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

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This Demonstration leads you through the construction of a temperature-composition (--) diagram step by step for vapor-liquid equilibrium (VLE) of an -hexane/-octane ideal mixture (Raoult's law). Click the "new problem" button to construct a diagram at a different pressure. After completing each step, check the "solution" box to see the correct solution and then click "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 (April 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 is the saturation pressure (bar); , and are Antoine constants; is temperature (°C); component is -hexane; and component is -octane.

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.

The saturation temperature is solved for by setting equal to and solving for .

The bubble point pressure is calculated using :


The dew point pressure 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]. (Apr 19, 2018)

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