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Clausius–Clapeyron Calculator

Find vapor pressure at another temperature.

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.

Worked example

Water boils at 100 °C where the vapor pressure is 101.325 kPa. Its enthalpy of vaporization is about 40.66 kJ/mol. What is the pressure at 80 °C? T1 = 373.15 K and T2 = 353.15 K. The difference 1/353.15 − 1/373.15 is about 0.00015178. Then −40,660 ÷ 8.3145 × 0.00015178 is about −0.7422. The ratio is e−0.7422, about 0.4761, and P2 = 101.325 × 0.4761, giving 48.2374 kPa, with a ratio of 0.4761.

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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%.

Sources

About this calculator

This calculator applies the Clausius–Clapeyron equation to find a substance's vapor pressure at a new temperature from a known reference point: ln(P2/P1) = −(ΔHvap/R)·(1/T2 − 1/T1), where ΔHvap is the molar enthalpy of vaporization and R is the gas constant. It describes how vapor pressure rises steeply with temperature along a liquid's phase boundary.

Chemistry students and chemical/process engineers use it to estimate boiling points at non-standard pressures (such as altitude adjustments or vacuum distillation), and to interpret vapor-pressure data from a reference table or Antoine-equation source.

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Comments (1)

  • Editorial team

    Water makes a good test: from 101.325 at 100 °C with 40.66 kJ/mol, the tool gives 70.63 at 90 °C, a ratio of 0.6971. The equation treats the enthalpy of vaporization as constant, so it works best over a modest temperature span and drifts the further you move from your reference point. Pressure comes back in the unit you entered for P1. Which substance and temperature range are you working with?

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