The correct order of $K_{a}$ is ?
(i) $C H_{3} - C H_{2} - C O O H$
(ii) $O H C - C H_{2} - C O O H$
(iii) $P h - C H_{2} - C O O H$

i i i > i i > i

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i > i i > i i i

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i i > i i i > i

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i > i i i > i i

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Solution

The correct option is D $i i > i i i > i$
Acetic acid $K_{a} = 1.7 \times 10^{- 5}$ , $- log K_{a} = - 4.76$ therefore $p K_{a} = 9.76$ and $p K_{a}$ of Phenylacetic acid is $4.28$ and $3 - o x o p r o p a n o i c$ acid is $3.6$ . So, according to $p K_{a}$ value $3 - o x o p r o p a n o i c$ acid is strong acid than phenylacetic acid and acetic acid. Phenylacetic acid is more acid than acetic acid. $p K_{a}$ is used which is defined as negative logarithm of $K_{a} (- log K_{a})$ . Also lower value of $p K_{a}$ of a acid, stronger it's acidity.
Hence the order is $i i > i i i > i$ .

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