Solutions · Raoult's Law

Vapour Pressure of Solutions

In an ideal solution each volatile liquid contributes a partial pressure equal to its pure vapour pressure times its mole fraction — so the total pressure is a straight line between the two pure liquids. Real mixtures deviate when A–B attractions differ from A–A and B–B. Slide the composition and watch the pressures build.

■ pA = xA·p°A ■ pB = xB·p°B ■ ptotal x-axis: mole fraction of A

Composition

Mole fraction xA 0.50
Pure V.P. p°A (more volatile) 80 kPa
Pure V.P. p°B 30 kPa

Behaviour

Ideal solution — A–B attractions match A–A and B–B, so ΔHmix ≈ 0 and each partial pressure follows Raoult's law exactly.

Ideal solution
at xA = 0.50
40.0
pA (kPa)
15.0
pB (kPa)
55.0
ptotal (kPa)
0.73
yA in vapour
Vapour is richer in the more volatile liquid

Try this

Non-volatile solute: drag p°B to its minimum — B barely evaporates, so ptotal ≈ pA and adding solute B only lowers the solvent's vapour pressure. That relative lowering is the root of every colligative property. Then switch to Positive deviation and note ptotal bulging above the ideal line (like ethanol + acetone).