NANTeL Chemistry Certification – Engineering Fundamentals Practice Test

In this titration, the equivalence point is determined by the behavior of the species present after all the weak acid has been converted to its conjugate base. The conjugate base of the weak acid reacts with water: A− + H2O ⇌ HA + OH−. This hydrolysis produces hydroxide ions, making the solution basic and giving a pH greater than 7. The reason this happens is tied to Ka and Kb: a weak acid (small Ka) has a conjugate base with the ability to accept protons (generate OH− in water) more readily than water itself, so the overall solution shifts toward OH−. If the conjugate acid were dominating, you’d expect additional H3O+ and a pH below 7, which isn’t the situation here. Water autoionization alone can’t override the base hydrolysis that occurs at equivalence, and a strong acid–strong base titration would yield pH 7 at equivalence, not this case.