Question

Vapour pressure of a pure solvent is 550 mm of H By addition of a non volatile solute it decreas to 510 mm of Hg. Calculate the mole fraction solute in solution.

• A. 0.215
• B. 0.072
• C. 0.610
• D. 0.512

Answer 1

Answer: (B)

0.072

Answer 2

Using Raoult's law for a non-volatile solute:

$$\frac{P^0 - P}{P^0} = x_{\text{solute}}$$

Here, $P^0 = 550\, \text{mmHg}$ and $P = 510\, \text{mmHg}$.

Calculate the relative lowering of vapor pressure:

$$x_{\text{solute}} = \frac{550 - 510}{550} = \frac{40}{550} \approx 0.0727$$

Rounded, the mole fraction of solute is approximately 0.072.

Relative lowering = $\frac{550-510}{550} = 0.072$; hence mole fraction of the solute ≈ 0.072.