Clausius–Clapeyron Calculator (Vapour Pressure and Boiling Point)
Vapour pressure of a liquid at another temperature, and its boiling point at another pressure, from one known point and the enthalpy of vaporisation.
Results
Vapour pressure at the new temperature
32.229
kPa
Boiling point at that pressure
89.76
°C
Ratio to the known pressure
0.3181
The dashed line marks the value you entered.
What this tool does
The Clausius–Clapeyron equation links how fast vapour pressure rises with temperature to the energy needed to evaporate the liquid. It explains why water boils at about 90 °C at 3000 m, where the pressure is near 70 kPa, and why a pressure cooker reaches 120 °C. It assumes the enthalpy stays constant and the vapour behaves as an ideal gas, so it is accurate over a few tens of degrees: water at 70 °C comes out at 32 kPa against the measured 31.2. For wide ranges the Antoine equation does better.
Formula
ln(P₂ ÷ P₁) = −(ΔH_vap ÷ R) × (1 ÷ T₂ − 1 ÷ T₁)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
p1 | Known vapour pressure | kPa |
t1 | At this temperature | °C |
dh | Enthalpy of vaporisation | kJ/mol |
t2 | New temperature | °C |
pt | Pressure to find the boiling point at | kPa |
P2 | Vapour pressure at the new temperature | kPa |
BP | Boiling point at that pressure | °C |
RT | Ratio to the known pressure | — |
Worked example
- Known vapour pressure101.325 kPa
- At this temperature100 °C
- Enthalpy of vaporisation40.65 kJ/mol
- New temperature70 °C
- Pressure to find the boiling point at70 kPa
- Vapour pressure at the new temperature32.229 kPa
- Boiling point at that pressure89.76 °C
- Ratio to the known pressure0.3181
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.