Chemistry

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

\bigl[\text{A}^-\bigr]\!/\bigl[\text{HA}\bigr]

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

\bigl[\text{A}^-\bigr]\!/\bigl[\text{HA}\bigr]

ratio is the dominant factor governing buffer capacity in this experiment. Support your answer with chemical reasoning tied to the data given.

Intuitive explanation

Buffer capacity—the ability to resist pH change when acid or base is added—depends on how much conjugate base is available to neutralize the incoming acid. Buffers A, B, and C all use the same weak acid–conjugate base pair, so they start at similar pH values (near the pK $_a$ ), but their amounts of propanoate ion differ dramatically. When HCl is added, it consumes propanoate; the solution with the most propanoate will lose the smallest fraction of it, and therefore experience the smallest pH drop. Compare the three buffers' capacities by tracking how much conjugate base each has to "spend."

Intuitive explanation

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