The vapor pressure of pure acetone (CH3COCH3) at 30 oC is 240 torr. The vapor pressure of pure water (H2O) at 30 oC is 40 torr. If the total vapor pressure of a mixture of water and acetone at 30 oC is 190 torr, what is the mol fraction of acetone?

Mole fraction for the acetone is 0.792

Explanation:

This is a typical problem of lowering vapor pressure. The formula is this:

P° - P' = P° . Xm

where P° → Vapor pressure of pure solvent

P' → Vapor pressure of solution

Xm → Mole fraction for the solute.

In this case:

Solvent → acetone

Solute →  water.

Let's write the data given in the formula:

240 Torr- 190 Torr = 240Torr . Xm

(240 Torr- 190 Torr) / 240 Torr = Xm → 0.208

This is the mole fraction for water, so.

Sum of mole fractions  = 1

Mole fraction of acetone → 1 - 0.208 =

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