Calculate vapor pressure at different temperatures using the Clausius-Clapeyron equation or Antoine coefficients. Supports 6 common substances with built-in coefficients and step-by-step working.
Water has a vapor pressure of 1 atm at its normal boiling point of 100°C (373.15 K). The enthalpy of vaporization is 40.65 kJ/mol.
Question (Clausius-Clapeyron): What is the vapor pressure of water at 80°C?
Given: P₁ = 1 atm, T₁ = 100°C (373.15 K), T₂ = 80°C (353.15 K), ΔH_vap = 40.65 kJ/mol
Using Clausius-Clapeyron: ln(P₂/1) = −(40650/8.314)(1/353.15 − 1/373.15)
Result: P₂ ≈ 0.473 atm
At 80°C, water's vapor pressure is about 0.47 atm.
Ethanol's Antoine coefficients (P in mmHg, T in °C): A = 8.20417, B = 1642.89, C = 230.300
Question: What is the vapor pressure of ethanol at 50°C?
Using Antoine: log₁₀(P) = 8.20417 − 1642.89 / (230.300 + 50) = 8.20417 − 1642.89 / 280.300 = 8.20417 − 5.8612 = 2.34297
Result: P = 102.34297 ≈ 220.3 mmHg ≈ 0.290 atm
At 50°C, ethanol has a vapor pressure of about 220 mmHg.
Ethanol has a normal boiling point of 78.37°C (351.52 K) and ΔH_vap = 38.56 kJ/mol.
Question: At what temperature does ethanol reach a vapor pressure of 2 atm?
Given: P₁ = 1 atm, T₁ = 78.37°C (351.52 K), P₂ = 2 atm, ΔH_vap = 38.56 kJ/mol
Result: T₂ ≈ 96.3°C (369.5 K)
Ethanol boils at about 96°C when the external pressure is 2 atm.
The Clausius-Clapeyron equation describes the relationship between vapor pressure and temperature for a substance. It is derived from thermodynamics and is fundamental for understanding phase transitions.
The Antoine equation is a semi-empirical correlation that relates vapor pressure to temperature with three substance-specific coefficients.
| Substance | Formula | A | B | C | Temp Range (°C) |
|---|---|---|---|---|---|
| Water | H₂O | 8.07131 | 1730.63 | 233.426 | 1–100 |
| Ethanol | C₂H₅OH | 8.20417 | 1642.89 | 230.300 | −3–78 |
| Acetone | C₃H₆O | 7.11714 | 1210.595 | 229.664 | −8–56 |
| Methanol | CH₃OH | 8.08097 | 1582.271 | 239.726 | −14–65 |
| Benzene | C₆H₆ | 6.90565 | 1211.033 | 220.790 | 8–80 |
| Toluene | C₇H₈ | 6.95464 | 1344.800 | 219.482 | 6–111 |
| Symbol | Meaning | Typical Units |
|---|---|---|
| P₁, P₂ | Initial and final vapor pressure | atm, kPa, mmHg, bar |
| T₁, T₂ | Initial and final temperature (absolute) | Kelvin (K) |
| ΔH_vap | Enthalpy of vaporization | kJ/mol |
| R | Universal gas constant | 8.314 J/(mol·K) |
| A, B, C | Antoine coefficients (substance-specific) | — |
The pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase at a given temperature in a closed system.
The heat energy required to convert one mole of liquid to gas at constant temperature and pressure. Higher ΔH_vap means stronger intermolecular forces.
The temperature at which the vapor pressure of a liquid equals the external pressure. At 1 atm, this is the normal boiling point.
Vapor pressure increases exponentially with temperature. The Clausius-Clapeyron equation models this over moderate ranges; Antoine is more accurate over wider ranges.
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. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or solid).
A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The vapor pressure of any substance increases non-linearly with temperature according to the Clausius-Clapeyron relation. At the normal boiling point of a liquid, the vapor pressure is equal to the standard atmospheric pressure (1 atm or 101.325 kPa).
Understanding vapor pressure is critical in many fields of science and engineering. In chemistry, vapor pressure determines how quickly a solvent evaporates and affects distillation processes. In meteorology, vapor pressure is essential for understanding humidity and weather patterns. In chemical engineering, vapor-liquid equilibrium data is fundamental for designing separation processes like distillation columns and evaporators.
The Clausius-Clapeyron equation is one of the most important relationships in physical chemistry because it connects the macroscopic property of vapor pressure to the molecular property of enthalpy of vaporization. The Antoine equation, with its empirical coefficients, provides more accurate vapor pressure data over wider temperature ranges for many common substances.
As temperature increases, the kinetic energy of molecules in a liquid increases, allowing more molecules to escape into the vapor phase. This causes the vapor pressure to rise. The relationship is exponential rather than linear — a small increase in temperature can cause a significant increase in vapor pressure. This is why water boils more vigorously at higher temperatures and why volatile liquids evaporate quickly even at room temperature.
At higher altitudes, atmospheric pressure is lower, so water boils at a lower temperature. The Clausius-Clapeyron equation explains why cooking times must be adjusted and why pressure cookers are effective at altitude.
Chemical engineers use vapor pressure data (often from the Antoine equation) to design distillation columns that separate mixtures based on differences in boiling points and volatilities.
Vapor pressure is directly related to relative humidity. Meteorologists use these relationships to predict cloud formation, precipitation, and storm intensity.
Steam turbines in power plants rely on the vapor pressure of water at high temperatures. Understanding the pressure-temperature relationship optimizes thermodynamic efficiency.
⚠️ Important Note: This Vapor Pressure Calculator is for educational and professional reference purposes. The Clausius-Clapeyron equation assumes constant enthalpy of vaporization and ideal gas behavior. The Antoine equation provides empirical accuracy within specified temperature ranges only. For precise engineering applications, consult experimental data or more advanced thermodynamic models. Always verify critical values with authoritative sources.