Raoult's Law (Page 2)

In cases of solutions where there is more than one component to be considered, Raoult's law permits us to calculate a component's vapor pressure mathematically:

In this equation, X is the mole fraction, Po is the vapor pressure of the pure solvent and pA is the vapor pressure contributed to the total vapor pressure by the A component. This is illustrated in the diagram at right for a mixture of two soluble liquids. Notice for the component A (red color) that the vapor pressure increases linearly as the mole fraction of A increases. The same is true for component B but on this scale the mole fraction increases from right to left.

By clicking on the horizontal axis in the diagram above you can examine the individual and total vapor pressures of the two component mixture. Note that the total vapor pressure is the combination of the vapor pressures for components A and B. Also note that when the mole fraction of either component approaches 1.0, the total vapor pressure is equal to the vapor pressure of that pure component.

Dalton's Law:

The composition of the vapor phase is given by Dalton's Law that says the mole fraction of any one component is the ratio of its vapor pressure to the total vapor pressure. Mathematically, Dalton's law is stated as:

By combining Dalton's law and Raoult's law it can be shown that the vapor will always be richer in the more volatile component than the liquid phase. Boiling point diagrams, like the one at right, are often used to illustrate differences in liquid and vapor phases.