Clausius-Clapeyron Calculator

This Clausius-Clapeyron calculator works out how the vapour pressure of a liquid changes between two temperatures, using the two-point form of the Clausius-Clapeyron equation that links vapour pressure to the enthalpy of vaporisation. You enter a known vapour pressure P1 at a reference temperature T1, the enthalpy of vaporisation of the liquid in joules per mole, and the second temperature T2 you want to check. From those four figures the calculator returns the predicted vapour pressure P2 at T2 in the same units as P1, plus the pressure ratio P2/P1 and the T2 value you entered, so you can see at a glance how much the vapour pressure has risen or fallen. This is the calculation chemistry students and lab workers reach for to estimate how a liquid's boiling point shifts with altitude or pressure, or to predict the vapour pressure of a solvent at a temperature you have not measured directly. Both temperatures must be entered in kelvin, not degrees Celsius, and the enthalpy of vaporisation in joules per mole, otherwise the result will be wrong. The equation assumes the enthalpy of vaporisation stays constant over the temperature range and that the vapour behaves as an ideal gas, which holds reasonably well over modest temperature spans but becomes less accurate over wide ranges, so treat results away from well-tabulated conditions as a useful estimate rather than an exact figure.

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Pressure P1

Temperature T1 (K)

Enthalpy of vap. (J/mol)

Temperature T2 (K)

Pressure ratio P2/P1

T2 (K)

The formula

The two-point Clausius-Clapeyron equation is the natural log of (P2 / P1) = minus the enthalpy of vaporisation over R, times (1/T2 minus 1/T1), with R = 8.314 J per mol per K. So P2 = P1 times the exponential of that. Temperatures are in kelvin.

Worked example

For water near boiling (P1 = 101325 Pa at T1 = 373.15 K, enthalpy 40700 J/mol), at T2 = 363.15 K the vapour pressure drops to about 70000 Pa, since the pressure ratio is about 0.70. Enter the defaults to confirm.

Frequently asked questions

What does the Clausius-Clapeyron equation do?

It relates a liquid’s vapour pressure at two temperatures using its enthalpy of vaporisation.

What units do I use?

Temperatures in kelvin, enthalpy of vaporisation in joules per mole. Pressures can be any consistent unit; the result matches P1.

Why does vapour pressure rise with temperature?

Higher temperature gives molecules more energy to escape the liquid, raising the vapour pressure, which the equation quantifies.

Who this calculator is for

This calculator is for chemistry students, teachers and lab workers.

What this calculator assumes

You enter values in the units shown.
Standard conditions unless stated.
Results are rounded for display.

Formula and sources

Related calculators

Molarity Calculator: concentration in mol/L.
pH Calculator: acidity from H+ concentration.
Ideal Gas Law Calculator: PV = nRT.

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