A student titrates 25 mL of a weak acid (HA) with NaOH and produces the curve shown. Between 5 and 15 mL of base added, the pH changes only slightly. The student is asked to identify the species responsible for resisting pH change in this region.
What chemical principle explains the flat region of the curve?
- A
Water autoionization buffers any added base
- B
NaOH precipitates out of solution and stops reacting
- Ccheck_circle
Conjugate base A- consumes added OH- protons indirectly while HA donates H+ to consume OH-
- D
The acid completely dissociates, so pH cannot change
Explanation
A buffer contains roughly equal HA and A-; HA neutralizes added OH- by donating H+, while A- would neutralize added H+, keeping pH nearly constant near the pKa.