Question

What 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 $25\%$. What will be the molality of the solution?

Answer 1

Using Raoult's law in the following form,

$\frac{P_0-P_s}{P_0}=\frac{w\times M}{W\times m}$

If $P_0=100$ mm, then $P_s=75$ mm

$\frac{100-75}{100}=\frac{w\times 18}{100\times 60}$

$w=83.33g$

Molality $=\frac{w\times 1000}{m\times W}=\frac{83.33\times 1000}{60\times 100}=13.89m$