Last updated: July 3, 2026
Raoult's Law Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
Raoult's law predicts ideal-solution vapor pressure from liquid mole fraction. For a nonvolatile solute, P_solution = x_solventP°_solvent and vapor-pressure lowering is ΔP = x_soluteP°_solvent. For an ideal binary volatile mixture, P_total = x_AP°_A + x_BP°_B.
Raoult's law says the vapor pressure of a solution component equals its liquid mole fraction multiplied by the vapor pressure of the pure component at the same temperature.
Key Takeaways
- Raoult's law says solution vapor pressure equals solvent mole fraction times pure solvent vapor pressure.
- For a nonvolatile solute, vapor-pressure lowering is ΔP = x_soluteP°_solvent.
- Ideal binary mixtures add both volatile contributions: P_total = x_AP°_A + x_BP°_B.
- Mole fraction, not mass percent, is the composition variable used in Raoult's law.
- Use pure-component vapor pressures at the same temperature and in the same pressure unit.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
P_solution = x_solvent × P°_solvent; ΔP = x_solute × P°_solvent; P_total = x_A × P°_A + x_B × P°_B
Where:
- P_solution=Vapor pressure of the solution above the solvent(same as P°)
- x_solvent=Mole fraction of solvent(dimensionless)
- P°_solvent=Vapor pressure of pure solvent(kPa, mmHg, atm, or chosen pressure unit)
- ΔP=Vapor-pressure lowering(same as P°)
- x_solute=Mole fraction of nonvolatile solute(dimensionless)
- P_total=Total vapor pressure of an ideal binary mixture(same as pure vapor pressures)
- x_A, x_B=Liquid-phase mole fractions of volatile components A and B(dimensionless)
- P°_A, P°_B=Pure-component vapor pressures of A and B(same pressure unit)
Worked Examples
Nonvolatile solute in a 100 kPa solvent
A solution has solvent mole fraction 0.90 and the pure solvent vapor pressure is 100 kPa.
- 1Apply Raoult's law: P_solution = x_solvent × P°_solvent.
- 2P_solution = 0.90 × 100 kPa = 90 kPa.
- 3The solute mole fraction is 1 − 0.90 = 0.10, so ΔP = 0.10 × 100 kPa = 10 kPa.
Ideal binary volatile mixture
Component A and B both contribute to the vapor pressure of an ideal liquid mixture.
- 1Calculate A contribution: P_A = 0.4 × 80 = 32 kPa.
- 2Calculate B contribution: P_B = 0.6 × 40 = 24 kPa.
- 3Add the contributions: P_total = 32 + 24 = 56 kPa.
Water-like solvent at 25 °C
Water has a vapor pressure near 23.8 mmHg at 25 °C; a solute lowers it according to the solvent mole fraction.
- 1Use P_solution = x_solvent × P°_solvent.
- 2P_solution = 0.75 × 23.8 mmHg = 17.85 mmHg.
- 3The pressure lowering is (1 − 0.75) × 23.8 = 5.95 mmHg.
Introduction
Raoult's law relates the vapor pressure above an ideal liquid solution to liquid-phase mole fraction. For a nonvolatile solute, the solvent vapor pressure is P_solution = x_solventP°_solvent, so adding solute lowers vapor pressure by ΔP = x_soluteP°_solvent. For ideal binary volatile mixtures, add each component contribution: P_total = x_AP°_A + x_BP°_B. Use this tool with the molarity calculator when preparing solutions by concentration, or compare gas-phase pressure concepts with the partial pressure calculator. The thermodynamic basis is covered in IUPAC solution terminology and LibreTexts colligative properties.
Raoult's law formula
For a solvent with a nonvolatile solute, the partial vapor pressure of the solvent equals its liquid mole fraction times the pure-solvent vapor pressure at the same temperature. The calculator reports both the remaining solvent vapor pressure and the vapor-pressure lowering.
P_solution = x_solvent × P°_solvent.
x_solute = 1 − x_solvent for a binary nonvolatile-solute solution.
ΔP = P°_solvent − P_solution = x_solute × P°_solvent.
Use the same pressure unit throughout; the result keeps that unit.
