# Van Laar equation to calculate activity coefficients

How do I use the Van Laar equation to calculate an activity coefficient for an equilibrium ethanol/water binary system with mole fractions of ethanol in liquid of: 0, 0.05, 0.1, 0.15, 0.2, 0.4, 0.6, 0.8, and 1? I'm looking at the Van Laar equation in Perry's Handbook where A12 is 1.6798 and A21 is 0.9227. I don't understand what these variables are, and I don't understand if ln of gamma 1 and ln of gamma 2 represent the vapor part and liquid part respectively (or vice versa) or if they represent the ethanol and water respectively (or vice versa). I want to make sure I know what x1 and x2 represent also. Thanks very much.

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