Fugacities in an Ideal Binary Mixture

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This Demonstration shows how the fugacities of benzene () and toluene () change with temperature and molar composition at constant pressure. The liquid mixture is an ideal solution and the gas phase is ideal, so Raoult's law models vapor-liquid equilibrium. Use sliders to vary the temperature and overall mole fraction of benzene. Use buttons to view the temperature-composition diagram (--), the fugacity-temperature plot or both plots at once.

Contributed by: Garrison J. Vigil and Rachael L. Baumann (July 2015)
Additional contributions by: John L. Falconer and Nick Bongiardina
(University of Colorado Boulder, Department of Chemical and Biological Engineering)
Open content licensed under CC BY-NC-SA



The saturation pressures were calculated using the Antoine equation:


where represents either benzene or toluene ( or ), is temperature (°C), and , and are Antoine constants.

The fugacity of a component depends on temperature and molar composition.

In the liquid region:


where is the liquid mole fraction and both and have units of bar.

In the vapor region:


where is the vapor molar composition and is the total pressure (bar).

When vapor and liquid are in equilibrium (VLE):


The screencast video at [1] explains how to use this Demonstration.


[1] Fugacities in an Ideal Binary Mixture [Video]. (Dec 1, 2016) www.colorado.edu/learncheme/thermodynamics/FugacitiesIdealBinaryMixture.xhtml.

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