d
Key ldea \(C H_{3} C O O H\) (weak acid) and \(C H_{3} C O O N a\) (conjugated salt) form acidic buffer and for acidic buffer

\(p H=p K_{a}+\log \frac{| \text {salt}|}{\text { lacid }}\) and \(\left[H^{+}\right]=-\) antilogp \(H\)

\(p H=-\log K_{a}+\log \frac{| \text {eg } t |}{| \text { accid }}\)

\(\left[\therefore p K_{a}=-\log K_{a}\right]\)

\(=-\log \left(1.8 \times 10^{-5}\right)+\log \frac{0.20}{0.10}\)

\(=4.74+\log 2\)

\(=4.74+0.3010=5.041\)

Now, \(\left[H^{+}\right]=\) antilog \((-5.045)\)

\(=9.0 \times 10^{-6}\, \mathrm{mol} / L\)