Raoult's Law Calculator

Raoult's law states that the vapor pressure of a solvent above an ideal solution equals the mole fraction of the solvent times the vapor pressure of the pure solvent: P = Xsolvent x P0solvent. For a mixture of two volatile components, it extends to Ptotal = XA x PA0 + XB x PB0, the sum of each component's partial pressure.

Vapor pressure lowering is the decrease in a solvent's vapor pressure caused by dissolving a non-volatile solute in it. Because some solvent molecules at the surface are replaced by solute molecules, fewer solvent molecules can escape into the vapor phase, so P = Xsolvent x P0solvent is always less than P0solvent whenever solute is present.

Multiply the mole fraction of the solvent by the pure solvent's vapor pressure: P = Xsolvent x P0solvent. For example, a solution with solvent mole fraction 0.9 and pure solvent vapor pressure 23.76 mmHg (water at 25 degrees C) gives P = 0.9 x 23.76 = 21.38 mmHg, a lowering of 2.38 mmHg from the pure solvent.

The single-solute mode assumes the solute is non-volatile, so only the solvent contributes vapor pressure, and the calculation gives vapor pressure lowering. The two-component mode assumes both liquids are volatile, so each contributes a partial pressure proportional to its own mole fraction and pure vapor pressure, and the total is the sum of both partial pressures.

A P-x diagram plots vapor pressure against liquid-phase composition for a two-component mixture. For an ideal solution following Raoult's law, the partial pressure of each component is a straight line from 0 to its pure vapor pressure, and the total pressure is also a straight line connecting the two pure-component vapor pressures.

Because each component's partial pressure depends on both its mole fraction and its own pure vapor pressure, the more volatile component (higher P0) contributes proportionally more to the vapor than its liquid mole fraction would suggest. Dalton's law gives the vapor mole fraction as yA = PA / Ptotal, which is greater than XA whenever component A is more volatile than B.

Raoult's law assumes an ideal solution, meaning intermolecular forces between different components are essentially the same as forces between molecules of the same component, so mixing causes no net change in those interactions. Real mixtures with strong specific interactions, such as hydrogen bonding differences, deviate positively or negatively from Raoult's law predictions.

A positive deviation occurs when the components interact more weakly with each other than with themselves, raising the actual vapor pressure above the Raoult's law prediction, as in ethanol and water mixtures. A negative deviation occurs when unlike molecules attract more strongly, such as through hydrogen bonding, lowering vapor pressure below the ideal prediction, as in chloroform and acetone.

Distillation relies on the fact that, under Raoult's law, the vapor above a mixture is enriched in the more volatile component compared with the liquid. Repeatedly vaporizing and condensing this vapor-enriched fraction, as in fractional distillation, progressively separates the mixture into components with different pure vapor pressures.

Yes. Vapor pressure lowering depends only on the mole fraction (the number) of solute particles dissolved, not on the chemical identity of the solute, as long as the solution is ideal and the solute is non-volatile and does not dissociate. This makes it one of the four classic colligative properties, alongside boiling point elevation, freezing point depression, and osmotic pressure.

Raoult's law works with any consistent pressure unit, since mole fractions are dimensionless. Common choices are mmHg, kPa, atm, or torr; this calculator accepts any unit as long as the pure vapor pressure inputs are entered in the unit you want the result reported in.