Raoult's Law Vapor Pressure Calculator
Raoult's law predicts how a dissolved solute lowers the vapor pressure of a solvent. For an ideal solution with a non-volatile solute, the vapor pressure equals the mole fraction of the solvent multiplied by the vapor pressure of the pure solvent. This calculator takes moles of solvent and moles of solute, computes the solvent mole fraction, and returns the solution vapor pressure and the vapor pressure lowering. Enter the pure-solvent vapor pressure at your temperature from a reference table.
Raoult's law formula
x_solvent = n_solvent / (n_solvent + n_solute)
x_solute = 1 - x_solvent
P_solution = x_solvent * P_pure
lowering = P_pure - P_solution = x_solute * P_pure
Here n is moles and P_pure is the vapor pressure of the pure solvent at the temperature of interest. The solution vapor pressure carries the same unit you enter for the pure-solvent pressure.
Worked example
- 10 mol solvent plus 1 mol solute gives a solvent mole fraction of 10 / 11 = 0.9091.
- With pure water at 23.76 mmHg (25 Celsius), solution pressure = 0.9091 times 23.76 = 21.60 mmHg.
- The lowering is 23.76 minus 21.60 = 2.16 mmHg.
- That equals the solute mole fraction 0.0909 times 23.76 mmHg.
- Use the pure-solvent vapor pressure that matches your temperature.
Raoult's law: frequently asked questions
What is Raoult's law?
Raoult's law states that the partial vapor pressure of each component of an ideal solution equals the vapor pressure of the pure component multiplied by its mole fraction in the mixture. For a single volatile solvent with a non-volatile solute, the solution vapor pressure is the solvent mole fraction times the pure-solvent vapor pressure.
What is a mole fraction?
A mole fraction is the number of moles of one component divided by the total moles of all components in the mixture. It ranges from 0 to 1, and the mole fractions of all components sum to 1. For a solvent with one non-volatile solute, the solvent mole fraction is moles of solvent divided by moles of solvent plus moles of solute.
What is vapor pressure lowering?
Adding a non-volatile solute lowers the solvent mole fraction below 1, so the solution vapor pressure falls below that of the pure solvent. The drop equals the solute mole fraction times the pure-solvent vapor pressure. This calculator reports both the solution vapor pressure and the amount of lowering.
When does Raoult's law fail?
Raoult's law is exact only for ideal solutions where the components have similar intermolecular forces. Real mixtures show positive or negative deviations when solute-solvent interactions differ from solvent-solvent interactions. It works best for dilute solutions and chemically similar components.
Official sources
- NIST Chemistry WebBook: pure-component vapor pressure data.
- IUPAC Gold Book: Raoult's law and mole fraction definitions.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.