The titration of a weak acid, HA ( K _a = 1.0×10^{- 4}), with NaOH requires 40.0 mL to reach the equivalence point. In order to calculate the pH of the system after 20 mL of NaOH is added, the best strategy is to recognize that the system is

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Since the weak acid is being titrated with NaOH, which is a strong base, the equivalence point will result in a basic solution. However, before the equivalence point, we still have a weak acid present. Therefore, the system is a buffer (a weak acid with its conjugate base) before the equivalence point. To calculate the pH after 20 mL of NaOH is added, we need to use the Henderson-Hasselbalch equation for a buffer solution.

The Titration of a Weak Acid, HA ( K a

The Titration of a Weak Acid, HA ( K _a