Question

A non-volatile solute, ?A? tetramerises in water to the extent of 80%. 2.5 g of ?A? in 100 g of water, lowers the freezing point by 0.3 ?C. the molar mass of ? in mol L-1 is (Kf for water = 1.86 K kg mol-1)

• A. 155
• B. 354
• C. 62
• D. 221

Answer 1

Answer: (C)

62

Answer 2

$\alpha=\frac{80}{100}=0.8$
$\alpha=\frac{1-i}{1-\frac{1}{n}}=\frac{1-i}{1-\frac{1}{4}}$
$0.8=\frac{1-i}{\frac{3}{4}}$
$i=0.4$
$\Delta T_{f}=0.3$
$\Delta T_{f}=\frac{i\times k_{f}\times w_{2}\times1000}{M_{2}\times100}$
$M_{2}=\frac{0.4\times1.86\times2.5\times10}{0.3}$
$M_{2}=62 \, g\, mol^{-1}$