## Chemistry: The Central Science (13th Edition)

Drawing number $(2)$ represents a situation that cannot arise from the addition of either acids or bases.
The sum: $[HA] + [A^-]$ has to be constant, because we are not removing anything. The only thing the addition of an acid or base can do, is to reduce the concentrations of one of them, and, in the same proportion, raise the concentration of the other one.