Last updated: July 3, 2026
Vapor Pressure Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
This vapor pressure calculator uses the integrated Clausius-Clapeyron equation to estimate P2 from a known pressure P1, two kelvin temperatures, and ΔHvap in J/mol. It also computes ideal Raoult's-law solution vapor pressure from solvent mole fraction and pure-solvent vapor pressure.
Vapor pressure at a new temperature is P one times e to the negative delta H vaporization over R times one over T two minus one over T one. Use kelvin temperatures and delta H in joules per mole.
Key Takeaways
- Vapor pressure at a new temperature is estimated with P2 = P1 × exp[-(ΔHvap/R)(1/T2 − 1/T1)].
- Use kelvin for T1 and T2 and J/mol for ΔHvap.
- The output pressure keeps the same unit as the known pressure P1.
- For a solution with a nonvolatile solute, Raoult's law gives P_solution = x_solvent × P°_solvent.
- The constant-ΔHvap approximation is best over modest temperature intervals.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
P2 = P1 × exp[-(ΔHvap/R)(1/T2 − 1/T1)]; P_solution = x_solvent × P°_solvent
Where:
- P1=Known vapor pressure at reference temperature(kPa, mmHg, atm, or consistent pressure unit)
- P2=Vapor pressure at target temperature(same as P1)
- T1=Reference absolute temperature(K)
- T2=Target absolute temperature(K)
- ΔHvap=Molar enthalpy of vaporization(J/mol)
- R=Universal gas constant(8.314 J mol⁻¹ K⁻¹)
- x_solvent=Solvent mole fraction for Raoult's-law lowering(dimensionless)
- P°_solvent=Pure-solvent vapor pressure at the same temperature(same pressure unit)
Worked Examples
Water cooled from boiling point to 80 °C
Estimate water vapor pressure at 353.15 K using 101.325 kPa at 373.15 K and ΔHvap = 40660 J/mol.
- 1Calculate the reciprocal-temperature change: 1/353.15 − 1/373.15 ≈ 0.0001517 K⁻¹.
- 2Compute the exponent: −(40660/8.314) × 0.0001517 ≈ −0.742.
- 3Multiply P1 by the exponential factor: 101.325 × exp(−0.742) ≈ 48.25 kPa.
Warmer liquid with a 35 kJ/mol enthalpy
A liquid has vapor pressure 20 kPa at 300 K. Estimate its vapor pressure at 330 K.
- 1Use P2 = P1 × exp[-(ΔHvap/R)(1/T2 − 1/T1)].
- 2The reciprocal-temperature term is negative because T2 is higher than T1.
- 3The exponential factor is greater than 1, so vapor pressure rises to about 71.62 kPa.
Raoult's-law vapor-pressure lowering
A solution has solvent mole fraction 0.80 and the pure solvent pressure is 100 kPa.
- 1For the solution context, apply Raoult's law: P_solution = x_solvent × P°_solvent.
- 2P_solution = 0.80 × 100 kPa = 80 kPa.
- 3The lowering is 100 − 80 = 20 kPa.
Introduction
Vapor pressure is the equilibrium pressure exerted by molecules escaping from a liquid into its vapor above the liquid. This calculator estimates a pure liquid's vapor pressure at a new temperature using the integrated Clausius-Clapeyron equation, then also shows ideal-solution lowering by Raoult's law. It is useful for boiling, evaporation, distillation, and thermodynamics problems. Compare solution lowering with the Raoult's law calculator, temperature sensitivity with the activation energy calculator, and reference data from the NIST Chemistry WebBook or IUPAC Gold Book.
Clausius-Clapeyron vapor pressure formula
For a narrow temperature range where ΔHvap is approximately constant, the integrated Clausius-Clapeyron equation is ln(P2/P1) = −(ΔHvap/R)(1/T2 − 1/T1). Rearranging gives P2 = P1 × exp[−(ΔHvap/R)(1/T2 − 1/T1)]. The pressure unit cancels in the ratio, so P2 is returned in the same unit as P1.
P1 and P2 may be kPa, mmHg, bar, or atm, as long as both use the same unit.
T1 and T2 must be absolute temperatures in kelvin.
ΔHvap must be entered in J/mol to match R = 8.314 J mol⁻¹ K⁻¹.
The equation predicts higher vapor pressure at higher temperature for positive ΔHvap.
How to calculate vapor pressure step by step
Start with a known vapor pressure P1 at a known temperature T1. Enter the target temperature T2 and the molar enthalpy of vaporization. The calculator computes the reciprocal-temperature difference, multiplies by −ΔHvap/R, exponentiates, and multiplies by P1. If your temperatures are in Celsius, convert them with T(K) = T(°C) + 273.15 before using the tool.
