Answer: a. about half of the buffer molecules are dissociated.
Explanation:
Buffers work like this in a general way:
HA ---- A- + H+
This reaction can go both ways, and that explains the buffer's capacity for regulating pH.
When about half the molecules are dissociated, it means the equation above is in an equilibrium. The buffer works best this way because it can easily go either way to balance out the extra OH- or H+ added.
If more than half of the molecules were dissociated, the equation would find it hard to go even further to the right and balance the H+ added.
If less than half of the molecules were dissociated, the equation would find it hard to go ever further left and balance the OH- added.
