How To Calculate Vapor Pressure

Clausius-Clapeyron Equation:

\[ P = P_0 \times \exp\left( -\frac{\Delta H_{vap}}{R} \times \left( \frac{1}{T} - \frac{1}{T_0} \right) \right) \]

Reference Pressure (P₀):

atm

Enthalpy of Vaporization (ΔH_vap):

J/mol

Reference Temperature (T₀):

Temperature (T):

Vapor Pressure (P):

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1. What is the Clausius-Clapeyron Equation?

The Clausius-Clapeyron equation describes how vapor pressure changes with temperature for a liquid-vapor phase transition. It's particularly useful for estimating vapor pressures at different temperatures when experimental data is limited.

2. How Does the Calculator Work?

The calculator uses the Clausius-Clapeyron equation:

\[ P = P_0 \times \exp\left( -\frac{\Delta H_{vap}}{R} \times \left( \frac{1}{T} - \frac{1}{T_0} \right) \right) \]

Where:

\( P \) — Vapor pressure at temperature T (atm)
\( P_0 \) — Reference vapor pressure at temperature T₀ (atm)
\( \Delta H_{vap} \) — Enthalpy of vaporization (J/mol)
\( R \) — Universal gas constant (8.314 J/mol·K)
\( T \) — Temperature for which to calculate vapor pressure (K)
\( T_0 \) — Reference temperature (K)

Explanation: The equation shows that vapor pressure increases exponentially with temperature, with the rate of increase determined by the enthalpy of vaporization.

3. Importance of Vapor Pressure Calculation

Details: Vapor pressure is crucial for understanding evaporation rates, boiling points, and phase equilibria. It's essential in chemical engineering, atmospheric science, and industrial process design.

4. Using the Calculator

Tips: Enter all values in the specified units. Ensure temperatures are in Kelvin (K = °C + 273.15). The reference pressure and temperature should be known values for the substance.

5. Frequently Asked Questions (FAQ)

Q1: What are typical values for ΔH_vap?
A: For water at 100°C it's 40.7 kJ/mol. Most liquids range between 20-50 kJ/mol.

Q2: How accurate is this equation?
A: It works best over small temperature ranges (50°C or less) and for substances with simple molecular interactions.

Q3: What's the difference between Antoine and Clausius-Clapeyron equations?
A: The Antoine equation is empirical and often more accurate, but requires three substance-specific parameters.

Q4: Can this be used for solids?
A: Yes, it can estimate sublimation pressure if you use the enthalpy of sublimation instead of vaporization.

Q5: Why must temperature be in Kelvin?
A: The equation involves reciprocal temperature differences, which would be incorrect in Celsius or Fahrenheit.