The line of uniform composition for regular solutions

Starting from (28.22), we may first replace a 2 by ocjRT, corresponding to the case of regular solutions, and then subtract the second equation from the first. On rearranging the terms we have 

The logarithm of the vapour pressure of a pure substance is very nearly 

If we know a and the vapour pressure curves of the two liquids we can calculate the azeotropic composition at a given temperature. In particular, if the vapour pressures of the two liquids are equal, then the azeotrope, if it exists, will have a mole fraction of 0-5 of each component. 

Equation (28.41) also enables us to deduce another general rule. For on differentiation with respect to temperature, we obtain  

If the two components have vapour pressures which are closely similar, this may be written approximately  

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