In a titration of a weak acid with a strong base, what characterizes the buffer region?

• A. The solution contains significant HA and A-, so pH changes slowly due to buffering.
• B. The base added is consumed quickly and pH stays near 7.
• C. The pH is fixed at the acid's pKa.
• D. The conjugate base dominates and pH climbs above 10.

Answer: (A)

During a titration of a weak acid with a strong base, the buffer region shows up when both the weak acid (HA) and its conjugate base (A−) are present in appreciable amounts. As the base is added, it converts some HA into A−, creating a mixture that resists pH change because the added hydroxide is largely consumed by this HA ⇌ H2O + A− equilibrium. The pH in this region follows the Henderson–Hasselbalch relationship, pH ≈ pKa + log([A−]/[HA]); when the two species are in comparable amounts, the ratio is near 1 and the pH sits around the acid’s pKa. The pH is not fixed at 7, and it does not stay perfectly constant; it changes gradually as the ratio shifts with continued addition of base. The other descriptions misstate the behavior: the pH won’t be fixed at the acid’s pKa nor held at 7, and the idea that the base is consumed instantly or that the pH climbs above 10 throughout the buffer region isn’t characteristic of this phase.