Construct a P-x-y Diagram for Vapor-Liquid Equilibrium (VLE)

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This Demonstration guides you through the construction of a pressure-composition (--) diagram step by step for vapor-liquid equilibrium (VLE) of an -hexane/-octane ideal mixture, assuming Raoult’s law. Click the "new problem" button to construct a diagram at a different temperature. After completing each step, check the "solution" box to see the correct result, and then "next" for the next step. Once you check "solution," you cannot change your answer. Also, you cannot return to the previous step once you click "next."

Contributed by: Rachael L. Baumann (March 2018)
Additional contributions by: John L. Falconer
(University of Colorado Boulder, Department of Chemical and Biological Engineering)
Open content licensed under CC BY-NC-SA



The saturation pressure of each component is calculated using the Antoine equation:


where for -hexane; for -octane; , and are the Antoine constants; is the saturation pressure (bar); and is the temperature (°C).

Raoult's law is used to calculate the bubble point and dew point pressures using factors:


where is the vapor mole fraction (), is the liquid mole fraction (), and is the total pressure (bar).

The bubble point pressure curve is calculated using :


The dew point pressure curve is calculated using :


View the screencast video at [2] to see another example of a -- diagram.


[1] J. R. Elliott and C. T. Lira, Introductory Chemical Engineering Thermodynamics, 2nd ed., Upper Saddle River, NJ: Prentice Hall, 2012 pp. 372–377.

[2] P-x-y and T-x-y Diagrams for Vapor-Liquid Equilibrium (VLE) [Video]. (Mar 16, 2018)

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