During an acid-base titration, when are equivalent quantities of hydronium ions and hydroxide ions present?

In an acid-base titration, equivalent quantities of hydronium ions and hydroxide ions are present at the endpoint. This is the point at which the amount of acid and base added together is stoichiometrically equivalent, resulting in complete neutralization.

At the endpoint, the reaction reaches a balanced state where the moles of hydronium ions (H3O+) from the acid neutralize the moles of hydroxide ions (OH−) from the base. This means that the solution contains neither excess acid nor excess base, and the resulting solution is usually neutral.

The midpoint, which was chosen as the answer, actually refers to a point in the titration where half of the acid has been neutralized when a weak acid is titrated with a strong base. At this point, the concentrations of the weak acid's conjugate base and remaining acid are equal, but the total amounts of hydronium and hydroxide ions are not equivalent.

In summary, at the endpoint, the system achieves a state of equality between the amounts of hydronium and hydroxide ions due to complete neutralization, which is the defining characteristic of this stage in the titration process.