Clausius–Clapeyron Calculator

How it is calculated

You enter a reference vapor pressure P1 at temperature T1, the molar enthalpy of vaporization ΔH in kJ/mol, and the target temperature T2. Both temperatures are typed in °C; the tool adds 273.15 to convert to kelvin. It then computes ln(P2 ÷ P1) = −(ΔH × 1000 ÷ R) × (1/T2 − 1/T1) with R = 8.314462618 J/(mol·K), and returns P2 = P1 × eln(P2/P1) along with the ratio P2/P1, both to four decimals. P2 comes out in whatever pressure unit you used for P1. P1 must be positive, and each temperature must be above absolute zero. The equation treats ΔH as constant and the vapor as an ideal gas, so it is an estimate that drifts over wide temperature ranges.

Common mistakes

• Entering temperatures in kelvin. Both fields are in °C, so typing 373 instead of 100 gives a wildly wrong answer.
• Entering the enthalpy in J/mol or kcal/mol. The field is kJ/mol; 40660 would be read as a thousand times too large.
• Mixing pressure units. P2 uses the same unit as P1, so a reference value in mmHg returns mmHg, not kPa.
• Using the result far from the reference point. A constant ΔH is an approximation; for a large gap use measured data or an Antoine equation.
• Reading the ratio as a percentage. A value of 0.4761 means P2 is about 47.6% of P1, not 0.48%.