The rational system of activities

In the rational system of activities, gfT, p) is identified with the chemical potential of the pure liquid, p°(T, p)  

As X - 1, the system comes nearer to being pure i, and pj must approach p°, so that 

Therefore the activity of the pure liquid is equal to unity. 

The rational activity coefficients are convenient for those systems in which the mole fraction of any component may vary from zero to unity mixtures of liquids such as acetone and chloroform, for example. 

The rational activity of volatile constituents in a liquid mixture can be readily measured by measuring the partial pressure of the constituent in the vapor phase in equilibrium with the liquid. Since at equilibrium the chemical potentials of each constituent must be equal in the liquid and the vapor phase, we have Using Eq. (16.4) for jU l) 

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