The equation of state used throughout Ansys Chemkin is that of an ideal, multi-fluid gas. The multi-fluid gas formulation allows for a temperature to be specified for each species, . This formulation collapses to the more usual thermal-equilibrium relation in the case where all species temperatures, , are equal to the gas temperature. The general equation of state is given by:

(2–5)

while the mean mass density is defined by:

(2–6)

The mean molecular weight may be defined variously as

(2–7)

or

(2–8)

or

(2–9)

As stated above, It is often convenient to represent a gas-mixture species composition as either mass fraction, mole fraction, or molar concentration. The following sections contain the formulas used to convert between these different ways of describing the mixture composition.