Vapor Pressure Calculator from Heat of Vaporization
Use the Clausius-Clapeyron equation to estimate vapor pressure at a new temperature using enthalpy of vaporization and one known pressure point.
How to Calculate Vapor Pressure Given Heat of Vaporization
If you know a liquid’s heat of vaporization and one reference vapor pressure at a known temperature, you can estimate vapor pressure at another temperature with high practical accuracy using the integrated Clausius-Clapeyron equation. This is one of the most important relationships in thermodynamics, chemical engineering, environmental modeling, and process safety.
Vapor pressure controls evaporation rates, distillation behavior, solvent losses, headspace concentration in storage tanks, and boil-off risk under heating. In atmospheric science and HVAC, vapor pressure is directly connected to humidity, condensation, and phase equilibrium. In short, when temperature changes, vapor pressure changes nonlinearly, and the heat of vaporization tells you how sensitive that change is.
The Core Equation
The two-point Clausius-Clapeyron form used in this calculator is:
ln(P2/P1) = -ΔHvap/R × (1/T2 – 1/T1)
- P1: known vapor pressure at temperature T1
- P2: unknown vapor pressure at target temperature T2
- ΔHvap: molar heat of vaporization (J/mol)
- R: universal gas constant = 8.314462618 J/(mol·K)
- T1, T2: absolute temperatures in kelvin
Rearranged for direct calculation: P2 = P1 × exp[-ΔHvap/R × (1/T2 – 1/T1)].
Why This Method Works
The Clausius-Clapeyron equation comes from combining equilibrium thermodynamics with the idealized temperature dependence of phase change. It assumes ΔHvap is approximately constant over the selected temperature range. That assumption is usually good for moderate ranges and often excellent for quick design calculations. Accuracy can decrease near the critical point, across very broad temperature intervals, or for strongly non-ideal fluids.
For engineering and laboratory use, this method is valued because it needs only one known pressure-temperature point plus a heat of vaporization. If your reference point is reliable and units are handled correctly, results are often close to tabulated values.
Step-by-Step Procedure
- Collect input values: ΔHvap, P1, T1, and T2.
- Convert ΔHvap to J/mol if needed.
- Convert T1 and T2 to K (kelvin).
- Use a consistent pressure basis for P1 and P2 during calculation (Pa is common internally).
- Evaluate the exponential expression and convert P2 into the desired output unit.
- Sanity-check the direction: if T2 is lower than T1, P2 should normally be lower.
Reference Data Table: Water Vapor Pressure vs Temperature
The table below gives representative saturation vapor pressures for pure water. These values are widely reported in thermodynamic references and are useful as checkpoints when validating calculator outputs.
| Temperature (°C) | Vapor Pressure (kPa) | Vapor Pressure (mmHg) |
|---|---|---|
| 0 | 0.611 | 4.58 |
| 20 | 2.338 | 17.54 |
| 40 | 7.385 | 55.4 |
| 60 | 19.946 | 149.6 |
| 80 | 47.373 | 355.1 |
| 100 | 101.325 | 760.0 |
Notice the strong curvature: pressure does not increase linearly with temperature. This is exactly why exponential models like Clausius-Clapeyron are so useful.
Comparison Table: Heat of Vaporization for Common Liquids
Different fluids respond very differently to heating because ΔHvap varies significantly by substance. Liquids with larger ΔHvap generally show slower vapor pressure increase per degree over moderate ranges, all else equal.
| Substance | Approx. ΔHvap at Normal Boiling Point (kJ/mol) | Normal Boiling Point (°C) |
|---|---|---|
| Water | 40.7 | 100.0 |
| Ethanol | 38.6 | 78.4 |
| Acetone | 29.1 | 56.1 |
| Benzene | 30.8 | 80.1 |
| Ammonia | 23.3 | -33.3 |
Worked Example
Suppose you know that water has a vapor pressure of 101.325 kPa at 100°C and you want an estimate at 25°C using ΔHvap = 40.65 kJ/mol.
- T1 = 100°C = 373.15 K
- T2 = 25°C = 298.15 K
- ΔHvap = 40.65 kJ/mol = 40650 J/mol
- P1 = 101.325 kPa
Plugging into Clausius-Clapeyron yields P2 near 3.8 kPa. The true saturation vapor pressure of water at 25°C is about 3.17 kPa, so this quick two-point estimate is in a reasonable range given the constant-ΔHvap assumption.
This demonstrates a key point: the method is powerful and fast, but reference-grade work may require temperature-dependent correlations (for example, Antoine constants over specific ranges) or direct database values.
When to Use Clausius-Clapeyron vs Other Models
Use Clausius-Clapeyron when:
- You have one trusted reference point and ΔHvap.
- You need a fast engineering estimate.
- The temperature span is moderate and away from the critical region.
Use other correlations when:
- You need high precision across a wide range.
- The fluid is non-ideal or near phase boundaries where assumptions weaken.
- You have access to fitted constants (Antoine, Wagner, DIPPR-type equations).
Common Errors and How to Avoid Them
- Using Celsius directly in 1/T terms: always convert to kelvin first.
- Ignoring ΔHvap unit conversions: kJ/mol must become J/mol for the gas constant in SI.
- Mixing pressure units: keep internal units consistent and convert only at the output stage.
- Large extrapolation: errors grow when T2 is far from T1.
- Assuming this is exact: it is an approximation, not a universal fit.
Practical Applications
In process design, vapor pressure informs condenser loading, vent sizing, and solvent recovery. In pharmaceuticals and specialty chemicals, it affects drying rates and storage stability. In environmental controls, it impacts emissions estimation and volatilization behavior. In meteorology, vapor pressure and saturation pressure underlie relative humidity calculations and dew point relationships.
A fast, transparent calculator helps teams move quickly in early-stage analysis while preserving physical rigor. It also improves communication between lab scientists and plant engineers by tying measurements directly to thermodynamic relationships.
Data Quality and Authoritative Sources
For production-level calculations, source your thermophysical data from recognized references. The following resources are widely used for validated properties and educational foundations:
- NIST Chemistry WebBook (.gov)
- NIST Chemistry WebBook Program Page (.gov)
- Purdue University Clausius-Clapeyron Guide (.edu)
Final Takeaway
To calculate vapor pressure given heat of vaporization, use the integrated Clausius-Clapeyron equation with disciplined units and a reliable reference state. For quick and credible estimates, this approach is excellent. For critical design or regulatory decisions, pair your estimate with high-quality property databases and range-appropriate correlations. Used correctly, this method gives you a thermodynamically grounded, high-value prediction in seconds.