Vapor Pressure Calculator

The Vapor Pressure Calculator computes vapor pressure changes with temperature using the Clausius-Clapeyron equation, or calculates solution vapor pressure using Raoult's law. Vapor pressure is the pressure exerted by vapor molecules in equilibrium with a liquid or solid.

Clausius-Clapeyron Equation

For phase transitions between liquid and gas, the relationship between pressure and temperature is:

$$\ln\frac{P_1}{P_2} = \frac{\Delta H}{R}\left(\frac{1}{T_2} - \frac{1}{T_1}\right)$$

where $\Delta H$ is molar enthalpy of vaporization (J/mol), $R = 8.3145$ J/(mol$\cdot$K), and temperatures must be in Kelvin. For water with $\Delta H_{\mathrm{vap}} = 40660$ J/mol at 280 K and 102325 Pa, the pressure at 263 K is about 33090 Pa.

Raoult's Law

For ideal solutions, vapor pressure is lowered by dissolved solute:

$$p_{\mathrm{solution}} = x_{\mathrm{solvent}} \cdot p^\circ_{\mathrm{solvent}}$$

Dissolving 100 g glucose in 500 g water at 37 °C (pure water $p^\circ = 47.1$ torr) gives $x_{\mathrm{water}} \approx 0.98$ and $p \approx 46.16$ torr.

Frequently Asked Questions

What is enthalpy of vaporization?

Enthalpy of vaporization is the energy required to convert one mole of liquid into gas at constant pressure. For water it is about 40660 J/mol at room temperature.

Why must temperature be in Kelvin?

The Clausius-Clapeyron equation uses absolute temperature because vapor pressure depends on kinetic energy, which scales with Kelvin. Celsius values can be entered and are converted automatically.

When should I use Raoult's law instead?

Use Raoult's law when calculating how a non-volatile solute lowers the vapor pressure of a solvent. Use Clausius-Clapeyron when temperature changes affect pure substance vapor pressure.

How does vapor pressure relate to boiling point?

A liquid boils when its vapor pressure equals the surrounding atmospheric pressure. Lower ambient pressure means a lower boiling temperature.

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Vapor Pressure Calculator