Question

At 25C, the vapour pressure of pure water is 25.0 mm Hg and that of an aqueous dilute solution of urea is 20 mm Hg. Calculate the molality of the solution.

Answer 1

Relative lowering in vapor pressure $=\frac{P^{\circ }-P_s}{P^{\circ }}=\frac{w_1\times M_2}{M_1\times w_2}$

Here $1$ denotes urea and $2$ denotes $H_{2}O$.

$\therefore \frac{25-20}{25}=\frac{w_1\times 18\times {10}^{-3}}{M_1\times w_2}\dots \dots \left[w_{2}inKg\right]$

$\therefore \frac{5}{25}\times \frac{1000}{18}=\frac{w_1}{M_1\times w_2=Molality}$

$\therefore Molality=11.11M$