In titrating a weak acid with a strong base, different calculation methods are applied at various stages. Initially, the pH of a weak acid like acetic acid is calculated using its dissociation constant (K_a) and an ICE table. Upon addition of a strong base such as sodium hydroxide, a buffer forms, and its pH is determined using the Henderson-Hasselbalch equation. As more base is added and the titration reaches the halfway point, the pH becomes equal to the pK_a of the acid, indicating equal concentrations of the acid and its conjugate base. At the equivalence point, all the acid is converted to its conjugate base, and the pH is calculated using the base's dissociation constant (K_b) and an ICE table. Beyond the equivalence point, the pH is governed by the concentration of the excess strong base.