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title: "A student prepares an aqueous buffer solution containing propanoic acid, \\(\\text{CH}_3\\text{CH}_2\\text{COOH}\\) (\\(\\text{p}K_a = 4.87\\)), and sodium propanoate, \\(\\text{NaCH}_3\\text{CH}_2\\text{COO}\\). The measured \\(\\text{pH}\\) of the buffer solution is \\(5.50\\). Which of the following correctly compares \\([\\text{CH}_3\\text{CH}_2\\text{COO}^-]\\) and \\([\\text{CH}_3\\text{CH}_2\\text{COOH}]\\) in the solution, and provides the correct justification?"
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url: "https://nerd-notes.com/ubq/119760/"
date_modified: "2026-08-21T08:12:15+00:00"
---

# A student prepares an aqueous buffer solution containing propanoic acid, \(\text{CH}_3\text{CH}_2\text{COOH}\) (\(\text{p}K_a = 4.87\)), and sodium propanoate, \(\text{NaCH}_3\text{CH}_2\text{COO}\). The measured \(\text{pH}\) of the buffer solution is \(5.50\). Which of the following correctly compares \([\text{CH}_3\text{CH}_2\text{COO}^-]\) and \([\text{CH}_3\text{CH}_2\text{COOH}]\) in the solution, and provides the correct justification?

A student prepares an aqueous buffer solution containing propanoic acid, \(\text{CH}_3\text{CH}_2\text{COOH}\) (\(\text{p}K_a = 4.87\)), and sodium propanoate, \(\text{NaCH}_3\text{CH}_2\text{COO}\). The measured \(\text{pH}\) of the buffer solution is \(5.50\). Which of the following correctly compares \([\text{CH}_3\text{CH}_2\text{COO}^-]\) and \([\text{CH}_3\text{CH}_2\text{COOH}]\) in the solution, and provides the correct justification?

- **A.** \([\text{CH}_3\text{CH}_2\text{COO}^-] > [\text{CH}_3\text{CH}_2\text{COOH}]\), because the \(\text{pH}\) of the solution is less than \(7.00\).
- **B.** \([\text{CH}_3\text{CH}_2\text{COO}^-] > [\text{CH}_3\text{CH}_2\text{COOH}]\), because \(\text{pH} > \text{p}K_a\), which requires \(\log\left(\dfrac{[\text{CH}_3\text{CH}_2\text{COO}^-]}{[\text{CH}_3\text{CH}_2\text{COOH}]}\right) > 0\).
- **C.** \([\text{CH}_3\text{CH}_2\text{COOH}] > [\text{CH}_3\text{CH}_2\text{COO}^-]\), because \(\text{pH} > \text{p}K_a\), which requires the concentration of the acid to exceed the concentration of the conjugate base.
- **D.** \([\text{CH}_3\text{CH}_2\text{COOH}] > [\text{CH}_3\text{CH}_2\text{COO}^-]\), because the \(\text{pH}\) of the solution is less than \(7.00\), indicating that the acid form must predominate.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119760/*