Fugacity can be thought of as the activity of a gas. The fugacity of a component in a mixture at equilibrium is the same whether in vapor or liquid phase. This equation defines the fugacity of a component in a vapor using the component characteristics of: mole fraction, system pressure, and fugacity coefficient of component in vapor.
The fugacity coefficient `hatvarphi_i`, for component `i` in the vapor is calculated from an equation of state (e.g., Virial). Sometimes it is approximated by a pure component value from a correlation. The fugacity coefficient is a correction for vapor phase non-ideality. Often at pressures close to atmospheric, `hatvarphi_i`= 1