Question text What is the mole fraction of benzene in a benzene-toluene solution whose vapor pressure is 51 torr at 20°C?

Mole fraction for benzene is 0.683

Explanation:

We need to determine the colligative property about vapour pressure which formula is:

ΔP = P° . Xm

where P° is vapour pressure of pure solvent.

ΔP = Vapor pressure of pure solvent - Vapor pressure of solution

In this case, our solvent is benzene and the solute is toluene. For this colligative property vapour pressure of solution is lower than pure solvent.

P° of benzene at 20°C is 74.7 Torr

Xm is the value for the mole fraction of solute.

Let's replace data: 74.7 Torr - 51 Torr = 74.7 Torr . Xm

Xm = 0.317 → That's the mole fraction for toluene.

Then, mole fraction for benzene will be: 1 - 0.317 → 0.683

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