To neutralize completely 20 mL of 0.1 M aqueous solution of phosphorous acid $(\mathrm{H}_{3} \mathrm{PO}_{3})$, the volume of 0.1 M aqueous KOH solution required is

JEE Main · 2004 · 24 Apr 2004 · Offline · Equilibrium

To neutralize completely 20 mL of 0.1 M aqueous solution of phosphorous acid

(H_{3} PO_{3})

, the volume of 0.1 M aqueous KOH solution required is

(A)10 mL
(B)60 mL
(C)40 mL✓ Correct
(D)20 mL

Answer: (C)

Solution

Phosphorous acid

H X_{3} PO X_{3}

looks tribasic but is really dibasic, because one of its three hydrogens is bonded straight to phosphorus and cannot ionise, giving an n-factor of 2. Equating the milliequivalents of acid and base,

2 \times 20 \times 0.1 = 1 \times V \times 0.1

, which solves to

V = 40 mL

of the potassium hydroxide.

JEE Main · 2004 · 24 Apr 2004 · Offline · Single correct option

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