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The amended Raoult's law description of the vapor pressure above the solution becomes:
The Margules function has the general form:
- fM,1 = exp(αx22+βx23+γx23+δx24+...)
The function always contains the opposite mole fraction x2 (= 1-x1 for a binary system). The numbers of Margules parameters α, β, γ, δ, .. varies. The larger the deviations from ideality the more parameters are required.
The Gibbs-Duhem relation can be used to fix the Margules function for the second component once the first has been chosen.
The case where only one parameter α is needed is known as the regular solution model.