Chemistry Calculators

#1 Free Vapor Pressure Calculator 2025

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2 mins
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Vapor Pressure Calculator

Calculates vapor pressure at a different temperature using the Clausius-Clapeyron equation.

Calculation Result:

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. It is a measure of a substance's tendency to evaporate.

Key Characteristics:

  • Temperature Dependent: Vapor pressure increases with increasing temperature. As temperature rises, more molecules gain sufficient kinetic energy to escape from the liquid (or solid) phase into the gas phase.
  • Intermolecular Forces (IMFs): Substances with weaker IMFs have higher vapor pressures at a given temperature because their molecules can escape more easily. Substances with stronger IMFs (e.g., hydrogen bonding) have lower vapor pressures.
  • Dynamic Equilibrium: In a closed container, an equilibrium is established where the rate of evaporation equals the rate of condensation. The pressure of the vapor at this equilibrium is the vapor pressure.
  • Boiling Point: A liquid boils when its vapor pressure equals the external (atmospheric) pressure.

The Clausius-Clapeyron equation describes the relationship between the vapor pressure of a liquid and its temperature. It allows us to estimate the vapor pressure at one temperature if we know it at another, along with the enthalpy of vaporization (ΔHvap).

A common two-point form of the equation is:

ln(P₂ / P₁) = - (ΔHvap / R) * (1/T₂ - 1/T₁)

Rearranging to solve for the new vapor pressure (P₂):

P₂ = P₁ * exp[ - (ΔHvap / R) * (1/T₂ - 1/T₁) ]

  • P₁ = Known vapor pressure at temperature T₁
  • T₁ = Known temperature (in Kelvin)
  • P₂ = New vapor pressure to be calculated (at temperature T₂)
  • T₂ = New temperature (in Kelvin)
  • ΔHvap = Enthalpy of vaporization of the substance (in J/mol)
  • R = Ideal gas constant (8.314 J/(mol·K))
  • exp = The exponential function (ex)

Assumptions for Clausius-Clapeyron:

  • ΔHvap is constant over the temperature range (reasonable for moderate ranges).
  • The vapor behaves as an ideal gas.
  • The volume of the liquid is negligible compared to the volume of the vapor.
  • The temperatures T₁ and T₂ are not drastically different.

1. Enter Known Values:

  • Known Vapor Pressure (P₁): Enter a known vapor pressure of the substance. Select its unit.
  • Temperature for P₁ (T₁): Enter the temperature at which P₁ was measured. Select its unit (°C, K, or °F).
  • Enthalpy of Vaporization (ΔHvap): Enter the substance's enthalpy of vaporization. Select its unit (kJ/mol or J/mol).
  • New Temperature (T₂): Enter the new temperature for which you want to calculate the vapor pressure. Select its unit.

2. Select Desired Output Unit for Pressure:

  • Choose the unit in which you want the new vapor pressure (P₂) to be displayed (atm, Pa, kPa, mmHg, bar).

3. Calculate:

  • Click the "Calculate New Vapor Pressure" button.

4. View Results:

  • The calculated vapor pressure (P₂) at the new temperature (T₂) will be displayed.
  • A detailed step-by-step solution will show how the result was obtained, including unit conversions and application of the Clausius-Clapeyron equation.
  • If there are errors in your input (e.g., non-numeric values, zero ΔHvap, non-positive temperatures in Kelvin), an error message will appear.

5. Reset:

  • Click the "Reset" button to clear all input fields and results.
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