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ION–ION INTERACTIONS 261

(observed) behavior of electrolytes in solution can be linked to the behavior expected
if there were zero electrostatic interactions between ions in solution.
During this presentation, the experimental mean activity coefficients showing up
in the deductions have been taken for granted—nothing has been said about how they
have been obtained. Obviously, one must be sure when dealing with theory that the
experimental values with which the theory is compared are soundly based.
It is time then that some account be given about the means by which experimental
values of activity coefficients are known. Only two methods will be presented because
the material contains no new ideas and is only presented so the reader is assured that
the ground is firm.

3.4.7. How to Obtain Solute Activities from Data on Solvent Activities
A characteristic of an ionic solution is that any vapor pressure due to the dissolved
electrolyte itself is effectively zero. The vapor pressure of the solvent in the solution
therefore falls with increasing concentration of the electrolyte in the solution. Thus,
the solvent vapor pressure in the solution will be less than the vapor pressure of the
pure solvent because the nonvolatile ions block out part of the surface from which, in
the pure solvent, solvent molecules would evaporate.
Now, there is nothing mysterious about a solvent activity. It is determined by

where is the vapor pressure of pure solvent and is the vapor pressure of the solvent
when it is a component of a solution. The relation of to in the ideal condition
will be governed by Raoult’s law, that is,