---
title: "| Weak Acid | Formula | \\(K_a\\) at \\(25^\\circ\\text{C}\\) | | :— | :— | :— | | Acetic acid | \\(\\text{CH}_3\\text{COOH}\\) | \\(1.8 \\times 10^{-5}\\) | | Dihydrogen phosphate ion | \\(\\text{H}_2\\text{PO}_4^-\\)| \\(6.2 \\times 10^{-8}\\) | | Ammonium ion | \\(\\text{NH}_4^+\\) | \\(5.6 \\times 10^{-10}\\) | | Hydrogen carbonate ion | \\(\\text{HCO}_3^-\\) | \\(4.7 \\times 10^{-11}\\) |  A biochemist is preparing a buffer solution to maintain a stable \\(\\text{pH}\\) of \\(7.40\\) for an in vitro enzyme assay. The table above lists the acid-dissociation constants (\\(K_a\\)) for four weak acids at \\(25^\\circ\\text{C}\\). Which of the following conjugate acid-base pairs is most suitable for preparing this buffer, and why?"
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url: "https://nerd-notes.com/ubq/123778/"
date_modified: "2026-09-28T12:30:16+00:00"
---

# | Weak Acid | Formula | \(K_a\) at \(25^\circ\text{C}\) |
| :— | :— | :— |
| Acetic acid | \(\text{CH}_3\text{COOH}\) | \(1.8 \times 10^{-5}\) |
| Dihydrogen phosphate ion | \(\text{H}_2\text{PO}_4^-\)| \(6.2 \times 10^{-8}\) |
| Ammonium ion | \(\text{NH}_4^+\) | \(5.6 \times 10^{-10}\) |
| Hydrogen carbonate ion | \(\text{HCO}_3^-\) | \(4.7 \times 10^{-11}\) |

A biochemist is preparing a buffer solution to maintain a stable \(\text{pH}\) of \(7.40\) for an in vitro enzyme assay. The table above lists the acid-dissociation constants (\(K_a\)) for four weak acids at \(25^\circ\text{C}\). Which of the following conjugate acid-base pairs is most suitable for preparing this buffer, and why?

| Weak Acid | Formula | \(K_a\) at \(25^\circ\text{C}\) |
| :--- | :--- | :--- |
| Acetic acid | \(\text{CH}_3\text{COOH}\) | \(1.8 \times 10^{-5}\) |
| Dihydrogen phosphate ion | \(\text{H}_2\text{PO}_4^-\)| \(6.2 \times 10^{-8}\) |
| Ammonium ion | \(\text{NH}_4^+\) | \(5.6 \times 10^{-10}\) |
| Hydrogen carbonate ion | \(\text{HCO}_3^-\) | \(4.7 \times 10^{-11}\) |

A biochemist is preparing a buffer solution to maintain a stable \(\text{pH}\) of \(7.40\) for an in vitro enzyme assay. The table above lists the acid-dissociation constants (\(K_a\)) for four weak acids at \(25^\circ\text{C}\). Which of the following conjugate acid-base pairs is most suitable for preparing this buffer, and why?

- **A.** \(\text{CH}_3\text{COOH} / \text{CH}_3\text{COO}^-\), because \(\text{CH}_3\text{COOH}\) has the largest \(K_a\) value among the choices, providing the greatest concentration of hydronium ions to stabilize the system against additions of base.
- **B.** \(\text{NH}_4^+ / \text{NH}_3\), because the \(pK_a\) of \(\text{NH}_4^+\) is greater than \(7.40\), ensuring that the conjugate base \(\text{NH}_3\) is present in higher concentration than \(\text{NH}_4^+\) to neutralize metabolic acids.
- **C.** \(\text{H}_2\text{PO}_4^- / \text{HPO}_4^{2-}\), because \(\text{H}_2\text{PO}_4^-\text{(aq)}\) is a polyprotic species capable of donating two protons simultaneously to neutralize added hydroxide ions.
- **D.** \(\text{H}_2\text{PO}_4^- / \text{HPO}_4^{2-}\), because the \(pK_a\) of \(\text{H}_2\text{PO}_4^-\) is approximately \(7.21\), which allows the ratio \([\text{HPO}_4^{2-}]/[\text{H}_2\text{PO}_4^-]\) to be close to \(1\) at \(\text{pH} = 7.40\), maximizing buffer capacity against both added acid and base.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123778/*