Fugacity:
In chemical thermodynamics, the fugacity of a real gas is an effective partial pressure which replaces the mechanical partial pressure in an accurate computation of the chemical equilibrium constant. It is equal to the pressure of an ideal gas which has the same chemical potential as the real gas. For example, nitrogen gas (N2) at 0 °C and a pressure of P = 100 atm has a fugacity of f = 97.03 atm.[1] This means that the chemical potential of real nitrogen at a pressure of 100 atm is less than if nitrogen were an ideal gas; the value of the chemical potential is that which nitrogen as an ideal gas would have at a pressure of 97.03 atm.
Fugacities are determined experimentally or estimated from various models such as a Van der Waals gas that are closer to reality than an ideal gas. The ideal gas pressure and fugacity are related through the dimensionless fugacity coefficient φ. [Wikipedia]
Sighted in Jack Vance’s The Palace of Love:
Some of my guests have complained of a gentle melancholy which hangs in the air; I agree that the mood exists. The explanation, I believe, arises from the fugacity of beauty, the tragic pavanne to which all of us step.
