Question

On dissolving 0.25 g of a non-volatile substance in 30 mL benzene (density 0.8 g / mL), its freezing point decreases by ${0.40}^{o}C$. Calculate the molecular mass of non-volatile substance ($K_f=5.12Kkgmo{l}^{-1}$).

Answer 1

Mass of benzene, W = volume X density
= 30 X 0.8 = 24 g
Given, $K_f=5.12Kkgmo{l}^{-1},w=0.25g,△T={0.40}^{o}C$.
We know that,
$m=\frac{1000K_f\times w}{W\times △T}$
$=\frac{1000\times 5.12\times 0.25}{24\times 0.40}=133.33$