For high-precision work over a wide temperature range, use Antoine or Wagner correlations fitted to experimental data instead of a single ΔHvap.
Solution vapor pressure from Raoult's law
When a nonvolatile solute is dissolved in a solvent, the solvent vapor pressure is lowered because fewer solvent molecules occupy the liquid surface. The ideal relation is P_solution = x_solvent × P°_solvent. Enter x_solvent and P°_solvent to compute this secondary output. For more mixture detail, use the partial pressure calculator together with Raoult's law in vapor-liquid equilibrium problems.
Mole fraction is dimensionless and must be between 0 and 1; do not enter mass percent directly.
Common reference values
Vapor pressure changes steeply with temperature, especially near the boiling point. These approximate values help check whether a result is plausible.
| Substance or condition | Approximate value | Use |
|---|---|---|
| Water at 100 °C | 101.325 kPa | Normal boiling reference |
| Water at 25 °C | 3.17 kPa | Room-temperature evaporation |
| Ethanol at 20 °C | 5.95 kPa | Volatile solvent estimate |
| Pure solvent mole fraction | x = 1 | No Raoult lowering |
| x_solvent = 0.80 | P_solution = 80% of P° | Ideal nonvolatile-solute case |
Assumptions and limitations
The Clausius-Clapeyron form used here assumes the vapor behaves ideally, the liquid molar volume is small compared with vapor volume, and ΔHvap is constant across the selected temperature interval. Real substances have temperature-dependent ΔHvap and may require empirical vapor-pressure equations for engineering design. Nonideal solutions may also deviate from Raoult's law because activities differ from mole fractions.
Avoid extrapolating far beyond the known data point.
Use consistent units and kelvin temperatures.
Expect larger error near the critical point or over very broad temperature ranges.
Use activity coefficients for electrolytes or strongly interacting mixtures.
Applications in chemistry and engineering
Vapor pressure controls boiling, evaporation rates, distillation feasibility, solvent selection, humidity calibration, and storage safety. It is connected to boiling-point shifts in the boiling point at altitude calculator and to kinetic temperature dependence through the Arrhenius equation calculator. Authoritative experimental vapor-pressure data are available from NIST and thermodynamics textbooks.
A quick sanity check: if T2 is lower than T1, P2 should be lower than P1 for normal liquids.
Quick Reference Card
Vapor Pressure — Quick Reference
Quick reference • Vapor Pressure Calculator
P2 = P1 exp[-(ΔHvap/R)(1/T2 − 1/T1)]; P_solution = x_solventP°Valid range: Positive pressures, T1 > 0 K, T2 > 0 K, ΔHvap > 0; best over modest temperature ranges away from the critical point
Common Values
⚠ Watch Out
- •Do not enter Celsius temperatures directly; convert to kelvin.
- •Convert ΔHvap from kJ/mol to J/mol before calculating.
- •Use one pressure unit consistently; the result is in the same unit as P1.
- •Avoid broad extrapolations with a constant ΔHvap.
- •Raoult's law assumes ideal solution behavior and mole fractions, not mass fractions.
Pro Tips
- →Check direction: heating should raise vapor pressure, cooling should lower it.
- →Use measured vapor-pressure data near your target temperature when available.
- →For solutions, calculate mole fraction from moles before applying Raoult's law.
- →Report the assumed ΔHvap and temperature interval with the result.
- →Use fitted correlations for design calculations or regulatory solvent data.
FAQs
What equation does this vapor pressure calculator use?
It uses the integrated Clausius-Clapeyron equation: P2 = P1 × exp[-(ΔHvap/R)(1/T2 − 1/T1)], with R = 8.314 J mol⁻¹ K⁻¹ and temperatures in kelvin.
Can I enter pressure in kPa, mmHg, bar, or atm?
Yes. Because the equation uses the ratio P2/P1, any pressure unit works as long as P1 and the reported P2 use the same unit.
Why must temperature be in kelvin?
The formula uses reciprocal absolute temperature. Celsius or Fahrenheit values are not absolute and will give physically meaningless exponent values.
What unit should ΔHvap use?
Enter ΔHvap in J/mol. If your source gives kJ/mol, multiply by 1000 before entering it so the units match R = 8.314 J mol⁻¹ K⁻¹.
How does Raoult's law fit into vapor pressure?
Raoult's law estimates ideal-solution vapor pressure as P_solution = x_solvent × P°_solvent. It is useful for nonvolatile solutes and ideal mixtures.
When is Clausius-Clapeyron not accurate enough?
Accuracy decreases over wide temperature intervals, near critical conditions, or when ΔHvap changes strongly. Use fitted Antoine, Wagner, or reference-data correlations for engineering design.