Activities and activity coefficients in a mixture of real gases

Activities and Activity Coefficients in a Mixture of Real Gases. 

Instead of starting from the equation for perfect gases 

It should be noticed that like Xi, is an intensive variable. 

Here then we have a non-ideal system (c/. chap. VII, 8) which may be compared with an ideal reference system consisting of a gas mixture of the same composition, and at the same temperature T but at a pressure sufficiently low that the mixture behaves as a perfect mixture. 

We can therefore apply all the formulae established in 8 of chap. VII. In particular we may introduce activity coefficients y defined by 

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