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 Å in mol L^-1 is (K_f for water = 1.86 K kg mol^-1)

• A. 155
• B. 354
• C. 62
• D. 221

Answer 1

Answer: (C)

$\alpha =\frac{80}{100}=0.8$
$\alpha =\frac{1-i}{1-\frac{1}{n}}=\frac{1-i}{1-\frac{1}{4}}$
$0.8=\frac{1-i}{\frac{3}{4}}$
$i=0.4$
$\Delta T_f=0.3$
$\Delta T_f=\frac{i\times k_f\times w_2\times 1000}{M_2\times 100}$
$M_2=\frac{0.4\times 1.86\times 2.5\times 10}{0.3}$
$M_2=62gmo{l}^{-1}$