The quasi-chemical method of modeling solutions

If we examine relation [3.31], it is exactly as though we had applied the law of mass aetion to a ehemical equilibrium, which can be written as  

The eonstant in this equilibriiun would be equal to 1, meaning that it is independent of the temperature. 

Similarly, by examining relation [3.75], we are led to write a law of mass action of the same equilibriiun [3R.1], but this time with an equilibrium constant of  

It is because of these similarities that Fowler and Guggenheim s model is known as the quasi-chemieal model. 

we can understand why the solution given by relation [3.31], which is the Bragg-Williams approximation, is also ealled the zero-order approximation, whereas Fowler and Guggenheim s, given by relation [3.75], 

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