Vapor Pressure Calculator
Vapor pressure at any temperature from boiling point data.
What it does
Vapor pressure climbs exponentially with temperature: water at 50 C pushes only 0.1313 atm but reaches a full atmosphere at its boiling point. The Clausius-Clapeyron relation draws that curve from two anchors, the boiling point and the heat of vaporization.
The tool above takes a liquid with temperature in C or F. It returns vapor pressure in atm, kPa, and torr. The default water at 50 C gives 0.1313 atm (13.3 kPa, 99.8 torr).
Forms match LibreTexts with NIST WebBook data behind them: ln(P1/P2) equals dHvap over R times the reciprocal temperature gap, with R at 8.3145. Water anchors at 373.17 K with 40.7 kJ/mol, ethanol at 351.5 K with 38.56, acetone at 329.3 K with 29.1.
Constant heat of vaporization is the working fiction. Real values drift with temperature, so measured water at 50 C reads near 92.5 torr against 99.8 computed; textbook examples land the same way, 0.6963 here against 0.697 printed.
How to use
- Pick water, ethanol, or acetone.
- Pick C or F to match the problem.
- Type the temperature.
- Read atm first, kPa and torr beside it.
- Treat results as estimates inside a few percent.
- Never run past decomposition temperatures.
Formula
- Tb
- Normal boiling point in kelvin at 1 atm.
- dHvap
- Heat of vaporization near the boiling point.
- R
- Gas constant 8.3145 J/mol/K.
Fahrenheit converts first ((F minus 32) x 5/9) into identical kelvin math. Solid to liquid transitions need the Clapeyron slope form instead.
Worked example
Example: water at 50 C
Metric defaults: water, 50 C.
- Kelvin: 50 + 273.15 = 323.15 K; Tb 373.17 K.
- Gap: 1/323.15 minus 1/373.17 = 0.00041465.
- Log: minus 40700/8.3145 x gap = minus 2.0297.
- Pressure: exp gives 0.1313 atm.
- Convert: 0.1313 x 760 = 99.8 torr; x 101.325 = 13.3 kPa.
- Tool shows 0.1313 atm, 13.3 kPa, 99.8 torr.
Reference table
| Liquid | Tb and dHvap | Check |
|---|---|---|
| Water | 373.17 K, 40.7 kJ/mol | 50 C: 0.1313 atm |
| Water | Same anchors | 90 C: 0.6963 atm |
| Water | Same anchors | 110 C: 1.4073 atm |
| Ethanol | 351.5 K, 38.56 kJ/mol | 40 C: 0.1987 atm |
| Acetone | 329.3 K, 29.1 kJ/mol | Near 56 C boils |
Limits
- dHvap held constant; real drifts apply.
- Pure liquids only; mixtures differ.
- Range minus 50 to 350 C.
- No decomposition or critical point math.
Common mistakes
- Plugging Celsius into the reciprocal gap.
- Using standard dHvap far from boiling.
- Reading torr as kPa.
- Forgetting 1 atm anchors the boiling point.
- Extrapolating to supercritical states.
Try it yourself
Quick checks. Answers sit tucked under each question.
Water at 90 C?
0.6963 atm (textbook prints 0.697).
Water at 110 C?
1.4073 atm (textbook prints 1.409).
500 C input?
Rejected: enter minus 50 to 350 C.
Questions
Default answer?
0.1313 atm (13.3 kPa, 99.8 torr) for water at 50 C.
Why estimate?
Heat of vaporization shifts with temperature; the form holds it fixed.
Measured 92.5 versus 99.8?
Expected gap from constant dHvap; textbooks show the same pattern.
Fahrenheit match?
Yes: 122 F converts to the same 50 C curve.
Ethanol at 40 C?
0.1987 atm (151 torr).
Gas law page versus this?
That relates P, V, n, T of a sample; this draws pressure versus temperature.