Raoult's Law Calculator
Use Raoult's law to estimate the vapor pressure of an ideal solution from the solvent mole fraction and the pure-solvent vapor pressure. The calculator also helps you work backward to a missing mole fraction or solvent amount.
Result
--
Quick Answer: Raoult's law for an ideal solution is p = x x p0, where x is the solvent mole fraction and p0 is the vapor pressure of the pure solvent. If you know the solution pressure and pure-solvent pressure, the mole fraction is simply x = p / p0.
How to Apply Raoult's Law
- Choose what to solve for: You can calculate solution pressure, mole fraction, pure-solvent pressure, or the missing solvent or solute amount.
- Enter mole amounts or pressures: For mole fraction from composition, use solvent and solute moles. For direct pressure work, enter p and p0.
- Keep the solution ideal: Raoult's law is most appropriate for ideal or near-ideal solutions.
- Interpret the result: A lower solvent mole fraction means a lower vapor pressure when the pure-solvent pressure is fixed.
Raoult's Law Calculator Formula
p = x x p0 ; x = n_solvent / (n_solvent + n_solute)
| Variable | Meaning | Unit |
|---|---|---|
| p | Vapor pressure of the solution | pressure |
| x | Mole fraction of the solvent | unitless |
| p0 | Vapor pressure of the pure solvent | pressure |
| n_solvent | Moles of solvent | mol |
| n_solute | Moles of solute | mol |
Worked Examples
Example 1 - Find solution vapor pressure
- Solute moles: 1.0 mol
- Solvent moles: 4.0 mol
- Pure-solvent pressure: 23.8 mmHg
Result: x = 0.80 and p = 19.04 mmHg
With 80 percent of the particles belonging to the solvent, the solution pressure is 80 percent of the pure-solvent pressure.
Example 2 - Find mole fraction from pressures
- Solution pressure: 3.0 mmHg
- Pure-solvent pressure: 5.0 mmHg
Result: x = 0.60
The solution retains 60 percent of the pure-solvent vapor pressure, so the solvent mole fraction is 0.60.
Example 3 - Find solvent moles
- Solute moles: 2.0 mol
- Mole fraction: 0.75
Result: n_solvent = 6.0 mol
Rearranging the mole-fraction formula shows that more solvent is needed as the target fraction approaches 1.
Raoult's Law Interpretation Table
| Range | Meaning | Action |
|---|---|---|
| x less than 0.25 | The solution is solute-rich, so vapor pressure is much lower than the pure solvent. | Expect a large lowering of vapor pressure if the ideal assumption is reasonable. |
| x from 0.25 to 0.75 | The solvent is still a major component but no longer dominant. | Check whether the ideal-solution assumption is still appropriate for your mixture. |
| x greater than 0.75 | The solution is solvent-rich. | The solution pressure stays closer to the pure-solvent pressure. |
Frequently Asked Questions
For an ideal solution, the partial vapor pressure of the solvent equals the pure-solvent vapor pressure multiplied by the solvent mole fraction.
If you know the solution vapor pressure and the pure-solvent vapor pressure, divide p by p0 to get the solvent mole fraction.
A nonvolatile solute reduces the fraction of solvent particles at the surface, so fewer solvent molecules escape into the vapor phase.
No. It is exact only for ideal solutions and can become inaccurate when intermolecular interactions cause strong positive or negative deviations.
Yes. If you know the solution vapor pressure and solvent mole fraction, the calculator can rearrange the equation to find p0.
Note: Results assume ideal-solution behavior. Real mixtures can deviate from Raoult's law depending on intermolecular interactions.
References
Last reviewed: March 14, 2026