Vapor Pressure Calculator
Calculate the vapor pressure of a liquid at any temperature using three methods: the Antoine equation (with built-in constants for water, ethanol, methanol, acetone, benzene, toluene, and hexane), the Clausius-Clapeyron equation (when you know vapor pressure at one temperature and the enthalpy of vaporization), or Raoult's law (for an ideal solution given the mole fraction of the solvent). Switch pressure units between mmHg, kPa, atm, Pa, and bar. Results update as you type.
What is vapor pressure?
Vapor pressure is the pressure exerted by the vapor (gas phase) of a liquid in equilibrium with the liquid at a given temperature, in a closed system. It is a measure of how readily molecules escape the liquid surface. Every pure liquid has a characteristic vapor pressure that rises steeply with temperature. When the vapor pressure equals the external pressure, the liquid boils. Water has a vapor pressure of about 3.17 kPa at 25 degC and 101.3 kPa (1 atm) at 100 degC, its normal boiling point.
The Antoine equation
The Antoine equation is an empirical three-parameter correlation that fits experimental vapor-pressure data remarkably well over moderate temperature ranges. In the Dortmund form used here: log10(P / mmHg) = A - B / (C + T / degC). The constants A, B, and C are substance-specific and have been tabulated for thousands of compounds by NIST and others. The equation captures the exponential rise of vapor pressure with temperature through a simple algebraic form. Each set of constants is valid only within its stated temperature range; extrapolating beyond it can introduce large errors.
The Clausius-Clapeyron equation
The Clausius-Clapeyron equation relates vapor pressure to temperature through the enthalpy of vaporization: ln(P2 / P1) = -(dHvap / R) * (1/T2 - 1/T1), where temperatures are in Kelvin and R is the gas constant (8.314 J / mol / K). It is useful when you know the vapor pressure at one temperature (for example, the normal boiling point) and the enthalpy of vaporization, and you need to estimate the pressure at a different temperature. The equation assumes dHvap is constant over the temperature range, which is reasonable for intervals smaller than about 50 K but introduces increasing error over wider ranges.
Raoult's law and solution vapor pressure
For an ideal liquid solution, Raoult's law states that the vapor pressure of the solution equals the mole fraction of the solvent times the vapor pressure of the pure solvent at the same temperature: P_solution = x_solvent * P*_solvent. This is why dissolving a non-volatile solute (such as salt or sugar) in water lowers the vapor pressure of the water, a colligative property called vapor-pressure lowering. The depression is proportional to the mole fraction of the solute. Raoult's law holds best for dilute solutions and for solutes and solvents with similar intermolecular forces. Non-ideal solutions require activity coefficients or excess Gibbs energy models.
Antoine constants and normal boiling points for common substances
| Substance | A | B | C | T range (degC) | Boiling point (degC, 1 atm) |
|---|---|---|---|---|---|
| Water | 8.07131 | 1730.63 | 233.426 | 1 to 100 | 100.0 |
| Ethanol | 8.20417 | 1642.89 | 230.300 | -57 to 80 | 78.4 |
| Methanol | 7.89750 | 1474.08 | 229.130 | -20 to 140 | 64.7 |
| Acetone | 7.11714 | 1210.595 | 229.664 | -13 to 55 | 56.1 |
| Benzene | 6.90565 | 1211.033 | 220.790 | 8 to 80 | 80.1 |
| Toluene | 6.95464 | 1344.800 | 219.482 | 6 to 137 | 110.6 |
| Hexane | 6.87601 | 1171.17 | 224.408 | -25 to 69 | 68.7 |
Constants in the Dortmund form: log10(P/mmHg) = A - B/(C + T/degC). Source: NIST Chemistry WebBook SRD 69.
Frequently asked questions
What is vapor pressure and why does it matter?
Vapor pressure is the pressure of a substance's gas phase above its liquid at equilibrium. It determines the boiling point, evaporation rate, and flammability of a liquid, and is central to distillation, refrigeration, environmental fate modeling, and safety assessments for volatile chemicals.
How does the Antoine equation calculate vapor pressure?
The Antoine equation uses three empirically fitted constants (A, B, C) for each substance: log10(P/mmHg) = A - B/(C + T), where T is in degrees Celsius. Raising 10 to the power of the right-hand side gives the vapor pressure in mmHg, which can then be converted to any other pressure unit.
What are the units for Antoine constants?
Antoine constants are empirical and their units depend on which form of the equation is used. The classic Dortmund form (used here) expresses P in mmHg and T in degrees Celsius. Other tabulations use bar, kPa, or atm for pressure and Kelvin for temperature, so always check which form your constants come from before using them.
When should I use Clausius-Clapeyron instead of Antoine?
Use Clausius-Clapeyron when you know the vapor pressure at one reference temperature and the enthalpy of vaporization, but do not have Antoine constants for your substance. It is also a good teaching tool for understanding why vapor pressure rises exponentially with temperature. Antoine is more accurate when constants are available, because it is fitted to experimental data over a wide range rather than derived from a single-parameter model.
What does Raoult's law tell us about boiling point elevation?
Raoult's law shows that adding a non-volatile solute reduces the solvent's vapor pressure. Since boiling requires the vapor pressure to reach the external pressure, the boiling point rises proportionally - a colligative property that does not depend on the identity of the solute, only its concentration.
Why does vapor pressure increase with temperature?
At higher temperatures, more molecules have enough kinetic energy to escape the liquid surface into the gas phase. The relationship is exponential (not linear), captured by both the Antoine and Clausius-Clapeyron equations. A small rise in temperature produces a large rise in vapor pressure, which is why flammability and evaporation hazards increase rapidly with heat.