The exact volumes of 1 M NaOH solution required to neutralize 50 mL of $1 \mathrm{M} \mathrm{H}_{3} \mathrm{PO}_{3}$ solution and 100 mL of $2 \mathrm{M} \mathrm{H}_{3} \mathrm{PO}_{2}$ solution respectively, are:

JEE Main · 2021 · 16 Mar 2021 · Shift 2 · Equilibrium

The exact volumes of 1 M NaOH solution required to neutralize 50 mL of

1 M H_{3} PO_{3}

solution and 100 mL of

2 M H_{3} PO_{2}

solution respectively, are:

Answer: (B)

Phosphorous acid,

H_{3} P O_{3}

, has one hydrogen bonded straight to phosphorus and two in

P - OH

groups, so only two are ionisable and the acid is dibasic. Hypophosphorous acid,

H_{3} P O_{2}

, has two

P - H

bonds and a single

P - OH

group, so it is monobasic. Neutralising

50 \times 1 = 50

mmol of the dibasic acid needs

100

mmol of alkali, which is 100 mL of the 1 M solution. Neutralising

100 \times 2 = 200

mmol of the monobasic acid needs

200

mmol of alkali, which is 200 mL. The volumes required are 100 mL and 200 mL.

JEE Main · 2021 · 16 Mar 2021 · Shift 2 · Single correct option