Vapor Pressure Calculator

Vapor pressure is the partial pressure of a vapor held in dynamic equilibrium with its own liquid or solid inside a closed system: evaporation and condensation run at equal rates, so the pressure stops changing. Because escaping molecules must overcome intermolecular attraction, the energetic fraction follows a Boltzmann factor, so vapor pressure climbs exponentially rather than linearly with temperature. The Clausius-Clapeyron form used here, ln(P2/P1) = -(dHvap/R)(1/T2 - 1/T1), follows from the exact Clapeyron relation after two approximations: the vapor is treated as an ideal gas, and the molar volume of the liquid is neglected against that of the vapor. Water sets the scale: 3.17 kPa at 25 degrees C, 47.4 kPa at 80 degrees C, 101.325 kPa at 100 degrees C, with dHvap near 40.7 kJ/mol. Weakly bound liquids sit far lower, methane near 8.2 kJ/mol. The Antoine form, log10(P) = A - B/(C + T), is empirical rather than derived, and its three fitted constants absorb curvature that a constant dHvap cannot reproduce. Constants are tabulated per compound and per temperature window, and they carry units with them: the common water set A = 8.07131, B = 1730.63, C = 233.426 returns mmHg for T in degrees C over roughly 1 to 100 degrees C. Common failures are extrapolating Antoine constants beyond their fitted range, mixing mmHg with kPa or Celsius with Kelvin, and applying either equation near the critical point, where dHvap falls toward zero (647 K, 22.06 MPa for water). Neither equation describes a mixture; solutions require Raoult's law with activity coefficients.

The thermodynamic backbone came from Benoit Paul Emile Clapeyron, the French engineer who in his 1834 memoir on the motive power of heat recast Sadi Carnot's arguments analytically and produced the relation linking the slope of a phase boundary to the latent heat and the volume change on transition. Rudolf Clausius rebuilt that result on the second law around 1850 and supplied the integrated exponential form that now carries both names once the vapor is idealized. The measurements being explained were older still: John Dalton and Joseph Louis Gay-Lussac tabulated the vapor pressure of water and other liquids in the early 1800s, and Henri Victor Regnault's meticulous mid-century steam data gave the equation something precise to be tested against. The empirical branch arrived later. Louis Charles Antoine, a French engineer, published his three-constant fit in the Comptes Rendus of the French Academy of Sciences in 1888, motivated by the practical need for a compact formula that tracked measured boiling curves better than a two-constant law could. Twentieth-century process engineering turned that convenience into infrastructure: compilations such as the DIPPR database and the NIST Chemistry WebBook now publish Antoine constants with explicit validity ranges for thousands of compounds. For wide-range work the Wagner equation, introduced in the early 1970s, has largely displaced Antoine, but Antoine constants remain the working currency of distillation column design and process simulation.