JEE Main · 2022 · 25 Jun 2022 · Shift 2 · Solutions

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Solute A associates in water. When 0.7 g of solute A is dissolved in 42.0 g of water, it depresses the freezing point by

0. 2^{\circ} C

. The percentage association of solute A in water, is
[Given : Molar mass of

A = 93 g mol^{- 1}

. Molal depression constant of water is

1.86 K kg mol^{- 1}

]

(A) $50%$
(B) $60%$
(C) $70%$
(D) $80%$ ✓ Correct

Answer: (D)

Solution

Using

Δ T_{f} = i \cdot k_{f} \cdot m

, where

m = \frac{0.7}{93} \times \frac{1000}{42}

. Solving:

0.2 = i \times 1.86 \times \frac{0.7 \times 1000}{93 \times 42}

gives

i = 0.60

. For association

2 A ⇌ A_{2}

, the van't Hoff factor is

i = 1 - \frac{α}{2}

. Setting

1 - \frac{α}{2} = 0.60

gives

α = 0.80

80%

JEE Main · 2022 · 25 Jun 2022 · Shift 2 · Single correct option

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