,

where is the residual Gibbs free energy, is the temperature, and is the universal gas-constant. As pressure approaches zero, the gas behaves like an ideal gas (i.e. ) and the fugacity coefficient approaches unity.

This Demonstration plots the gas-phase fugacity coefficient for the gas propylene versus temperature (in K) using either the Soave–Redlich–Kwong (SRK) or the Peng–Robinson (PR) equation of state (EoS) [1, 2]. Use the slider to set the value of the pressure (in bar). At we get excellent agreement between the results of our calculations (red curve for the SRK EoS, blue for the PR EoS) and the corresponding values obtained with the Aspen Plus process simulator [3] (green squares).