K in Terms of Composition

Once the value of the equilibrium constant K for the reaction under consideration - and the temperature of interest - is determined, the equilibrium conversion can be obtained. To this purpose we first express K in terms of compositions, which in turn are expressed in terms of the progress of the reaction variable (see Section 12.5.3). The latter is then calculated from the known K value and, consequently, the equilibrium conversion can be determined. 

For a gas-phase reaction the activity of component i of the reacting mixture is given by  

Since fiigacity coefficients can be determined as functions of composition through an equation of state - such as the virial - the equilibrium constant K has thus been expressed in terms of composition. 

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