Answer :
Final answer:
At the half-equivalence point of a titration involving a weak acid and a strong base, the pH equals pKa, which occurs at 12.3 mL of the added base since this is half of the volume at the equivalence point.
Explanation:
The pH of a solution at which the concentration of the weak acid is equal to the concentration of its conjugate base is known as the half-equivalence point. At this point, pH is equal to pKa.
Since the equivalence point in this titration occurs at 24.6 mL of strong base, the half-equivalence point would be half that volume, at 12.3 mL.
Therefore, the pH equals pKa at 12.3 mL of added base.
In a titration involving a weak acid and a strong base, the point where pH = pKa is precisely when half the amount of base required to reach the equivalence point has been added because half of the weak acid has been neutralized to form its conjugate base, resulting in the formation of a buffer system.