A laboratory comparison study examines the behavior of three aqueous solutions when…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A laboratory comparison study examines the behavior of three aqueous solutions when each is challenged with the same quantity of strong acid.
• Buffer A: 0.30 M propanoic acid (pK_a = 4.87) and 0.30 M sodium propanoate, 1.0 L total volume
• Buffer B: 0.10 M propanoic acid and 0.30 M sodium propanoate, 1.0 L total volume
• Buffer C: 0.30 M propanoic acid and 0.10 M sodium propanoate, 1.0 L total volume
• Control Solution: 0.40 M NaCl, 1.0 L total volume (contains no acid–base pair)
Each solution receives a single addition of 0.010 mol HCl(aq).
(a) Rank Buffer A, Buffer B, Buffer C, and the control solution from the smallest to the largest expected change in pH after the HCl addition. Justify your ranking qualitatively without performing numerical calculations.
(b) For Buffer B and Buffer C, both of which have the same total concentration of the conjugate acid–base pair, explain how the initial ratio of [A−]/[HA] influences the magnitude and direction of the pH change upon addition of the strong acid.
(c) Identify whether the absolute concentration of the conjugate species or the [A−]/[HA] ratio is the dominant factor governing buffer capacity in this experiment. Support your answer with chemical reasoning tied to the data given.