Vapor Pressure Calculator
Calculate vapor pressure for water, ethanol, and methanol using the Antoine equation. Free chemistry calculator with Clausius-Clapeyron and interactive charts.
About This Calculator
The Vapor Pressure Calculator computes the vapor pressure of liquids using two powerful thermodynamic methods: the Antoine equation and the Clausius-Clapeyron equation. Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase at a given temperature — a critical property in chemistry, chemical engineering, and materials science.
For the Antoine equation method, select from eight supported substances (water, ethanol, methanol, acetone, benzene, toluene, n-hexane, and isopropyl alcohol) and enter a temperature within the valid range for each substance's Antoine coefficients. The vapor pressure is calculated using the formula log₁₀(P) = A − B/(C + T), where P is in mmHg and T in °C, using standard literature Antoine coefficients. Results are converted to kPa, mmHg (torr), atm, and bar for convenient use.
For the Clausius-Clapeyron equation method, enter the initial temperature and pressure, the final temperature, and the molar enthalpy of vaporization (ΔH_vap) in J/mol. This thermodynamic relation ln(P₂/P₁) = −ΔH_vap/R · (1/T₂ − 1/T₁) describes how vapor pressure changes with temperature and is valid for any substance where you know the enthalpy of vaporization. The gas constant R = 8.3145 J/(mol·K) is used automatically.
The interactive chart displays the vapor pressure vs. temperature curve around your input (Antoine method) or a unit comparison bar chart (both methods), helping visualize the strong exponential relationship between temperature and vapor pressure.
Regional Notes
Global: Vapor pressure is a fundamental physical property independent of region. The calculator supports multiple pressure units (kPa, mmHg, atm, bar) used worldwide. In scientific and engineering contexts worldwide, kPa and bar are preferred (SI system), while mmHg (torr) is common in medical and vacuum applications, and atm is widely used in chemistry education.
Frequently Asked Questions
What is vapor pressure and how is it calculated?
Vapor pressure is the pressure exerted by the vapor of a liquid or solid in equilibrium with its condensed phase at a given temperature. Our calculator uses the Antoine equation log₁₀(P) = A − B/(C + T) for common substances like water, ethanol, and methanol, and the Clausius-Clapeyron equation ln(P₂/P₁) = −ΔH_vap/R · (1/T₂ − 1/T₁) for general temperature-dependent vapor pressure calculations.
What substances does the Antoine equation calculator support?
The calculator supports water (H₂O), ethanol (C₂H₅OH), methanol (CH₃OH), acetone (C₃H₆O), benzene (C₆H₆), toluene (C₇H₈), n-hexane (C₆H₁₄), and isopropyl alcohol (C₃H₈O). Each substance has validated Antoine coefficients for accurate vapor pressure computation within its specified temperature range.
What is the difference between the Antoine equation and Clausius-Clapeyron equation?
The Antoine equation is an empirical formula that gives vapor pressure directly from temperature for a specific substance using three fitted constants (A, B, C). The Clausius-Clapeyron equation is a thermodynamic relation that calculates how vapor pressure changes between two temperatures using the enthalpy of vaporization (ΔH_vap). Use Antoine for quick single-point calculations with built-in substances, and Clausius-Clapeyron when you know the enthalpy of vaporization or need to calculate for any substance.
What units does the vapor pressure calculator support?
Results are displayed in four common pressure units: kilopascals (kPa), millimeters of mercury (mmHg or torr), atmospheres (atm), and bars (bar). The interactive chart shows the vapor pressure curve across a temperature range around your input value.
How accurate are the Antoine equation vapor pressure calculations?
The Antoine coefficients used are standard literature values that provide accurate vapor pressure estimates within the specified temperature range for each substance. Typical accuracy is within 1–2% of experimental values within the valid temperature range. Results are displayed with 4 decimal places for kPa and atm, and 2 decimal places for mmHg.
What is the enthalpy of vaporization and how do I find it?
The enthalpy of vaporization (ΔH_vap) is the energy required to vaporize one mole of a liquid at its boiling point. For water it is 40,660 J/mol at 100 °C, for ethanol 38,560 J/mol, and for methanol 35,210 J/mol. Standard values can be found in chemistry reference tables or the NIST Chemistry WebBook. The calculator accepts any positive value in J/mol.
Can this calculator be used for educational purposes?
Yes, this calculator is ideal for chemistry students learning about phase equilibria, vapor pressure, and the Clausius-Clapeyron equation. The curve chart visually demonstrates how vapor pressure varies exponentially with temperature, and the multiple unit conversions help students understand pressure unit relationships.
Why does vapor pressure increase with temperature?
As temperature increases, molecules gain kinetic energy and more molecules can escape from the liquid surface into the vapor phase. This increases the vapor density and therefore the vapor pressure. The relationship is exponential — the Clausius-Clapeyron equation shows that ln(P) is proportional to 1/T, meaning vapor pressure rises rapidly with temperature.