Clausius-Clapeyron Vapor Pressure Calculator

It describes how the vapor pressure of a liquid changes with temperature, using the enthalpy of vaporization. Vapor pressure rises exponentially as temperature increases.

Enter both temperatures in kelvin and the heat of vaporization in kilojoules per mole. The result comes out in the same pressure units you entered for the known pressure.

It goes up, and exponentially. Because more molecules gain enough energy to escape the liquid as it warms, a modest temperature rise can multiply the vapor pressure. In the worked example a 20 K increase raised the pressure from 100 kPa to about 177 kPa.

The two-point integrated form used here assumes the enthalpy of vaporization stays constant across the temperature range, which is only an approximation. In reality ΔH decreases as temperature rises and falls to zero at the critical point, so the estimate drifts over wide temperature spans and near the critical temperature. The equation also treats the vapor as an ideal gas and neglects the liquid's volume. For the best results, keep the two temperatures close together and well below the critical point.