CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A diprotic acid, H₂A, undergoes two sequential deprotonation steps in aqueous solution:
H₂A ⇌ H⁺ + HA⁻ (first deprotonation)
HA⁻ ⇌ H⁺ + A²⁻ (second deprotonation)
A chemist prepares a solution in which the equilibrium concentrations are determined to be
[H₂A] = 0.010 M, [HA⁻] = 0.020 M, [A²⁻] ≈ 0 M (below detection limits).
Use the information provided to investigate the relative magnitudes of the solution pH, pKa1, and pKa2.
(a) Compare the first and second deprotonation steps in terms of their extent in this solution. Cite the concentration data as evidence.
(b) Determine whether the solution pH is numerically closer to pKa1 or to pKa2. Support your answer by discussing the observed distribution of protonation states.
(c) Explain why the negligible concentration of A²⁻ places a constraint on the possible numerical relationship among pH, pKa1, and pKa2.