Clausius-Clapeyron Vapor Pressure Calculator

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).