Thermodynamics: Fugacity

Fugacity (fi) is a thermodynamic property defined to correlate behavior of real gases with ideal gases.  It is used to evaluate the most useful property Gibbs Free energy (chemical potential) for real species/component either in pure or in solution.

If we have pure species, for ideal gas fugacity is equal to pressure of the gas, and for ideal gas solution fugacity of the species/component is equal to partial pressure of the species/component and in ideal liquid solution it is equal to fuacity x mole fraction (fi.xi) (Lewis/Randall Rule)

The fugacity can be calculated by comparing the Gibbs free energy for real conditions to its ideal value. The gas will behave like an ideal gas at lowest possible pressure at a given temperature. At ideal conditions the fugacity can be calculated from its conditions for pure species or in solution. The difference between the Gibbs free energy can be calculated from its fundamental equation of (∆H-T∆S).
The fugacity can be calculated using equation of state or compressiblility factor versus pressure data.
In case of steam we can use saturated steam table and super heated steam table to find fugacity.