Raoult's Law Calculator

Find the vapor pressure lowering of a solution.

How to use

  1. Enter your values in the fields above.
  2. Press Calculate to see your result instantly.
  3. Use the Share button to copy a link to your result.

About this calculator

Raoult's law states that the vapor pressure of a solvent above an ideal solution equals the vapor pressure of the pure solvent multiplied by its mole fraction in the solution: P = x·P₀. Because adding a solute always lowers the solvent's mole fraction below 1, dissolving something in a liquid always lowers its vapor pressure — one of the four classic colligative properties, alongside boiling-point elevation, freezing-point depression, and osmotic pressure.

Chemists and chemical engineers use Raoult's law to predict vapor-liquid equilibrium in distillation columns, to model how antifreeze or dissolved salts change a solution's evaporation behavior, and as the baseline "ideal" case against which real (non-ideal) mixtures with positive or negative deviations are measured. Enter the solvent's mole fraction and its pure vapor pressure to find the vapor pressure above the solution.

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