---
title: "A student prepares a series of buffer solutions containing ethanoic acid, \\(\\text{CH}_3\\text{COOH}\\), and sodium ethanoate, \\(\\text{CH}_3\\text{COONa}\\), keeping the total concentration of the conjugate pair constant at \\(0.20\\text{ M}\\). The \\(\\text{p}K_a\\) of \\(\\text{CH}_3\\text{COOH}\\) is \\(4.74\\). Which of the following best explains why the buffer solution with \\(\\text{pH} = 4.74\\) has the greatest capacity to resist \\(\\text{pH}\\) changes upon the addition of equal amounts of either strong acid or strong base?"
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url: "https://nerd-notes.com/ubq/119877/"
date_modified: "2026-08-21T08:16:51+00:00"
---

# A student prepares a series of buffer solutions containing ethanoic acid, \(\text{CH}_3\text{COOH}\), and sodium ethanoate, \(\text{CH}_3\text{COONa}\), keeping the total concentration of the conjugate pair constant at \(0.20\text{ M}\). The \(\text{p}K_a\) of \(\text{CH}_3\text{COOH}\) is \(4.74\). Which of the following best explains why the buffer solution with \(\text{pH} = 4.74\) has the greatest capacity to resist \(\text{pH}\) changes upon the addition of equal amounts of either strong acid or strong base?

A student prepares a series of buffer solutions containing ethanoic acid, \(\text{CH}_3\text{COOH}\), and sodium ethanoate, \(\text{CH}_3\text{COONa}\), keeping the total concentration of the conjugate pair constant at \(0.20\text{ M}\). The \(\text{p}K_a\) of \(\text{CH}_3\text{COOH}\) is \(4.74\). Which of the following best explains why the buffer solution with \(\text{pH} = 4.74\) has the greatest capacity to resist \(\text{pH}\) changes upon the addition of equal amounts of either strong acid or strong base?

- **A.** The solution is neutral, because the concentration of \(\text{H}_3\text{O}^+\) is equal to the concentration of \(\text{OH}^-\) at this \(\text{pH}\).
- **B.** The concentration of \(\text{CH}_3\text{COOH}\) is much greater than that of \(\text{CH}_3\text{COO}^-\), because an excess of weak acid is required to prevent large \(\text{pH}\) shifts upon adding a strong base.
- **C.** The weak acid \(\text{CH}_3\text{COOH}\) has completely dissociated, because all weak acid molecules are converted into conjugate base when \(\text{pH} = \text{p}K_a\).
- **D.** The concentrations of \(\text{CH}_3\text{COOH}\) and \(\text{CH}_3\text{COO}^-\) are equal, because the molar ratio \(\dfrac{[\text{CH}_3\text{COO}^-]}{[\text{CH}_3\text{COOH}]} = 1\) allows the buffer to neutralize both added \(\text{OH}^-\) and added \(\text{H}_3\text{O}^+\) with equal effectiveness.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119877/*