Question

Solute A associates in water. When 0.7 g of solute A is dissolved in 42.0 gof water, it depresses the freezing point by 0.2°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%

Answer 1

Answer: (A)

50%

Answer 2

The correct option is(D): 80%

Since, ΔTf = ikfm

$m=]frac{0.7}{93}×\frac{1000}{42}$

$0.2=i×1.86×\frac{0.7×1000}{93×42}$

i = 0.6

$α=\frac{i=1}{\frac{1}{n}-1}=\frac{0.6-1}{\frac{1}{2}-1}=0.8$

Hence, percentage association of solute A is 80%.