Relate vapour pressure to temperature with the Clausius–Clapeyron equation: from P₁ at T₁ and a second temperature T₂, solve for P₂ or for the enthalpy of vaporisation.
Open in the full calculator → — with every other tool, the AI tutor and PNG export.
From the article Clausius–Clapeyron Equation — Vapour Pressure and Boiling Point.
Question: Water has ΔHvap ≈ 40.7 kJ mol⁻¹ and a vapour pressure of 101.3 kPa at its normal boiling point, 373.15 K. Estimate its vapour pressure at 353.15 K (80 °C).
Prediction first: lower temperature → lower vapour pressure. The answer must come out below 101.3 kPa.
Step 1 — ΔH in joules: ΔHvap/R = 40 700 / 8.314 = 4895.4
Step 2 — reciprocal temperatures:
1/T₂ = 1/353.15 = 2.831658 × 10⁻³
1/T₁ = 1/373.15 = 2.679887 × 10⁻³
(1/T₂ − 1/T₁) = 1.51771 × 10⁻⁴
Step 3 — apply the equation:
ln(P₂/P₁) = −4895.4 × 1.51771 × 10⁻⁴ = −0.74297
Step 4 — exponentiate:
P₂/P₁ = e⁻⁰·⁷⁴²⁹⁷ = 0.4757
P₂ = 101.3 × 0.4757 = 48.19 kPa
P₂ ≈ 48.2 kPa
Honest note: the measured vapour pressure of water at 80 °C is close to 47.4 kPa. Our estimate is about 2% high, and the reason is worth knowing: we treated ΔHvap as constant between 80 °C and 100 °C, and it is not quite. A 2% error over a 20 K range is exactly the accuracy this equation is designed to give.
Worked in full in Clausius–Clapeyron Equation — Vapour Pressure and Boiling Point.
This is one of the tools in the Chemistry Calculator Suite, a free calculator built for IIT-JAM, CSIR-NET, GATE and CUET (Chemical Science) preparation. Nothing is behind a login and no result is stored on a server — the arithmetic runs in your own browser.
The Clausius–Clapeyron calculator above is the live tool — type your values and press its button; the answer appears in the same box. The link under it opens the full suite with this tool already selected, next to the others.