Question

A current of dry air is passed through a bulb containing $5\text{g}$ of a solute in $100\text{g}$ of water and then through water alone. The loss in weight of the solution and pure water were respectively $0.78\text{g}$ and $0.02\text{g}$. Calculate the relative lowering of vapour pressure.

• A. 2.04
• B. 1.05
• C. 0.025
• D. 0.09

Answer 1

The correct option is C 0.025

loss in weight of pure water $\propto P_o-P_S$ (i)
and loss in weight of solution ∝ $P_S$ (ii)
ratio of (i) and (ii) will give relative lowering of vapour pressure
Using $\frac{P_o-P_S}{P_s}=\frac{0.02}{0.78}=0.0256$