For an ideal binary solution of volatile liquids A and B: P_A = x_A * P_A_standard and P_B = x_B * P_B_standard. Total vapour pressure: P_total = x_A * P_A_standard + (1 - x_A) * P_B_standard = P_B_standard + (P_A_standard - P_B_standard) * x_A. This is a straight line in x_A. Vapour composition: y_A = P_A/P_total. The more volatile component (higher P_standard) is enriched in the vapour phase. Raoult's law is the liquid-phase analogue of Dalton's law. It assumes: (i) similar-sized molecules, (ii) similar intermolecular forces (A-A ≈ A-B ≈ B-B). Examples: benzene + toluene, n-hexane + n-heptane, chlorobenzene + bromobenzene.