Half the base needed for equivalence has been added to a weak acid. What is the pH of the mixture?

1. Half the equivalence volume converts half the acid into its conjugate base. 2. The mixture therefore holds equal moles of the weak acid and its conjugate base. 3. Both sit in the same volume, so their concentrations are equal as well. 4. That mixture is a buffer, so the Henderson and Hasselbalch equation applies. 5. The equation gives pH as pKa plus the logarithm of the base to acid ratio. 6. The ratio is one, and the logarithm of one is zero. 7. So the pH equals the pKa exactly, which for acetic acid is 4.74. 8. This half equivalence point is the standard way to read a pKa off a titration curve. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.7 Acid-Base Titrations_