Decreasing order of acidity of p-methoxy benzoic acid (A), p-nitrobenzoic acid (B) and benzoic acid (C) is :

B > C > A

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A > B > C

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C > A > B

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None

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Solution

The correct option is B B > C > A
Substitution of $- O C H_{3}$ on benzoic acid decreases its acidic strength because it is an activating group having $+ M$ effect which increases electron density and destabilizes benzoate carbanion. The $- N O_{2}$ group has $- M$ effect which withdraws electron density and forms stable benzoate carbanion and makes it a strong acid. Therefore, the order of acidic strength is $p - n i t r o b e n z o i c a c i d > b e n z o i c a c i d > p - m e t h o x y b e n z o i c a c i d$ .

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