The expression for Z is dependent on the media/phase/compartment. The following list gives the fugacity capacities for common medias:
* Air (under ideal gas assumptions): Z = 1/RT
* Water: Z = 1/H
* Octanol: Z = K/H
* Pure phase of target chemical: Z = 1/Pv
Where: R is the Ideal gas constant (8.314 Pa·m/mol·K); T is the absolute temperature (K); H is the Henry's law constant for the target chemical (Pa/mmol); K is the octanol-water partition coefficient for the target chemical (dimensionless ratio); P is the vapor pressure of the target chemical (Pa); and v is the molar volume of the target chemical (m/mol). 
Notice that the ratio between Z-values for different media (e.g. octanol and water) is the same as the ratio between the concentrations of the target chemical in each media at equilibrium. 
When using a fugacity capacity approach to calculate the concentrations of a chemical in each of several medias/phases/compartments, it is often convenient to calculate the prevailing fugacity of the system using the following equation if the total mass of target chemical (M) and the volume of each compartment (V) are known:
Alternatively, if the target chemical is present as a pure phase at equilibrium, its vapor pressure will be the prevailing fugacity of the system.