Raoult’s Law Calculator

Calculate component mole fractions, partial vapor pressures, and total vapor pressure for an ideal volatile liquid mixture.

Ideal-solution vapor pressure

Enter two to five volatile components with positive mole amounts and pure vapor pressures.

Number of Components

Pressure Unit

Moles of Component 1

Pure Vapor Pressure of Component 1 (mmHg (torr))

Moles of Component 2

Pure Vapor Pressure of Component 2 (mmHg (torr))

About Raoult’s law

Raoult’s law relates the vapor pressure above an ideal liquid solution to the composition of that solution. For each volatile component, the partial vapor pressure equals its liquid-phase mole fraction multiplied by the vapor pressure of the pure component at the same temperature. Adding all partial pressures gives the total pressure above the mixture. This calculator performs those steps for two to five components and keeps every pressure in the selected unit. The mole fraction of a component is its number of moles divided by the total moles of all entered components. Mole fractions are dimensionless and add to one. If a solution contains two moles of ethanol and three moles of water, their mole fractions are 0.4 and 0.6. Multiplying each fraction by the corresponding pure vapor pressure produces its partial pressure. With pure pressures of 44.6 and 23.8 mmHg, the contributions are 17.84 and 14.28 mmHg, giving 32.12 mmHg in total. Temperature is not entered directly because the calculator expects pure-component vapor pressures already evaluated at one common temperature. Pure vapor pressure changes strongly with temperature, so values copied from tables or calculated with an Antoine equation must all correspond to the actual system temperature. Pressure units must also be consistent. Selecting kPa or mmHg changes the displayed label but does not convert entered numbers, allowing users to work directly with a uniform source dataset. Raoult’s law is exact for an ideal solution, where interactions between unlike molecules resemble interactions between like molecules. Chemically similar mixtures, such as some hydrocarbon pairs, can approximate this behavior. Real solutions often show positive or negative deviations. Weaker unlike interactions can increase vapor pressure above the ideal prediction, while stronger attractions can decrease it. Activity coefficients are needed when those deviations are important, and some mixtures form azeotropes that simple ideal calculations cannot predict correctly. The method is widely used as a first model for distillation, flash calculations, solvent formulation, and evaporation analysis. A distillation engineer can compare partial pressures to understand which component enriches the vapor. Laboratory users can estimate solvent loss, and students can connect liquid composition with Dalton’s law for the gas phase. The result is an equilibrium estimate rather than a rate of evaporation; airflow, area, heat transfer, and mass-transfer resistance determine how quickly equilibrium is approached. Only volatile liquid components should be represented as ordinary entries. A nonvolatile solute such as salt or sugar contributes effectively no vapor pressure, although it lowers the solvent mole fraction. For rigorous work, include all species when determining liquid mole fractions, use reliable pure vapor pressure data, and assess non-ideality. This calculator is best used for transparent ideal-mixture calculations, classroom checks, and preliminary comparisons before applying an activity-coefficient or equation-of-state model.

Raoult’s law examples

These ideal mixtures demonstrate weighted pure-component vapor pressures.

Mixture	Total vapor pressure	Calculation
2 mol at 44.6 mmHg and 3 mol at 23.8 mmHg	32.12 mmHg	Ethanol-water teaching example with mole fractions 0.4 and 0.6.
1.5 mol at 53.3 kPa and 2.5 mol at 18 kPa	31.2375 kPa	Binary benzene-toluene ideal estimate.
1 mol at 80, 2 mol at 60, and 1.5 mol at 40 mmHg	57.7778 mmHg	Three volatile components weighted by a total of 4.5 moles.
2.2 mol at 38.7 kPa and 1.8 mol at 21.2 kPa	30.825 kPa	Binary acetone-chloroform ideal estimate.

How to use Raoult’s law

Choose the number of volatile components in the liquid mixture.
Select one pressure unit and obtain every pure vapor pressure at the same temperature.
Enter the positive mole amount and pure vapor pressure for each component.
Select Calculate Vapor Pressure to view every mole fraction and partial pressure.
Check whether ideal-solution behavior is reasonable before applying the total pressure.

Raoult’s law FAQ

When does Raoult’s law work best?

It works best for ideal or nearly ideal liquid mixtures whose molecules have similar intermolecular interactions. Strongly associating or dissimilar compounds may require activity coefficients.

Why must pure vapor pressures use the same temperature?

Pure vapor pressure changes with temperature, and equilibrium requires one system temperature. Combining values from different temperatures gives partial pressures that do not describe one physical state.

Does the pressure-unit selector convert values?

No. It labels all entered and calculated pressures in the chosen unit. Enter every pure vapor pressure in that same unit.

Can I include a nonvolatile solute?

A nonvolatile solute can affect solvent mole fractions but contributes essentially no partial vapor pressure. For a careful calculation, account for its moles while treating its pure vapor pressure contribution appropriately.

What causes deviations from Raoult’s law?

Differences between like and unlike molecular attractions cause non-ideal behavior. Positive deviations raise vapor pressure, while negative deviations lower it relative to the ideal prediction.