Vapor Pressure Calculator

About the Vapor Pressure Calculator

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its liquid (or solid) phase at a given temperature. It is a fundamental physical property that governs boiling, evaporation, distillation, and the behavior of volatile substances in mixtures.

The Antoine equation is the most widely used empirical correlation for vapor pressure as a function of temperature: log₁₀(P) = A − B / (C + T). For thermodynamic derivations, the Clausius-Clapeyron equation relates vapor pressure to enthalpy of vaporization: ln(P₂/P₁) = −ΔHvap/R × (1/T₂ − 1/T₁).

This calculator supports both equations with built-in Antoine parameters for 30+ common substances, estimates boiling points at non-standard pressures, computes vapor pressures of ideal mixtures via Raoult's Law, and converts between mmHg, kPa, atm, bar, and psi.

When This Page Helps

Vapor pressure calculations are central to chemical engineering (distillation design), environmental science (VOC evaporation), pharmaceutical stability studies, weather prediction (humidity), and industrial safety (flash point estimation). Consistent pressure modeling improves process planning and reduces errors when translating formulas between different unit systems and mixed regulatory documentation standards during design, reporting, and routine operations.

How to Use the Inputs

1. Select a substance from the preset list or enter custom Antoine parameters.
2. Choose the calculation mode: Antoine equation, Clausius-Clapeyron, or Raoult's Law.
3. Enter the temperature to calculate vapor pressure, or enter pressure to find boiling point.
4. For Clausius-Clapeyron, provide two P-T data points and the enthalpy of vaporization.
5. For Raoult's Law, enter mole fractions and pure-component vapor pressures.
6. Review vapor pressure in multiple units and the temperature-pressure curve.
7. Check the reference table for Antoine constants of common substances.

Example Calculation

Tips & Best Practices

• Antoine parameters are only valid within their specified temperature range — extrapolation is unreliable.
• For mixtures, positive deviations from Raoult's Law indicate weaker solute-solvent interactions than pure components.
• The Clausius-Clapeyron equation is exact only if ΔHvap is truly constant over the temperature range.
• Convert all temperatures to the unit expected by Antoine parameters (usually °C, sometimes K).
• At the critical temperature, the vapor pressure curve ends — above Tc, no liquid phase exists.
• Compare calculated boiling points against NIST data to validate your Antoine parameters.

Antoine Equation Details

The Antoine equation is a semi-empirical modification of the Clausius-Clapeyron equation. The three parameters (A, B, C) are fitted to experimental data and are valid only within a specified temperature range. The NIST WebBook is the most authoritative source for Antoine parameters.

Different sources may use different forms: some use log₁₀ with T in °C, others use ln with T in K. Always verify the convention before using published parameters.

Vapor-Liquid Equilibrium

For ideal mixtures, Raoult's Law connects vapor and liquid compositions through the relative volatility α = P°₁/P°₂. Distillation separations rely on α > 1 to enrich the more volatile component in the vapor phase. The larger α, the easier the separation.

Environmental and Safety Significance

Vapor pressure determines how quickly solvents evaporate, contributing to VOC emissions. OSHA uses vapor pressure to classify chemicals for workplace exposure limits. High-vapor-pressure substances (acetone, gasoline) require proper ventilation and ignition source control.

Frequently Asked Questions

• What is the Antoine equation?

  It is an empirical correlation of the form log₁₀(P) = A − B/(C+T) that accurately describes vapor pressure over a limited temperature range. Parameters A, B, C are substance-specific.

• When should I use Clausius-Clapeyron instead of Antoine?

  Use Clausius-Clapeyron when you have thermodynamic data (ΔHvap) and two reference P-T points but not Antoine parameters. It is thermodynamically grounded but assumes constant ΔHvap.

• What is Raoult's Law?

  Raoult's Law states that the partial vapor pressure of each component in an ideal solution equals the product of its mole fraction and its pure-component vapor pressure. It applies to ideal or nearly ideal solutions.

• How does altitude affect boiling point?

  At higher altitudes, atmospheric pressure decreases, lowering the boiling point. At 2000 m, water boils at about 93 °C. This calculator can find the boiling point at any pressure.

• What are typical units for vapor pressure?

  Common units include mmHg (torr), kPa, bar, atm, and psi. 1 atm = 760 mmHg = 101.325 kPa = 1.01325 bar = 14.696 psi.

• Why do substances have different volatilities?

  Volatility depends on intermolecular forces. Weak London dispersion forces (e.g., diethyl ether) give high vapor pressure, while hydrogen bonding (e.g., water) gives lower vapor pressure at the same MW.