Raoult's LawJEE Main

Vapour pressure of an ideal solution — interactive Chemistry simulation for IIT-JEE.

Concept

In an ideal solution each component contributes vapour in proportion to how much of it is present: $P_i = x_iP_i^°$ . Total pressure is a straight line between the two pure pressures. The vapour is always richer in the more volatile component — the principle behind distillation.

Key formula

P_A = x_AP_A^°, \qquad P_{total} = x_AP_A^° + x_BP_B^°, \qquad y_A = \frac{P_A}{P_{total}}

Derivation

Escaping tendency of A is proportional to its surface fraction, i.e. its mole fraction. Dalton then gives the vapour composition $y_A = P_A/P_{total}$ .

Since $y_A/y_B = (x_A/x_B)(P_A^°/P_B^°)$ , whichever component has the larger $P^°$ enriches the vapour. Repeated vaporize-condense cycles = fractional distillation.

Scenarios to explore

Raoult's Law — Ideal-solution vapour pressure & why distillation works.

Real-world applications

Fractional distillation of crude oil & liquor.
Relative lowering of vapour pressure → molar-mass determination.
Azeotropes = real solutions deviating from Raoult.

JEE exam tips

Positive deviation (A–B weaker than A–A/B–B): P > ideal, e.g. ethanol + acetone; negative: acetone + chloroform.
Non-volatile solute: ΔP/P° = x_solute — the first colligative property.
y_A > x_A exactly when P_A° > P_total — quick vapour-enrichment check.

Common mistakes

Using mass fraction instead of mole fraction.
Equating vapour composition with liquid composition.
Applying Raoult to non-volatile solutes both ways (only the solvent term survives).

Exam traps to avoid

Minimum-boiling azeotropes come from POSITIVE deviation.
The P_total–x line is straight only when plotted against LIQUID composition, not vapour.