Vapor Pressure Calculator

Calculate vapor pressure using the Clausius-Clapeyron equation

Vapor Pressure Calculator

Clausius-Clapeyron Equation: ln(P₂/P₁) = -(ΔHvap/R)(1/T₂ - 1/T₁)
Relates vapor pressure to temperature and enthalpy of vaporization

atm, mmHg, kPa (must match P₂)

Kelvin (K)

Kelvin (K)

kJ/mol

Common ΔHvap Values:

• Water: 40.7 kJ/mol
• Ethanol: 38.6 kJ/mol
• Benzene: 30.7 kJ/mol
• Acetone: 29.1 kJ/mol

Understanding Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. The Clausius-Clapeyron equation relates vapor pressure to temperature, providing a powerful tool for predicting phase behavior and understanding intermolecular forces.

The Clausius-Clapeyron Equation

ln(P₂/P₁) = -(ΔHvap/R)(1/T₂ - 1/T₁)

P₁, P₂ = vapor pressures at temperatures T₁ and T₂

ΔHvap = enthalpy (heat) of vaporization (J/mol or kJ/mol)

R = universal gas constant (8.314 J/(mol·K))

T₁, T₂ = absolute temperatures (K)

Example: Water Vapor Pressure

Problem: Calculate the vapor pressure of water at 90°C, given:

  • P₁ = 1.00 atm at T₁ = 373 K (100°C)
  • ΔHvap = 40.7 kJ/mol
  • T₂ = 363 K (90°C)

Solution:

ln(P₂/1.00) = -(40,700/8.314)(1/363 - 1/373)

ln(P₂) = -(4,895)(0.002755 - 0.002681)

ln(P₂) = -(4,895)(0.000074)

ln(P₂) = -0.362

P₂ = e^(-0.362)

P₂ = 0.696 atm

Heat of Vaporization Values

SubstanceΔHvap (kJ/mol)Boiling Point
Water40.7100°C
Ethanol38.678°C
Methanol35.265°C
Benzene30.780°C
Acetone29.156°C
Diethyl Ether26.535°C

Key Relationships

Temperature Effect

Vapor pressure increases exponentially with temperature. Higher T means more molecules have enough energy to escape the liquid phase.

ΔHvap Significance

Higher ΔHvap indicates stronger intermolecular forces. Water has high ΔHvap due to extensive hydrogen bonding.

Applications

  • 🌡️
    Boiling Point Prediction: Determine boiling point at different pressures (e.g., high altitude cooking)
  • 🏭
    Distillation: Design and optimize separation processes based on vapor pressure differences
  • 💧
    Evaporation Rates: Predict how quickly liquids will evaporate at various temperatures
  • ☁️
    Weather Prediction: Understanding humidity and cloud formation

💨Quick Reference

R constant:

8.314 J/(mol·K)

Boiling occurs:

When P = atmospheric pressure

Higher ΔHvap:

Stronger intermolecular forces

🔗Related Calculators

📐Related Formulas

🎯Where It's Used

  • 🏭

    Distillation

    Separation processes

  • 🌡️

    Climate

    Weather prediction