Vapor Pressure Calculator
Compute vapor pressure at a temperature or find the temperature for a target vapor pressure using the Clausius–Clapeyron relation or the Antoine equation.
Calculation Steps
Practical Guide
Vapor Pressure Calculator: Clausius–Clapeyron & Antoine Made Simple
This guide shows how to use the Vapor Pressure Calculator correctly, choose the right equation, and interpret results for chemistry, mechanical, and process-engineering work. You’ll see what inputs matter most, how units interact with constants, and two worked examples you can mirror in your own problems.
Quick Start
- 1 Decide what you need: vapor pressure at a temperature or temperature at a target vapor pressure. Set Solve For accordingly.
- 2 Pick a method in Mode: Clausius–Clapeyron if you have a reference point and \(\Delta H_{vap}\), or Antoine if you have constants \(A, B, C\).
- 3 Enter the “given” variables only. The calculator automatically hides and disables the row for the unknown.
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4
Choose units carefully:
- For Clausius–Clapeyron, any pressure/temperature units are fine because the calculator converts internally to SI.
- For Antoine, your pressure unit must match the constants (often mmHg or bar) and temperature is assumed in °C.
- 5 Confirm the temperature range makes sense for your constants or enthalpy assumption.
- 6 Read the main result, then use Quick Stats to check ratios, logarithms, and estimated 1-atm boiling point.
- 7 If your result seems off by orders of magnitude, recheck units and log base (natural log vs. \(\log_{10}\)).
Tip: Antoine constants are substance-specific and temperature-range-specific. Always verify the range (e.g., “valid 1–100°C”) in the data source you pulled them from.
Assumption alert: Clausius–Clapeyron here assumes \(\Delta H_{vap}\) is constant over the temperature span. That’s usually fine near the reference temperature, but accuracy drops far away.
Choosing Your Method
Method A — Clausius–Clapeyron (integrated form)
Use this when you know a reliable reference vapor pressure \(P_1\) at temperature \(T_1\), and you have (or can estimate) the enthalpy of vaporization \(\Delta H_{vap}\).
- Works with a single reference point.
- Units are flexible; the calculator converts to SI internally.
- Great for engineering estimates across moderate temperature spans.
- Assumes \(\Delta H_{vap}\) is constant over the range.
- Less accurate near the critical point or far from \(T_1\).
- Needs a trustworthy \(P_1, T_1\) pair.
Method B — Antoine Equation
Use this when you have Antoine constants \(A, B, C\) for your substance and a temperature within their validity range. The Antoine form is empirical and often highly accurate within its band.
- Very accurate in its fitted temperature range.
- No need for \(\Delta H_{vap}\) or a reference point.
- Commonly tabulated for industrial fluids.
- Pressure unit must match the constants (mmHg, bar, kPa, etc.).
- Temperature expected in °C in most tables.
- Extrapolation outside the range can be wildly wrong.
If you’re doing a quick feasibility check or you only have one trusted data point, start with Clausius–Clapeyron. If you’re doing a detailed design and you have constants from a reputable source in the correct range, Antoine is usually better.
What Moves the Number the Most
Vapor pressure is exponentially sensitive to temperature. A small \(5–10^\circ C\) shift can change \(P\) by 2× or more for many liquids.
In Clausius–Clapeyron mode, any error in the reference pair propagates directly to the result. Use data close to your target temperature when possible.
Higher \(\Delta H_{vap}\) means vapor pressure rises more slowly with temperature. If you only have a value at the normal boiling point, expect more uncertainty away from that point.
Constants are fitted over a specific range and unit system. Mixing constants from one source with units from another is the #1 cause of bad Antoine results.
Antoine uses \(\log_{10}\), Clausius–Clapeyron uses natural log (\(\ln\)). Swapping these accidentally produces order-of-magnitude errors.
Near critical temperature, both methods degrade. If you’re near \(T_c\), use a more advanced EOS or tabulated data.
Worked Examples
Example 1 — Find vapor pressure using Clausius–Clapeyron
- Substance: water (engineering estimate)
- Reference: \(T_1 = 25^\circ C\), \(P_1 = 23.8\ \text{mmHg}\)
- Enthalpy: \(\Delta H_{vap} = 40.7\ \text{kJ/mol}\)
- Target: \(T_2 = 60^\circ C\)
- Solve For: \(P_2\)
This is close to published saturation vapor pressure for water around \(60^\circ C\) (about 19–20 kPa), with the difference reflecting the constant-\(\Delta H_{vap}\) approximation. In the calculator, you can test sensitivity by nudging \(\Delta H_{vap}\) or using a reference point closer to 60°C.
Example 2 — Find temperature using Antoine constants
- Substance: water (Antoine fit)
- Constants: \(A = 8.07131,\ B = 1730.63,\ C = 233.426\)
- Valid range: roughly 1–100°C (source-dependent)
- Target pressure: \(P = 120\ \text{mmHg}\)
- Solve For: \(T_2\)
Antoine equation: \[ \log_{10}(P)=A-\frac{B}{C+T} \] Rearranged: \[ T = \frac{B}{A-\log_{10}(P)} – C \]
So a vapor pressure of 120 mmHg corresponds to about \(55^\circ C\) for water within this Antoine range. If you switch the pressure unit away from mmHg without changing constants, your result will no longer be meaningful.
Common Layouts & Variations
Vapor pressure problems show up in different engineering contexts. The table below summarizes typical setups, which method is most appropriate, and common gotchas.
| Use case | Typical inputs | Preferred method | Notes / pitfalls |
|---|---|---|---|
| Boiling point at given pressure | \(P_2\), constants or \(P_1,T_1,\Delta H_{vap}\) | Antoine if constants valid; CC for estimate | Be sure pressure unit matches constants; boiling point often defined at 1 atm. |
| Evaporation rate / drying models | \(T_2\) and saturation \(P_2\) | CC near ambient; Antoine for accurate saturation curve | Saturation pressure is an input to mass-transfer correlations. |
| Vacuum distillation design | \(T\) range, vapor pressures | Antoine (multiple ranges) | Use constants for the correct temperature band (often multiple Antoine sets). |
| Refrigerant property checks | \(P\)-\(T\) points | Neither (use EOS / refprop) | Many refrigerants are non-ideal; Antoine may exist but check validity carefully. |
| Environmental / pollutant volatility | \(P_1,T_1,\Delta H_{vap}\) or Antoine constants | CC for screening; Antoine for reporting | Regulations often cite vapor pressure at 20–25°C. |
- Confirm your substance data source and temperature range.
- Check whether constants are for mmHg, bar, or kPa.
- Stay within ~±30–50°C of \(T_1\) for CC when possible.
- Use absolute temperature (K) for CC, not °C.
- Use \(\log_{10}\) for Antoine; \(\ln\) for CC.
- If pressure is very low, expect larger uncertainty.
Specs, Logistics & Sanity Checks
Typical Data Sources
Antoine tables (CRC, NIST WebBook, DIPPR), property packages, or journal fits. For CC, reference points come from saturation tables or measurement at a known temperature.
How to Sanity-Check Results
- At higher temperature, vapor pressure should increase monotonically.
- For water, expect ~3 kPa at 25°C and ~20 kPa at 60°C as a rough mental check.
- Boiling point at 1 atm should be close to known values (if within range).
When Not to Trust These Equations
- Near critical temperature or for strongly associating fluids.
- Across very large temperature spans with CC.
- When constants are extrapolated outside their fit range.
If you’re using this in design, treat outputs as property estimates unless validated against a database or EOS. Many workflows use Antoine/CC for quick selection, then a full thermophysical package for final sizing.