CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Carbonic acid, H₂CO₃(aq), is a diprotic acid with dissociation constants K_a1 = 4.3 × 10⁻⁷ and K_a2 = 4.8 × 10⁻¹¹ at 25 °C. A 0.100 M solution of carbonic acid is prepared as an analyte for a titration, and the chemist records the pH before any titrant is introduced.
(a) Describe qualitatively why the first dissociation step, not the second, is expected to dominate the initial pH of the solution.
(b) Using only the dominant dissociation, calculate the initial pH of the 0.100 M carbonic acid solution. Show all reasoning and approximations.
(c) Without performing an additional full calculation, explain how including the second dissociation would affect the calculated pH and why the magnitude of that effect is small.