ConverterHub

Clausius–Clapeyron Calculator

Find vapor pressure at a new temperature.

How it is calculated

The tool applies the Clausius–Clapeyron equation in the form P2 = P1 × exp(−ΔHvap ÷ R × (1/T2 − 1/T1)). Here P1 is the known vapor pressure at T1, T2 is the new temperature, ΔHvap is the molar enthalpy of vaporization in J/mol and R = 8.314 J/(mol·K). Both temperatures must be absolute, in kelvin, and above zero, as must P1. The result carries the label Pa, so enter P1 in pascals; another pressure unit gives a correct number but the wrong label. The enthalpy is treated as constant between the two temperatures, which is a good approximation only over a modest range.

Worked example

Water boils at 373.15 K where its vapor pressure is 101,325 Pa. What is it at 363.15 K (90 °C), using ΔH = 40,660 J/mol? First ΔH ÷ R = 40,660 ÷ 8.314 ≈ 4,890.5. Next 1/363.15 − 1/373.15 ≈ 0.00007380. Their product is 0.3609, so the factor is exp(−0.3609) ≈ 0.6970. Then P2 = 101,325 × 0.6970 ≈ 70,628.40 Pa, about 70.6 kPa.

Try this example →

Common mistakes

  • Entering temperatures in °C. Typing 100 and 90 gives a wildly wrong result; add 273.15 to get kelvin.
  • Entering the enthalpy in kJ/mol. The equation expects J/mol, so 40.66 kJ/mol must be typed as 40660.
  • Swapping the reference and target values, which turns the factor upside down.
  • Taking ΔH as fixed over a very wide temperature range. The estimate drifts when T1 and T2 are far apart.
  • Reading the result as Pa when P1 was entered in kPa or atm. The output keeps the same unit as P1 although it is labelled Pa.

About this calculator

The Clausius–Clapeyron equation, ln(P₂/P₁) = −ΔHvap/R × (1/T₂ − 1/T₁), links a liquid's vapor pressure at two different temperatures through its molar enthalpy of vaporization. Because vapor pressure rises steeply — and roughly exponentially — with temperature, knowing it at one reference point lets you predict it at another without a new measurement.

Chemists and chemical engineers use it to predict boiling points at non-standard pressures, size distillation columns, and read vapor-pressure charts used in solvent selection; it also explains why water boils at a lower temperature on a mountaintop and shows up in meteorology when relating dew point to humidity. Culinary science even borrows it to explain why recipes need adjusting at altitude.

Was this helpful?

You can pick up to 3 reactions.

Comments (0)

  • Be the first to comment.

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators