Vapor Pressure Calculator
Our chemical thermodynamics calculator computes vapor pressure accurately. Enter measurements for results with formulas and error analysis.
Reviewed by Manoj Kumar, Mathematics Educator
Formula
ln(P2/P1) = (-deltaHvap/R)(1/T2 - 1/T1) | log(P) = A - B/(C+T)
The Clausius-Clapeyron equation relates vapor pressure change to temperature through the enthalpy of vaporization. The Antoine equation uses three empirical constants (A, B, C) for more accurate estimates over wider temperature ranges. Both allow calculation of vapor pressure at any temperature.
Worked Examples
Example 1: Water Vapor Pressure at 80 C
Problem:Calculate the vapor pressure of water at 80 C (353.15 K) given P = 101.325 kPa at 100 C (373.15 K) and deltaHvap = 40700 J/mol.
Solution:ln(P2/101.325) = (-40700/8.314)(1/353.15 - 1/373.15)\nln(P2/101.325) = (-4893.4)(-0.000152) = -0.7426\nP2 = 101.325 x e^(-0.7426) = 48.15 kPa
Result:P = 48.15 kPa (361.2 mmHg) at 80 C
Example 2: Ethanol Using Antoine Equation
Problem:Find the vapor pressure of ethanol at 50 C using Antoine constants A = 8.20417, B = 1642.89, C = 230.300.
Solution:log10(P) = 8.20417 - 1642.89/(230.300 + 50)\nlog10(P) = 8.20417 - 5.8602 = 2.3440\nP = 10^2.3440 = 220.8 mmHg = 29.44 kPa
Result:P = 220.8 mmHg (29.44 kPa) at 50 C
Frequently Asked Questions
What is vapor pressure?
Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. When a liquid is placed in a sealed container, molecules continuously escape from the liquid surface into the vapor phase (evaporation) and return from the vapor to the liquid (condensation). At equilibrium, these rates are equal, and the pressure of the vapor above the liquid is the vapor pressure. Every substance has a characteristic vapor pressure that increases with temperature because higher temperatures give more molecules enough kinetic energy to escape into the vapor phase.
Why is vapor pressure important in chemistry?
Vapor pressure is crucial in many chemical and industrial processes. It determines the boiling point of a substance (a liquid boils when its vapor pressure equals atmospheric pressure), governs evaporation rates, and controls the behavior of solutions through Raoult's law. In environmental science, vapor pressure dictates how quickly pollutants evaporate into the atmosphere. In pharmacy, it affects drug formulation and storage stability. Industrial applications include distillation design, vacuum system engineering, and refrigeration cycle optimization. Understanding vapor pressure is also essential for safety, as volatile substances with high vapor pressures pose greater fire and explosion risks.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy