Question

Calculate the mole fraction of solute, if the vapour pressure of pure benzene at certain temperature is 640 mmHg and vapour pressure of solution of a solute in benzene is 600 mmHg.

Hint:

Raoult’s law applies to non-volatile solutes; mole fractions sum to 1.

Answer 1

Using Raoult’s law for a non-volatile solute: \( P = P_0 x_A \), where \( P_0 = 640 \, \text{mmHg} \), \( P = 600 \, \text{mmHg} \), \( x_A \) is mole fraction of solvent (benzene).
\[ x_A = \frac{P}{P_0} = \frac{600}{640} = 0.9375. \] Mole fraction of solute \( x_B = 1 - x_A = 1 - 0.9375 = 0.0625 \).
Answer: Mole fraction of solute = 0.0625.