P_total = χ_A × P°_A + χ_B × P°_B
Raoult's law states that the partial vapor pressure of each component in an ideal solution equals the product of its mole fraction and its pure component vapor pressure: Pᵢ = χᵢP°ᵢ. The total vapor pressure is P_total = Σ Pᵢ. An ideal solution forms when intermolecular forces between A-B are similar to A-A and B-B (e.g., benzene-toluene). Positive deviations (P > Raoult) occur when A-B interactions are weaker (e.g., ethanol-hexane); negative deviations (P < Raoult) when A-B interactions are stronger (e.g., acetone-chloroform). Raoult's law is the basis of distillation: the more volatile component is enriched in the vapor (y_A > x_A if P°_A > P°_B). An azeotrope occurs where vapor and liquid have the same composition.