In a titration of a weak acid with a strong base, how is the pH determined before reaching the equivalence point?

• A. The pH is determined by the concentration of water.
• B. The pH is determined by the concentration of the strong base alone.
• C. The pH is determined by the buffer system formed by the weak acid and its conjugate base.
• D. The pH is determined by the concentration of the weak acid alone.

Answer: (C)

Verified step by step guidance

1. Understand that in a titration of a weak acid with a strong base, a buffer system is formed before reaching the equivalence point. This buffer system consists of the weak acid and its conjugate base.
2. Recognize that the pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation: $p_H=p_{K}a+log\left(\frac{\mathrm{[A^-]}}{\mathrm{[HA]}}\right)$, where $p_{K}a$ is the acid dissociation constant, $\mathrm{[A^-]}$ is the concentration of the conjugate base, and $\mathrm{[HA]}$ is the concentration of the weak acid.
3. Calculate the concentrations of the weak acid and its conjugate base. As the strong base is added, it reacts with the weak acid to form its conjugate base. Use stoichiometry to determine the amounts of acid and conjugate base present.
4. Apply the Henderson-Hasselbalch equation using the calculated concentrations to find the pH of the solution. This equation accounts for the buffer system and provides a way to determine the pH before reaching the equivalence point.
5. Remember that the pH will gradually increase as more strong base is added, but the buffer system will resist drastic changes in pH until the equivalence point is approached.