Question

The vapour pressure of water is $15.6\text{kPa}$ at $333\text{K}$. Calculate the vapour pressure of 3 molal solution of a solute in it.

• A. $15.4\text{kPa}$
• B. $15.0\text{kPa}$
• C. $14.8\text{kPa}$
• D. $14\text{kPa}$

Answer 1

The correct option is C $14.8\text{kPa}$

3 molal solution means 3 mole of the solute in 1 kg of the water.
$\therefore$ Mole fraction of solute = $\frac{3}{3+55.5}=\frac{2}{39}$
Now, $\frac{P^0-P_s}{P^0}=x_2$
i.e., $\frac{15.6-P_s}{15.6}=\frac{2}{39}$
or, $P_s=14.8\text{kPa}$