The Activity Coefficient
 
While we have defined fugacity coefficients of individual species in a liquid solution by 6.153, we may define yet another parameter called activity coefficient in order to describe the nonideality of a liquid solution, especially at low to moderate system pressure.
We have by eqn. 6.155: And from eqn. 6.158:  
Using the above equations:  
The left side of this equation is the partial excess Gibbs energy ; the dimensionless ratio appearing on the right is the activity coefficient of species i in solution, represented by the symbol Thus, by definition:  
Whence,
 
But
