In the above analogy, the undissociated and dissociated couples represented solutes in an equilibrium and the little kids represented spectator ions. Note that for the little kids to affect the equilibrium, there had to be an attraction between the little kids and the individual adult skaters. Ions in solution have electrostatic interactions with other ions. Neutral solutes do not have such interactions. When the concentrations of ions in a solution is greater than approximately 0.001 M, a shielding effect occurs around ions similar to the little kids around individual adults.
Cations tend to be surrounded by nearby anions and anions tend to be surrounded by nearby cations.
This shielding effect becomes significant at ion concentrations of 0.01 M and greater. Note that the charge is squared so that positive and negative charges do not cancel. Now, we could tabulate equilibrium constants for acid dissociation equilibria at different ionic strengths, but that would be a lot of work and create huge tables. These activity coefficients serve as correction factors so that equilibrium constants measured in an ideal solution can be used to predict equilibria in non-ideal solutions. Since benzoic acid is a weak acid, it will exist mostly as C 6 H 5 COOH in water, so the concentration of ions will be very small.