If you will recall, the definition of the activity coefficient is the deviation of the activity of the current solution to some reference solution. Since the activity coefficient is only valid at low concentrations, the function being used is close to or moving towards the reference system. Therefore, the activity coefficient is going towards 1 and is usually put in the equation that way. So Allowing the activity to equal our function makes the calculations much easier. Using the function as our activity is a nice and convenient calculation, but it is not the true value.
Real systems can be very complex.
The next slide shows a reference system verses a possible real system. In the real system we would have to take into account the interactions between ions and the structure of the solvent in response to all the ions. But for the purposes of class, we will only worry about the calculations which involve a simpler system that is similar to the reference system.