What is the Clausius-Clapeyron equation?

The Clausius-Clapeyron equation ln(P2/P1) = −(ΔHvap/R)·(1/T2 − 1/T1) relates the vapor pressure of a liquid to temperature. ΔHvap is the molar enthalpy of vaporization, R is the universal gas constant (8.314 J/mol·K), and T1, T2 are absolute temperatures in Kelvin.

Why must temperature be in Kelvin?

The equation uses absolute temperature so the reciprocal 1/T is mathematically valid. Celsius values are automatically converted by adding 273.15 when you select °C as input unit.

What units should I use for ΔHvap?

ΔHvap is most commonly expressed in kJ/mol or J/mol. This calculator accepts both — select the appropriate unit before entering the value, and the conversion is handled internally. For water, ΔHvap ≈ 40.7 kJ/mol at 100 °C.

Can I solve for temperature instead of pressure?

Yes. Switch the solve mode to "Solve for T2". Enter P1, T1, ΔHvap, and the target pressure P2, and the calculator will return the temperature T2 at which that vapor pressure occurs.

What are the assumptions of this equation?

The Clausius-Clapeyron equation assumes ΔHvap is constant over the temperature range, the vapor behaves as an ideal gas, and the molar volume of the liquid is negligible compared to the vapor. These assumptions hold reasonably well for moderate temperature ranges away from the critical point.

What is the normal boiling point?

The normal boiling point is the temperature at which vapor pressure equals 1 atm (101325 Pa). You can find it by setting P2 = 1 atm and solving for T2 given any reference P1/T1 point and ΔHvap.

Clausius-Clapeyron Calculator

Name: Clausius-Clapeyron Calculator
Availability: InStock
Author: Nham Vu

Enter a reference vapor pressure, two temperatures, and enthalpy of vaporization to find the vapor pressure at the new temperature.

Clausius-Clapeyron Inputs

Solve For

Temperature Unit

Pressure Unit

Reference Vapor Pressure P1

Reference Temperature T1

Enthalpy of Vaporization ΔHvap

Target Temperature T2

Enter values on the left and click Calculate

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Summary

Enter a reference vapor pressure, two temperatures, and enthalpy of vaporization to find the vapor pressure at the new temperature.

How it works

Enter the reference vapor pressure P1 and its temperature T1.
Enter the enthalpy of vaporization ΔHvap in kJ/mol or J/mol.
Enter the target temperature T2 (or target pressure P2 to find T2).
Select your preferred temperature unit (K or °C) and pressure unit (Pa, kPa, atm, mmHg, bar).
Click Calculate to get the result along with a step-by-step breakdown.
Click Reset to clear all fields and start a new calculation.

Use cases

Predict the vapor pressure of water or a solvent at a non-standard temperature.
Find the boiling point of a liquid at reduced pressure (e.g., in a vacuum distillation).
Calculate the vapor pressure of refrigerants at different operating temperatures.
Estimate flash points and evaporation behavior in chemical process design.
Verify physical chemistry homework involving phase equilibria.
Determine the altitude at which water boils at a given ambient pressure.
Analyze distillation column conditions in chemical engineering problems.
Prepare for AP Chemistry or university thermodynamics exams.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu