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.
Vapor Pressure
The Clausius-Clapeyron equation describes how vapor pressure changes with temperature.
Formula:
ln(P₂/P₁) = (ΔH_vap / R) × (1/T₁ - 1/T₂)
Or equivalently:
P₂ = P₁ × exp[(ΔH_vap / R) × (1/T₁ - 1/T₂)]
Where:
- P₁, P₂ = vapor pressures at temperatures T₁ and T₂
- ΔH_vap = enthalpy of vaporization (J/mol)
- R = 8.314 J/mol·K (gas constant)
- T₁, T₂ = absolute temperatures (Kelvin)
Common ΔH_vap values at the normal boiling point:
| Liquid | bp (°C) | ΔH_vap (kJ/mol) |
|---|---|---|
| Water | 100°C | 40.65 |
| Ethanol | 78.4°C | 38.56 |
| Benzene | 80.1°C | 30.72 |
| Methanol | 64.7°C | 35.21 |
| Acetone | 56.1°C | 31.27 |
| Diethyl ether | 34.6°C | 26.52 |
Normal boiling point: By definition, a liquid boils when its vapor pressure equals external pressure (1 atm = 760 mmHg). At high altitude (lower pressure), liquids boil at lower temperatures. At the top of Mt. Everest (pressure ≈ 253 mmHg), water boils at about 70°C.
Applications:
- Cooking at altitude
- Chemical distillation
- Pressure cookers (raise boiling point to 120°C at 2 atm)
- Understanding weather (water vapor in atmosphere)