State Raoult's law for a solution of two volatile liquids. Derive the expression for the total vapour pressure and draw the graph of vapour pressure against mole fraction for an ideal solution.

1. 1 · State the law. For a solution of volatile liquids, the partial vapour pressure of each component is directly proportional to its mole fraction.
2. 2 · Write it for each component. p₁ = p₁° x₁ and p₂ = p₂° x₂, where p° is the vapour pressure of the pure liquid.
3. 3 · Use Dalton's law. p_total = p₁ + p₂ = p₁° x₁ + p₂° x₂.
4. 4 · Substitute x₁ = 1 − x₂. p_total = p₁° + (p₂° − p₁°) x₂, a straight line between p₁° and p₂°.
5. 5 · Draw the graph. Composition on the x-axis (x₁ from 1 to 0, x₂ from 0 to 1) and vapour pressure on the y-axis. Three straight lines: p₁ falling from p₁° to 0, p₂ rising from 0 to p₂°, and p_total joining p₁° to p₂°. Label all three.
6. 6 · Add the vapour composition. y₁ = p₁ ÷ p_total; the vapour is richer in the more volatile component.
7. 7 · Close in one line. So in an ideal solution, the total vapour pressure lies between the two pure vapour pressures and changes linearly with composition.

Scoring tip: Draw the three lines with a ruler and label p₁°, p₂°, p₁, p₂ and p_total. An unlabelled graph loses its mark.