In a solution composed of benzene and toluene that has a vapor pressure of 32 torr at 20°C, what is the mole fraction of benzene?

The mole fraction of benzene if 0.43

Explanation:

Let's apply the colligative property of vapor pressure lowering.

P° - Ps = P°. Xm

Where P° is vapor pressure of pure solvent

Ps is vapor pressure of solution

Xm is mole fraction of solute

Determine the solute and the solvent:

Solute: Toluene

Solvent: Benzene (Vapor pressure of pure bencene at 20°C, is 74.7Torr)

74.7 Torr - 32 Torr = 74.7 Torr. Xm

42.7 Torr / 74.7 Torr = 0.57 → Xm

Sum of mole fraction = 1

Mole fraction of bencene + Mole fraction of toluene = 1

Mole fraction of bencene + 0.57 = 1 ⇒ 1 - 0.57 = 0.43