Question

A solution of lactose containing 8.45 g of lactose in 100 g of water has a vapour pressure of 4.559 mm of Hg at 0${}^o$C. If the vapour pressure of pure water is 4.579 mm of Hg, calculate the molecular weight of lactose.

Answer 1

Given : Weight of lactose w $=8.45g$

Weight of water W $=100g$

Vapour pressure of pure water $=4.579mm$ of Hg

Vapour pressure of solution $=4.559mm$ of Hg

Now, according to Roast's law.
$\frac{\Delta P}{P}=\frac{w\times M}{m\times W}$

$\frac{4.579-4.559}{4.579}=\frac{8.45\times 18}{m\times 100}$

$m=\frac{8.45\times 18\times 4.579}{0.02\times 100}$

$=348.23\approx 348$
Molecular mass of lactose is 348.