Raoult's Law Calculator
Calculate the vapour pressure of a solution from mole fraction and pure component pressures.
About Raoult's Law Calculator
Each component contributes its pure vapour pressure scaled by its mole fraction. The vapour is always richer in the more volatile component than the liquid, which is exactly what makes distillation work. The difference between liquid and vapour composition is the driving force of every separation column, and Raoult’s law is the ideal-case starting point.
Formula
Raoult's Law
P_total = x_A P°_A + x_B P°_B for an ideal solution
- x
- Mole fraction in the liquid phase
- P°
- Vapour pressure of the pure component
Worked example
With the default values below, Raoult's Law Calculator returns Total Vapour Pressure of 4,052.100. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Mole fraction of solventaccepts 0–1
- 0.9
- Vapour pressure of the pure solventminimum 0 Pa
- 3,169 Pa
- Mole fraction of volatile soluteaccepts 0–1
- 0.1
- Vapour pressure of the pure soluteminimum 0 Pa
- 12,000 Pa
Results
- Total Vapour Pressure
- 4,052.100 Pa
- Partial Pressure of the Solvent
- 2,852.100 Pa
- Partial Pressure of the Solute
- 1,200.000 Pa
- Lowering Against the Pure Solvent
- 316.900 Pa
- Mole Fraction of Solute in the Vapour
- 0.29614
Step by step
- 1Values used
- 2Raoult's Law
- 3Total Vapour Pressure= 4,052.100 Pa
- 4Partial Pressure of the Solvent= 2,852.100 Pa
- 5Partial Pressure of the Solute= 1,200.000 Pa
- 6Lowering Against the Pure Solvent= 316.900 Pa
- 7Mole Fraction of Solute in the Vapour= 0.29614
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Raoult's Law calculated?
P_total = x_A P°_A + x_B P°_B for an ideal solution Each component contributes its pure vapour pressure scaled by its mole fraction. The vapour is always richer in the more volatile component than the liquid, which is exactly what makes distillation work.
Why does Raoult's Law matter?
The difference between liquid and vapour composition is the driving force of every separation column, and Raoult’s law is the ideal-case starting point.
What values do I need to enter?
This calculator takes 4 inputs: Mole fraction of solvent, Vapour pressure of the pure solvent, Mole fraction of volatile solute, Vapour pressure of the pure solute. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.