Activities and Standard States of Strong Electrolytes

This behavior suggests as a convenient standard state, a hypothetical solution that obeys equation (6.112) when m = 1. In this standard state 

In the limit of infinitely dilute solutions, where equation (6.112) holds, m2 —f2/k2. If we maintain this ratio as our definition of activity, a2. then a2 = m2 in these solutions. For solutions which are not in the limiting region, we writecc 

The limiting law dependence of a2 on nr and a better understanding of the nature of 72 can be obtained by considering the equilibrium that is present when the electrolyte dissociates. For HC1, this equilibrium is 

It is the H+ and Cl that exist as independent species in solution. They can each be assigned a Henry s law standard state. That is 

The activity of the HC1 (aq) is obtained from the equilibrium process represented by equation (6.115). The free energy change for this reaction is given by 

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