CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A diprotic acid H₂A (pKa₁ = 2.19, pKa₂ = 6.32) is titrated at 25 °C with 0.150 M NaOH. A 35.0 mL aliquot of 0.0750 M H₂A is placed in a flask and titrated under constant stirring.
(a) Calculate the volume of NaOH required to reach the first stoichiometric equivalence point of the titration.
(b) A second titration is performed under identical conditions, and exactly 26.3 mL of NaOH are added. Demonstrate quantitatively that this volume places the system halfway between the first and second equivalence points.
(c) Determine the pH of the solution after the 26.3 mL addition. Show all reasoning, including any stoichiometric and equilibrium considerations used.
(d) Explain why the pH obtained in part (c) is largely independent of the initial concentration of H₂A, provided the titration mixture remains in the region between the first and second equivalence points.