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JEE Main levelSingle correct optionDifficulty 2 of 3g2033

The pKa values of acetic acid and phenol in water are 4.76 and 10.0. The ratio $K_{a} (acetic acid) / K_{a} (phenol)$ is closest to

A $5.8 \times 1 0^{5}$
B $1.7 \times 1 0^{5}$
C $5.2$
D $1.7 \times 1 0^{- 5}$

Answer: B

The ratio of Ka values equals

1 0^{p K_{a} (phenol) - p K_{a} (acetic)} = 1 0^{5.24} = 1.74 e + 05

, i.e. about

1.7 \times 1 0^{5}

. A common slip is to quote the pKa difference (5.24) itself as the ratio.

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