K _ a for butyric acid ( C _ 3 H _ 7 COOH) is 2 × 10 ^ - 5 . The pH...

${K}_{a}$ of Butyric acid $⇒ 2 \times {10}^{- 5}{pK}_{a} = - {logK}_{a} = 4 . 7$

$pH$ of weak acid solution

$pH = \dfrac{1}{2}{pK}_{a} - \dfrac{1}{2}logC$

$= \dfrac{1}{2}\left(4 \cdot 7\right) - \dfrac{1}{2}\log \left(0 . 2\right)$

$= 2 . 35 + 0 . 35 = 2 . 7$

$pH = 27 \times {10}^{- 1}$