In what scenario is the Henderson-Hasselbalch equation not applicable?

The Henderson-Hasselbalch equation is primarily used to estimate the pH of buffer solutions, which contain a weak acid and its conjugate base or a weak base and its conjugate acid. It is relevant for scenarios involving weak acids and bases in equilibrium with their respective conjugate species.

At the equivalence point in a titration, a strong acid has completely reacted with a strong base, resulting in a solution that contains only the salt formed from the reaction or possibly additional ions from the titrant. At this stage, the pH is determined by the properties of the resulting salt and the ions present in the solution, not by the concentrations of the original weak acid and conjugate base. Since the Henderson-Hasselbalch equation relies on having both a weak acid and its conjugate base in solution, it is not applicable at the equivalence point where these components are no longer in the solution in the form needed for the equation to be valid.

For the other options, the equation is applicable: determining the pH of a buffer solution involves using the concentrations of the weak acid and its conjugate base; calculating pH at various points in a titration with a buffer can be done if not at the equivalence point;