Vapor Pressure Calculator

Vapor pressure is the pressure exerted by vapor molecules of a substance in equilibrium with its liquid phase. It increases with temperature and is a key property for understanding phase transitions and volatility.

The Clausius-Clapeyron equation relates vapor pressure to temperature using the enthalpy of vaporization. It's expressed as ln(P₂/P₁) = -ΔHvap/R × (1/T₂ - 1/T₁), where R is the gas constant.

The enthalpy of vaporization (ΔHvap) is the energy required to convert one mole of liquid into vapor at constant temperature and pressure. It's typically expressed in kJ/mol and varies by substance.

Vapor pressure increases exponentially with temperature. As temperature rises, more molecules have sufficient kinetic energy to escape the liquid phase, resulting in higher vapor pressure.

The Antoine equation is an empirical formula that relates vapor pressure to temperature: log₁₀(P) = A - B/(C + T). Constants A, B, and C are substance-specific and valid only within certain temperature ranges.

A liquid boils when its vapor pressure equals atmospheric pressure (760 mmHg at sea level). You can use either equation to find the temperature where vapor pressure equals 1 atmosphere.

Use Clausius-Clapeyron when you have enthalpy of vaporization data and need thermodynamically accurate results. Use Antoine equation when you have the empirical constants and need high accuracy within the valid temperature range.

Different fields use different pressure units: mmHg in medicine and chemistry, atm in general chemistry, kPa and bar in engineering, and psi in industrial applications. The calculator converts between all common units.