Partial pressure of a volatile electrolyte

In 10 10 it was shown that the equilibrium of an electrol3rte in a saturated solution with its pure solid phase depends on /e, the chemical potential of the electrolyte as a whole. Similar considerations apply to the equilibrium of a volatile electrolyte, such as hydrochloric acid, with its vapour. 

Let fi be the chemical potential of hydrogen chloride in the vapour phase. For equilibrium with the solution 

The quantities on the right-hand side of this equation are independent of composition. Hence 

In principle, equation (10 85) can be used for the calculation of y as soon as this limit has been determined. The difficulty is to determine K with any certainty because the partial pressures become almost immeasurably small in a region of concentration where y. is still appreciably different from unity. f On the other hand, if values of for dilute solutions are taken from some independent source (e.g. e.m.f. measurements at the same temperature), the equation can be used for the calculation of y in more concentrated solutions, using the measured partial pressures. 

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