percent dissociation
This is one of those questions where I think you'll need to know more chemistry than you probably have been taught. 

The freezing point depression formula is only usable (i.e. linear) for "dilute" solutions. As the concentrations of solutes become larger, the relationship becomes non-linear. 

So when $\ce{NaCl}$ is dissolved in water all the sodium and chloride ions are truly dissociated. There are no "molecules" of $\ce{NaCl}$ floating around in the aqueous solution. 

Imagine the solution this way. Instead of the water molecules bouncing around in the solution more or less randomly, the positive charged $\ce{Na^+}$ ion attracts the more negative oxygen atom of the water molecule leaving the positive hydrogen atoms on the outside of this first "shell" of ligands. Now a second "shell" of water molecules will be attracted to the first (Oxygen to hydrogens) and so on. Now each shell has more and more water molecules but the binding energy decreases as the shell number increases. So the gist is that there is an exponential decrease in the ordering of the water molecules. Ultimately the ordering becomes random again as in the bulk of the water. 

Now in dilute solutions these spheres of water around each ion are separated by so much water that they never interact. But as the solution becomes more concentrated then these spheres of water do start to interact. The interaction is measured by what is known as an activity coefficient.