Raoult's Law Calculator

About the Raoult's Law Calculator

Raoult's law states that the vapor pressure of a component in an ideal solution equals the product of its mole fraction in the liquid phase and its pure-component vapor pressure: Pᵢ = χᵢ × P°ᵢ. This fundamental relationship connects solution composition to physical behavior and is a cornerstone of chemical thermodynamics, distillation design, and colligative property calculations.

For a solution with a non-volatile solute (like salt or sugar in water), only the solvent contributes to vapor pressure. The total vapor pressure drops by an amount equal to the solute mole fraction times the pure solvent vapor pressure — this is vapor pressure lowering, one of the four colligative properties. For binary mixtures of two volatile liquids (like benzene and toluene), both components contribute and the total pressure is the sum of the partial pressures.

This calculator computes the partial vapor pressures, total vapor pressure, vapor pressure lowering, and vapor-phase composition for any ideal binary solution. Enter the mole fractions (or calculate them from moles) and the pure-component vapor pressures, and the calculator produces a complete Raoult's law analysis with a composition-vs-pressure table and visual comparison.

When This Page Helps

Raoult's law calculations require careful handling of mole fractions and vapor-phase composition. It gives a complete analysis and a composition table that would take considerable time to construct by hand.

How to Use the Inputs

1. Enter the mole fraction of the solute (or enter moles of solute and solvent).
2. Enter the pure vapor pressure of the solvent in mmHg.
3. If the solute is volatile, enter its pure vapor pressure too.
4. Review partial pressures, total pressure, and vapor pressure lowering.
5. Check the composition table for how pressure varies across the full mole fraction range.
6. Use presets for common ideal solution systems.

Example Calculation

Tips & Best Practices

• Water's vapor pressure at 25°C is 23.76 mmHg (3.17 kPa). Temperature significantly affects P°.
• For non-volatile solutes (sugar, NaCl), set P°_solute to 0.
• Real solutions with strong positive deviations can form minimum-boiling azeotropes (e.g., ethanol-water at 95.6%).
• Negative deviations produce maximum-boiling azeotropes (e.g., HCl-water).
• Use Antoine equation to find P° at any temperature: log P° = A − B/(C + T).

Ideal vs. Non-Ideal Solutions

Raoult's law is exact only for ideal solutions. In practice, deviations are common. Positive deviations (P_total > Raoult prediction) occur when A-B interactions are weaker than A-A and B-B interactions — the molecules "want to escape" more readily. Negative deviations (P_total < Raoult prediction) occur when A-B interactions are stronger, as in acetone-chloroform where hydrogen bonding between the two species stabilizes the liquid phase.

Applications in Distillation

The vapor-liquid equilibrium (VLE) predicted by Raoult's law is the foundation of distillation design. The relative volatility α = P°A/P°B determines how easily two components can be separated. When α is close to 1 (similar vapor pressures), many theoretical stages are needed. When α >> 1, separation is straightforward. Azeotropes occur where vapor and liquid compositions are equal, making simple distillation insufficient.

Colligative Property Connection

Vapor pressure lowering (ΔP = χ_solute × P°) is one of four colligative properties, all stemming from the same thermodynamic origin: the solute reduces the chemical potential of the solvent. From this single effect, boiling point elevation, freezing point depression, and osmotic pressure all follow through the Clausius-Clapeyron and van't Hoff equations.

Frequently Asked Questions

• What is Raoult's law?

  Raoult's law states that the partial vapor pressure of each component of an ideal solution equals its mole fraction multiplied by the pure component vapor pressure. This keeps planning practical and lowers the chance of preventable errors.

• When does Raoult's law fail?

  Raoult's law applies strictly to ideal solutions where intermolecular forces between all species are equal. Real solutions show positive deviations (weaker A-B forces, like ethanol-hexane) or negative deviations (stronger A-B forces, like acetone-chloroform).

• What makes a solution "ideal"?

  An ideal solution has ΔH_mix = 0 and ΔV_mix = 0. The intermolecular forces between unlike molecules are the same as between like molecules. Mixtures of similar molecules (benzene/toluene, hexane/heptane) approximate ideal behavior.

• How does vapor pressure lowering relate to boiling point elevation?

  Lowering the vapor pressure means the solution must be heated to a higher temperature to reach atmospheric pressure and boil. Both are colligative properties proportional to solute concentration.

• What is the difference between Raoult's law and Henry's law?

  Raoult's law describes the solvent (or major component) at high mole fractions. Henry's law describes the solute at low mole fractions, using an empirical constant KH instead of the pure vapor pressure.

• Can Raoult's law predict distillation?

  Yes. For ideal mixtures, Raoult's law combined with Dalton's law gives the vapor-liquid equilibrium (VLE) data needed to design distillation columns. The relative volatility is α = P°A / P°B.