4-Hydroxybenzoic acid (HO-C6H4-COOH; carboxylic acid pK_a 4.58, phenolic OH pK_a 9.3 after the carboxylate has formed) is treated with an excess of (i) aqueous NaHCO3 and (ii) aqueous NaOH (pK_a of H2CO3 6.4, of H2O 15.7). The number of moles of base consumed per mole of acid in (i) and (ii) respectively is

JEE Advanced levelSingle correct optionDifficulty 3 of 3g2132

4-Hydroxybenzoic acid ( $HO - C X_{6} H X_{4} - COOH$ ; carboxylic acid $p K_{a}$ 4.58, phenolic OH $p K_{a}$ 9.3 after the carboxylate has formed) is treated with an excess of (i) aqueous $NaHCO X_{3}$ and (ii) aqueous $NaOH$ ( $p K_{a}$ of $H X_{2} CO X_{3}$ 6.4, of $H X_{2} O$ 15.7). The number of moles of base consumed per mole of acid in (i) and (ii) respectively is

With

HCO X_{3} X^{-}

: the

COOH

reacts (

lo g K = 6.4 - 4.58 = 1.8

) but phenolic OH does not (

6.4 - 9.3 = - 2.9

), so 1 mole. With

OH X^{-}

both react (

15.7 - 4.58

and

15.7 - 9.3

are both large and positive), so 2 moles.