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In thermodynamics, the fugacity is a state function of any isothermal system. The fugacity, which has units of pressure, represents the tendency of a fluid to escape or expand isothermally. For gases at low pressures where the ideal gas law holds, fugacity is equal to pressure. The ratio  \phi = f/P \, between fugacity f\, and pressure P\, is called the fugacity coefficient; therefore,  \phi = 1 \, for an ideal gas.

The fugacity  f\, is defined at fixed temperature  T\, by the following differential relation:

 d \ln f = {dG \over RT} = {{V dP} \over RT} \,

where G\, is the Gibbs free energy, R\, is the molar gas constant, and V\, is the fluid's volume. For any two isothermal physical states, represented by subscripts 1 and 2, the ratio of the two fugacities is as follows:

 f_2 / f_1 = exp \left ({1 \over RT} \int_{G_1}^{G_2} dG \right) = exp \left ({1 \over RT} \int_{P_1}^{P_2} V\,dP \right) \,