Chemical fugacity ($f$)

An effective gas pressure defined by $\mu ={\mu }^{\circ }+RT\ln (f/p^{\circ })$ in a specified standard-state convention. It approaches the gas pressure at low density and differs from the dimensionless fugacity.

The dimensionless ratio of a gas species' chemical fugacity to its pressure in the pure-fluid convention. Its thermodynamic activity relative to a gas reference pressure $P^{\circ }$ is $\varphi P/P^{\circ }$. For mixtures the pressure denominator uses the species' partial pressure. The ideal-gas limit has $\varphi =1$.