What is Raoult's Law?

Raoult's Law states that the partial vapor pressure of a solvent above a solution equals the mole fraction of that solvent (x_solvent) multiplied by the vapor pressure of the pure solvent (P_pure): P_solution = x_solvent × P_pure. It applies to ideal solutions where solute–solvent interactions are similar in strength to solvent–solvent interactions.

What is the mole fraction of the solvent?

The mole fraction of the solvent (x_solvent) is the ratio of the number of moles of solvent to the total number of moles of all species in the solution. It is a dimensionless number between 0 and 1. For a pure solvent x = 1; adding solute reduces x below 1.

When does Raoult's Law not apply?

Raoult's Law describes ideal solutions. Non-ideal solutions — where solute–solvent attractions differ significantly from solvent–solvent attractions — show positive or negative deviations. Strong solute–solvent interactions (e.g., hydrogen bonding between ethanol and water) cause negative deviations; weaker interactions cause positive deviations.

What is vapor pressure lowering?

Vapor pressure lowering is the decrease in a solvent's vapor pressure caused by dissolving a non-volatile solute. By Raoult's Law, ΔP = x_solute × P_pure. Because x_solvent < 1 for any solution, P_solution is always less than P_pure for an ideal solution with a non-volatile solute.

Can I use this for volatile solutes?

For solutions of two volatile liquids, each component follows Raoult's Law independently: P_A = x_A × P_pure_A and P_B = x_B × P_pure_B. The total vapor pressure is the sum of both partial pressures. This calculator handles one component at a time; run it twice (once per component) and add the results for the total pressure.

What pressure units are supported?

The calculator accepts pressure in atm, kPa, mmHg (Torr), and Pa. Both vapor pressure fields use the same selected unit, and results are reported in that same unit so no manual conversion is needed.

Raoult's Law Calculator

Enter any two of the three variables — solution vapor pressure, mole fraction, and pure solvent vapor pressure — to solve for the third using Raoult's Law.

Enter Known Values

Select which variable to solve for, then fill in the other two.

P_solution = x_solvent × P_pure

Solve for

Solution Vapor Pressure (P_solution) Mole Fraction of Solvent (x_solvent) Pure Solvent Vapor Pressure (P_pure)

Pressure Unit

Solution Vapor Pressure (P_solution)

mmHg

Mole Fraction of Solvent (x_solvent)

(dimensionless)

Pure Solvent Vapor Pressure (P_pure)

mmHg

Fill in two values and click Calculate

Result

All Variables Summary

Symbol	Description	Value

Step-by-Step Solution

Vapor Pressure Lowering

ΔP = P_pure − P_solution

Relative Lowering (x_solute)

Summary

Raoult's Law states that the vapor pressure of a solution equals the mole fraction of the solvent multiplied by the vapor pressure of the pure solvent: P_solution = x_solvent × P_pure. This calculator lets you solve for any one of the three variables — solution vapor pressure, solvent mole fraction, or pure solvent vapor pressure — given the other two. All calculations are performed client-side with no data sent to any server.

How it works

Select which variable you want to solve for: solution vapor pressure, mole fraction of solvent, or pure solvent vapor pressure.
Enter values for the two known variables in the input fields.
Choose the pressure unit (atm, kPa, mmHg, or Pa) for vapor pressure fields.
Click Calculate — the tool applies P_solution = x_solvent × P_pure and solves for the unknown.
Review the result, the full variable summary, and the step-by-step algebraic working.
Click Reset to clear all fields and start a new calculation.

Use cases

Calculate the vapor pressure lowering of a saltwater or sugar solution.
Find the mole fraction of solvent from measured vapor pressure data.
Determine the pure solvent vapor pressure from solution data.
Understand colligative properties in general chemistry coursework.
Verify lab distillation or evaporation rate estimates.
Prepare for AP Chemistry or university physical chemistry exams.
Model ideal solution behavior for mixtures of similar liquids.
Predict boiling point elevation and osmotic pressure indirectly.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu