Raoult's law
Summary
Raoult's law says that in an ideal solution, the partial vapor pressure of a component equals its mole fraction in the liquid times its pure vapor pressure. A nonvolatile solute lowers the solvent's vapor pressure. Real solutions deviate when A–B attractions differ from A–A and B–B.
Detail
Meaning: Raoult's law says the partial vapor pressure of a component above a solution equals its mole fraction in the liquid times its pure-liquid vapor pressure. A nonvolatile solute lowers the solvent's mole fraction, so fewer solvent molecules escape into the gas phase. This underlies vapor pressure lowering and boiling point elevation. Ideal solutions obey it because A–B attractions equal A–A and B–B attractions. Weaker A–B attractions give a higher vapor pressure than predicted, and stronger ones give a lower vapor pressure.
Equation: PA = XA P°A, where PA is the partial vapor pressure of A above the solution (Pa), XA is the mole fraction of A in the liquid (unitless), and P°A is the vapor pressure of pure A at the same temperature (Pa). The lowering is ΔP = Xsolute P°solvent (Pa).
On the MCAT: A passage gives a pure vapor pressure plus moles of solute and solvent. You compute the mole fraction and then the solution's vapor pressure, or you rank solutions by vapor pressure or boiling point. For salts, count the dissolved particles, since each ion lowers the solvent's mole fraction.
Common confusion: Using the solute's mole fraction in PA = XA P°A gives the drop in vapor pressure, not the vapor pressure itself. Use the solvent's mole fraction to get the solution's vapor pressure.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Physical Chemistry (Atkins)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.