Clausius-Clapeyron Calculator
This Clausius-Clapeyron calculator predicts how the vapour pressure of a liquid changes with temperature. The Clausius-Clapeyron equation links the vapour pressures at two temperatures through the enthalpy of vaporisation. Enter a known pressure and temperature, the enthalpy of vaporisation, and a second temperature, and the calculator returns the vapour pressure at that second temperature. Temperatures must be in kelvin and the enthalpy in joules per mole. This is how you estimate boiling points at altitude or the vapour pressure of a solvent at a new temperature. The formula, a worked example and the assumptions are below.
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\u2019s 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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