Relationship of Solute Activity to Single Ion Activities

This defines the equilibrium constant for the reaction between component HCl and the ions and Cl . It follows then that for the reaction 

As usual, it is best to see the truth of a relationship by nnderstanding it rather than by seeing no fault with its derivation. In this case this can be accomplished by realizing that in the ideal one molal standard state to which ArG° refers, the solute component HCl consists in solution entirely as H and Cl, therefore the G of component HCl has no choice but to be identical to Gh+ + Gci, from which it follows that A G° = 0. 

In the solution chemistry literature it is usual to refer to the solvent in a binary system as component I, and to the solute (as normally written) as component 2. Therefore in this case onci could also be referred to as 02. 

We can now see why ohojCIos is called a ,Hci- It comes from the concept of individual ion activities, because if 

The great convenience of being able to think in terms of individual ionic properties means that we always look at ionic solutions this way, rather than as a simple changing of solute component. 

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