Solutions, perfect partial vapour pressures

Fig. 21.1 shows the partial vapour pressures and the total pressure of a perfect solution as a function of composition. In the case of a binary solution (21.8) may be written... 

Fig. 21.1. Total and partial vapour pressure of a perfect solution at constant T.

Greater physical insight into a particular problem may often be obtained by making direct application, not of the phase rule itself, but of the conditions of equilibrium on which it is based. As an example consider the system discussed under case (c) above. Let T and p be the temperature and total pressure respectively and let partial pressures in the vapour and let etc., be the mole fractions in the liquid phase. If the gas phase may be assumed for simplicity to be a perfect mixture and if the liquid phase is an ideal solution, then the eight variables are related by the following six equations Pa+Pb+Pc=P. 

As mentioned in 7 3 it is always possible to discuss the equilibrium of a reaction in solution in terms of the partial pressures in the saturated vapour above the solution (provided that this vapour is a perfect mixture). However, for many purposes it is more useful to express the equilibrium constant of a liquid phase reaction directly in terms of the composition of the liquid. This is done by substituting in equation (10 1) any of the appropriate expressions for the chemical potential of a component of a solution which have been developed in the last two chapters. It will save space if the equations are developed in a general form applicable to a non-ideal solution. The limiting forms of these expressions appljdng to reaction equilibrium in an ideal solution may be obtained by putting the activity coefficients equal to unity. 

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