Fugacity as a Driving Force for Mass Transfer

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Two 1-liter flasks are at different temperatures; initially the left flask contains 500 mL of water with dissolved salt and the right flask contains 200 mL of pure water. You can control the temperature of the left flask and the amount of salt dissolved with sliders. Click the play button "go to equilibrium" to remove the caps on each flask. Water transfers from one flask to the other to try to make water fugacities equal in each flask. For some conditions, all the water transfers to the left or right flask. Check "show fugacities" to display the water fugacities in each flask.

Contributed by: Rachael L. Baumann (March 2015)
Additional contributions by: Garrison J. Vigil, 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 pressure of water is calculated using the Antoine equation:


where , and are Antoine constants, is temperature (°C) and is in units of kPa.

The fugacity of pure water is equal to its saturation pressure:


For water with dissolved salt, the fugacity of the solution (kPa) is given by:


where is the mole fraction of water.

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


[1] Fugacity as a Driving Force for Mass Transfer [Video]. (Sep 1, 2016) www.colorado.edu/learncheme/thermodynamics/FugacityDrivingForceMassTransfer.html.

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