Clausius-Clapeyron Calculator
Vapour pressure at a second temperature.
Work out Clausius-Clapeyron. Vapour pressure at a second temperature. Says where the method stops being reliable.
Any pressure unit — the answer comes back in the same one, because only the ratio matters.
Water is 40.66, ethanol 38.56, benzene 30.72.
Assumes the enthalpy of vaporisation stays constant between the two temperatures. Accuracy degrades over wide ranges.
Vapour pressure at the second temperature
361.81
0.4761× the pressure you started from
Vapour pressure rises steeply and non-linearly with temperature, which is why a liquid can go from barely evaporating to boiling over a small range. A liquid boils when its vapour pressure reaches the surrounding pressure — so this equation also tells you the boiling point at altitude. The equation assumes ΔHvap is constant over the range, which it is not: it falls as you approach the critical point. Over a 20–30 °C span the error is small; over a hundred degrees it is not.
How the Clausius-Clapeyron Calculator works
Give a known vapour pressure at one temperature plus the enthalpy of vaporisation, and this returns the vapour pressure at any other temperature. Pressure comes back in whatever unit you entered, because only the ratio matters to the equation.
Also known as: vapour pressure calculator · vapor pressure at temperature calculator · boiling point at altitude calculator · enthalpy of vaporization calculator
Frequently asked questions
What is the Clausius-Clapeyron equation?
ln(P₂/P₁) = −ΔHvap/R × (1/T₂ − 1/T₁). It relates vapour pressure at two temperatures through the enthalpy of vaporisation, with temperatures in kelvin.
Why does water boil at a lower temperature up a mountain?
Because boiling happens when vapour pressure equals the surrounding pressure. Atmospheric pressure at 3,000 m is about 0.7 atm, and water reaches that vapour pressure at roughly 90 °C rather than 100.
Can I use this to find the enthalpy of vaporisation?
Yes — measure vapour pressure at several temperatures, plot ln P against 1/T, and the slope is −ΔHvap/R. That is the standard laboratory method.
How accurate is it?
Good over a 20–30 °C range. It assumes ΔHvap is constant, but ΔHvap actually falls as the temperature approaches the critical point, so error grows over wide spans.
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