Clausius-Clapeyron Vapor Pressure Calculator
Calculate vapor pressure at a new temperature using the Clausius-Clapeyron equation.
Find how vapor pressure changes with temperature for any liquid.
How the Clausius-Clapeyron Equation Is Used
The Clausius-Clapeyron equation relates the vapor pressure of a liquid to temperature, governed by the enthalpy of vaporization. It’s used to predict boiling points at different pressures and to measure vaporization enthalpies.
Clausius-Clapeyron Equation:
ln(P2/P1) = −(ΔH_vap / R) × (1/T2 − 1/T1)
Where:
- P1, P2 = vapor pressures at temperatures T1 and T2 (same units)
- ΔH_vap = molar enthalpy of vaporization (J/mol)
- R = gas constant = 8.314 J/mol·K
- T1, T2 = temperatures in Kelvin
Worked Example — Water at High Altitude: Water boils at 100°C (373 K) at 1 atm (101,325 Pa). What is the boiling point at 0.75 atm (Denver altitude)?
Given: ΔH_vap(water) = 40,700 J/mol
- ln(0.75/1.0) = −(40,700/8.314) × (1/T2 − 1/373)
- −0.2877 = −4,895 × (1/T2 − 0.002681)
- 1/T2 = 0.002681 + 0.0000588 = 0.002740
- T2 = 1/0.002740 = 365 K = 91.8°C
Water boils about 8°C lower in Denver — matching the altitude boiling point formula result.
ΔH_vap Reference Values:
- Water: 40,700 J/mol (44,000 at 25°C)
- Ethanol: 38,600 J/mol
- Acetone: 31,300 J/mol
- Benzene: 30,800 J/mol
Applications: distillation column design, vacuum evaporation in food processing, pressure cooking optimization, pharmaceutical lyophilization (freeze-drying).
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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