Question

Mass of non-volatile solute (urea) needs to be dissolved in 100 g of water in order to decrease the vapour pressure of water by 30%. What will be the molality of solution?

• A. 23.88 m
• B. 28.88 m
• C. 25.88 m
• D. 35.88 m

Answer 1

The correct option is A 23.88 m

Using Raoult’s law in the following form,
$\frac{P^{\circ }-P_S}{P_S}=\frac{W_{2}M{w}_1}{W_{1}M{w}_2}$
If $P^{\circ }=100mm,thenPs=70mm$
$\frac{100-70}{70}=\frac{W_2\times 18}{100\times 60}$
$0.43=\frac{w_2\times 18}{100\times 60}\Rightarrow W_2=143.33g$
$Molality=\frac{W_2\times 1000}{M{w}_2\times W_1}=\frac{143.33\times 1000}{60\times 100}=23.88m$