CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A weak diprotic acid, H2A, is titrated with 0.100 mol L⁻¹ KOH. A 40.00 mL aliquot of 0.0500 mol L⁻¹ H2A is placed in a beaker, and the titration is carried out at constant temperature.
The titration curve shows two distinct equivalence points corresponding to the stepwise neutralization of each acidic proton.
(1) Derive the theoretical volume of KOH (in mL) required to reach the first equivalence point, at which only HA− remains in solution.
(2) Using your result from part (a), calculate the additional volume of KOH needed to reach the second equivalence point, where the solution contains only A2−. Express the cumulative volume added from the start of the titration.
(3) Explain qualitatively why the two equivalence points occur at different pH values, even though the stoichiometric amounts of KOH added between them are equal.