 M = 9,54·10⁻⁵ M.