All calculatorsVapor Pressure Calculator
Compute the vapor pressure of a liquid at a specified temperature using the Antoine equation (now supports solving P(T) or T(P)) or the Clausius–Clapeyron equation. Includes a liquid selector and quick picks.
Background
Vapor pressure increases with temperature. The Antoine equation (base-10 logarithm) fits specific liquids over limited temperature ranges. Clausius–Clapeyron provides a thermodynamic link using ΔHvap. Always match units and stay within the valid range.
How to use this calculator
Antoine method
- Enter T (°C or K) and constants A, B, C (make sure they match the units shown). We compute log₁₀ P = A − B/(C + T).
- Choose output units (mmHg, atm, kPa, bar). We convert from mmHg basis.
Clausius–Clapeyron method
- Provide a reference point (T₁, P₁), ΔHvap, and a target temperature T₂. We compute ln(P₂/P₁) = −ΔH/R (1/T₂ − 1/T₁) and convert to your selected units.
- Use T in Kelvin (we auto-convert °C → K), ΔH in J/mol (we accept kJ/mol and convert), R = 8.314 J·mol⁻¹·K⁻¹.
Example Problems & Step-by-Step Solutions
Example 1 (Antoine, water at 25 °C)
A=8.07131, B=1730.63, C=233.426; T=25.00 °C → log₁₀P = 8.07131 − 1730.63/(233.426+25.00) → P ≈ 23.8 mmHg.
Example 2 (Clausius–Clapeyron, water)
T₁=373.15 K, P₁=760 mmHg, ΔH=40.65 kJ/mol, T₂=298.15 K. ln(P₂/760)=−(40650)/(8.314)(1/298.15−1/373.15) → P₂ ≈ 23.8 mmHg.
Frequently Asked Questions
Q: Which equation should I use?
Use Antoine when you have constants for your liquid and your temperature is within the valid range. Use Clausius–Clapeyron when you know one (T,P) point and ΔHvap.
Q: Why do different sources list different Antoine constants?
Constants depend on the fit range and units. Always check the reference temperature range and whether T is in °C or K and P in mmHg, bar, etc.
Q: Can I compute temperature from a desired vapor pressure?
Not in this version. We currently compute P(T). If you need T(P), we can add that inversion for Antoine.
Q: Are results valid near the critical point?
No—both models break down near phase boundaries like the critical point; use an EOS model instead.