Ideal binary mixtures
If both liquid components are volatile and the mixture is ideal, each component obeys Raoult's law independently. Component A contributes x_AP°_A, component B contributes x_BP°_B, and the total vapor pressure is their sum. The calculation assumes x_A + x_B is close to 1.
| Quantity | Expression | Meaning |
|---|---|---|
| A contribution | P_A = x_AP°_A | Partial vapor pressure from component A |
| B contribution | P_B = x_BP°_B | Partial vapor pressure from component B |
| Total | P_total = P_A + P_B | Ideal-mixture vapor pressure |
| Check | x_A + x_B ≈ 1 | Complete binary liquid composition |
How to calculate vapor pressure lowering
First identify whether the solute is effectively nonvolatile at the temperature of interest. Then find the solvent mole fraction and the vapor pressure of the pure solvent at that same temperature. Multiplying gives the solution vapor pressure; subtracting from P° gives the lowering.
Convert composition to mole fraction, not mass percent.
Look up P° at the exact temperature of your solution.
Multiply x_solvent by P° for the solvent vapor pressure.
For lowering, multiply x_solute by P° or subtract from pure vapor pressure.
Why mole fraction matters
Raoult's law is based on the fraction of surface molecules that are solvent or component A/B in an ideal liquid. Mole fraction is therefore the natural composition variable. If your data are in mass percent, use the mass percent calculator first, convert mass to moles, and then calculate mole fractions before applying Raoult's law.
Assumptions and limitations
Raoult's law is exact for ideal solutions and a good approximation for many dilute nonelectrolyte solutions. Real mixtures can deviate because molecular interactions differ between unlike and like pairs. Positive deviations raise vapor pressure above the ideal line; negative deviations lower it. Activity coefficients are needed for accurate nonideal work.
Strong electrolytes, associating liquids, azeotropes, and highly concentrated mixtures often need activity-based models instead of simple mole fractions.
Applications in solution chemistry
Vapor-pressure lowering is one of the classic colligative properties, alongside boiling-point elevation, freezing-point depression, and osmotic pressure. Raoult's law also underpins distillation diagrams, solvent evaporation estimates, humidity corrections over solutions, and first-pass vapor-liquid equilibrium calculations.
For a volatile binary mixture, plot total pressure against x_A to visualize ideal vapor-liquid behavior and distillation driving force.
Quick Reference Card
Raoult's Law — Quick Reference
Quick reference • Raoult's Law Calculator
P_solution = x_solventP°; ΔP = x_soluteP°; P_total = x_AP°_A + x_BP°_BValid range: Mole fractions 0–1; best for ideal solutions, dilute nonelectrolytes, and similar volatile liquids
Common Values
⚠ Watch Out
- •Do not use mass fraction directly; convert to mole fraction first.
- •Pure vapor pressures must be measured or looked up at the same temperature.
- •Strongly nonideal mixtures may need activity coefficients.
- •For binary volatile mixtures, x_A + x_B should be close to 1.
- •Use one consistent pressure unit for all pure vapor pressures.
Pro Tips
- →For dilute nonvolatile solutes, x_solute is often small, so ΔP is small but measurable.
- →Check limiting cases: x_solvent = 1 gives pure-solvent vapor pressure; x_solvent = 0 gives zero solvent vapor pressure.
- →Pair this calculation with osmotic pressure, freezing-point depression, or boiling-point elevation for colligative-property problems.
- →In distillation, combine Raoult's law with Dalton's law to estimate vapor composition.
FAQs
What does Raoult's law calculate?
Raoult's law calculates the vapor pressure contribution of a solution component by multiplying its liquid mole fraction by its pure-component vapor pressure at the same temperature.
How do I calculate vapor-pressure lowering?
For a nonvolatile solute, find x_solute = 1 − x_solvent and multiply by the pure solvent vapor pressure: ΔP = x_soluteP°_solvent.
Can pressure be entered in kPa, mmHg, or atm?
Yes. Raoult's law is proportional, so any pressure unit works as long as every pure vapor pressure uses the same unit. The output is in that same unit.
What is an ideal binary mixture?
It is a two-component liquid mixture whose molecules interact similarly enough that each volatile component follows P_i = x_iP°_i and the total vapor pressure is the sum of the two contributions.
Why do real solutions deviate from Raoult's law?
Real solutions deviate when unlike molecular interactions differ from pure-component interactions. Activity coefficients, association, dissociation, or azeotrope formation can make vapor pressure higher or lower than the ideal prediction.
Is Raoult's law the same as Dalton's law?
No. Raoult's law links liquid composition to vapor pressure above a solution; Dalton's law adds gas-phase partial pressures. They are often combined in vapor-liquid equilibrium calculations.