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°C. If the vapour pressure of pure water is 4.579 mm of Hg, calculate the molecular weight of lactose.

Answer 1

Weight of lactose w = 8.45 g

Weight of water W = 100 g

Vapour pressure of pure water (p°) = 4.579 mm of Hg

Vapour pressure of solution (ps) = 4.559 mm of Hg

Now, according to Raoult’s’ law

The molecular mass of lactose is 348.