In a later section we discuss mass-action kinetics, where the rate-of-progress of reactions depends on molar concentrations either in the gas phase or on surface sites and activities in the bulk phases. However, for the purposes of formulating and solving the conservation equations that describe physical situations, it is often more natural to use gas-phase mass fractions and surface site fractions as dependent variables. Therefore, it is important to establish the rules for converting between the different ways to describe the composition of the gas and the surface.

For the gas-phase species the molar concentrations (in mole/cm3) are written as

(4–37)

where the are the mass fractions, is the gas-phase mass density, and the are the molecular weights.

On the surface sites we can describe the composition in terms of an array of surface species site fractions . This array is of length . It is composed of subunits of the site fractions of each of the species on a given site (phase) . The site fractions on each site are normalized

(4–38)

The sum in Equation 4–38 runs from the first species in phase to the last species in phase . The surface molar concentration of a species is then

(4–39)

where is the density of sites of phase (in mole/cm²) and is the number of sites that each species occupies. For the sake of parallelism, we adopt the nomenclature for bulk species

(4–40)

It is at best an approximation that bulk species form an ideal solution. Therefore, the concept of an activity (and the standard state to which it refers) must be introduced at the outset. In the limiting case of an ideal solution, the activity of a species is equal to its mole fraction. KINetics assumes that the activity, , of bulk species is used in all chemical rate expressions.