Talk:Fugacity

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Definition of Fugacitiy

Fugacity is a pressure-like property, that allows us to apply the same equations from the perfect gas to a real gas.

In a perfect gas, the molecules are so distant apart, such that collisions only occur between molecules and walls.

However, in a real gas, interference between molecules does happen, and statistical properties such as entropy can no longer be linearly added together (eg: say, for a two fluid mixture).

The fugacity definition solves the problem, as it directly relates to the compressibility factor (Z), which quantifies how far our assumptions are from the perfect gas, pV = ZRT. In turn, it represents a way to "correct" the idealized pressure, so as to remain able to use the classic equations.

The compressibility factor can be found in charts for different substances.

The beauty and generality of it lies on that, for different substances, if the whole chart is normalized regarding a critical pressure (easily identified by an inflection point), all charts (of a given molecule shape) will look alike.

Contributed by E.Hwang, March 22nd, 2006.