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Vapour Pressure (Raoult Law) Calculator

Work out vapour pressure (raoult law) instantly with clear inputs, formula shown and shareable results.

About Vapour Pressure (Raoult Law) Calculator

Raoult's law states that the vapour pressure above an ideal solution is the pure solvent's vapour pressure scaled by the solvent's mole fraction. A non-volatile solute contributes no vapour of its own, so it simply dilutes the solvent at the surface. Water at 25 C (23.8 mmHg) with 1 mol of solute per 10 mol of water drops to 21.6 mmHg, a lowering of 2.16 mmHg.

Formulas

Raoult's law

P(solution) = x(solvent) x P0(solvent)

Lowering

dP = P0 - P = x(solute) x P0

Worked example

With the default values below, Vapour Pressure (Raoult Law) Calculator returns Vapour Pressure Above The Solution of 21.636. Change any input above and every figure here is recalculated in your browser.

Inputs used

Vapour pressure of the pure solventminimum 0 mmHg
23.8 mmHg
Moles of solventminimum 0 mol
10 mol
Moles of non-volatile soluteminimum 0 mol
1 mol

Results

Vapour Pressure Above The Solution
21.636 mmHg
Solvent Mole Fraction
0.9091
Lowering Of Vapour Pressure
2.164 mmHg
Solute Mole Fraction
0.0909

Step by step

  1. 1Values used
  2. 2Raoult's law
  3. 3Lowering
  4. 4Vapour Pressure Above The Solution= 21.636 mmHg
  5. 5Solvent Mole Fraction= 0.9091
  6. 6Lowering Of Vapour Pressure= 2.164 mmHg
  7. 7Solute Mole Fraction= 0.0909

Assumptions and limits

  • The result is derived only from the 3 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 4 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

Why does the solute lower the vapour pressure?

Solute particles occupy part of the surface, so fewer solvent molecules can escape per second while condensation is unchanged. Equilibrium settles at a lower pressure.

What if the solute is volatile too?

Then each component contributes its own partial pressure by Raoult's law and the total is their sum. This calculator assumes a non-volatile solute.

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