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JEE Advanced levelSingle correct optionDifficulty 3 of 3g2053

The pKa values in water are 4-nitrobenzoic acid 3.44, benzoic acid 4.20 and 4-methoxybenzoic acid 4.47. The ratio $K_{a} (4-nitrobenzoic acid) / K_{a} (4-methoxybenzoic acid)$ is closest to

A5.8
B1.9
C10.7
D0.09

Answer: C

The ratio is

1 0^{4.47 - 3.44} = 1 0^{1.03} = 10.7

. The value 5.8 would be the ratio of the nitro acid to benzoic acid (

1 0^{0.76}

), and 1.9 the ratio of benzoic acid to the methoxy acid (

1 0^{0.27}

), while 0.09 is the inverted ratio.

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