Question

A non-volatile solute ‘X’ (molar mass = 50 g mol^–1 ) when dissolved in 78 g of benzene reduced its vapour pressure to \(90\%\). Calculate the mass of X dissolved in the solution.

Answer 1

Let W gm of solute X is dissolved in 78 g of benzene.

Mole fraction of solute \(X = \frac{\frac{W}{50}}{\frac{W}{50}+\frac{78}{78}} = \frac{W}{W+50}\)

\(\text{Relative lowering of vapour pressure} = \frac{100-90}{100}\) 

\(= \frac{10}{100} = \frac{W}{W+50}\) 

\(\therefore \ W = 5.55 \ \text{gm}\)