Vapour Pressure of Liquid Solutions and Raoult's Law

When two volatile liquids, A and B, are mixed to form a solution, both components contribute molecules to the vapour above the liquid, and the composition of that vapour is governed by Raoult's law. Raoult's law states that, for a solution of two (or more) volatile liquids, the partial vapour pressure of each component in the vapour phase is directly proportional to its mole fraction in the liquid solution, with the pure liquid's own vapour pressure as the proportionality constant:

Here $p_A^{\circ}$ and $p_B^{\circ}$ are the vapour pressures of pure A and pure B at the same temperature, and $x_A$ and $x_B$ are their respective mole fractions in the liquid solution ($x_A + x_B = 1$). By Dalton's law of partial pressures, the total vapour pressure of the solution is simply the sum of the two partial pressures:

This total vapour pressure varies linearly with composition between the two pure-component vapour pressures, $p_A^{\circ}$ and $p_B^{\circ}$, whenever the solution genuinely obeys Raoult's law across the whole composition range (an ideal solution, discussed in the next section). …