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: $y_A$ = $\frac{P_A}{P_total}$. 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.