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title: "A student prepares a water-quality sensor calibration solution containing both \\(0.100\\text{ M}\\) \\(\\text{CH}_3\\text{COOH}\\) and \\(0.100\\text{ M}\\) \\(\\text{CH}_3\\text{COO}^-\\) in a total volume of \\(100.0\\text{ mL}\\). The equilibrium in the solution is represented by the following equation.  \\(\\text{CH}_3\\text{COOH(aq)}+\\text{H}_2\\text{O(l)}\\rightleftharpoons\\text{H}_3\\text{O}^+\\text{(aq)}+\\text{CH}_3\\text{COO}^-\\text{(aq)}\\)  The student adds \\(0.50\\text{ mL}\\) of \\(0.100\\text{ M}\\) \\(\\text{HCl(aq)}\\) to the solution. Which statement best predicts the change in pH and explains the buffer’s response?"
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date_modified: "2026-08-19T12:40:59+00:00"
---

# A student prepares a water-quality sensor calibration solution containing both \(0.100\text{ M}\) \(\text{CH}_3\text{COOH}\) and \(0.100\text{ M}\) \(\text{CH}_3\text{COO}^-\) in a total volume of \(100.0\text{ mL}\). The equilibrium in the solution is represented by the following equation.

\(\text{CH}_3\text{COOH(aq)}+\text{H}_2\text{O(l)}\rightleftharpoons\text{H}_3\text{O}^+\text{(aq)}+\text{CH}_3\text{COO}^-\text{(aq)}\)

The student adds \(0.50\text{ mL}\) of \(0.100\text{ M}\) \(\text{HCl(aq)}\) to the solution. Which statement best predicts the change in pH and explains the buffer’s response?

A student prepares a water-quality sensor calibration solution containing both \(0.100\text{ M}\) \(\text{CH}_3\text{COOH}\) and \(0.100\text{ M}\) \(\text{CH}_3\text{COO}^-\) in a total volume of \(100.0\text{ mL}\). The equilibrium in the solution is represented by the following equation.

\(\text{CH}_3\text{COOH(aq)}+\text{H}_2\text{O(l)}\rightleftharpoons\text{H}_3\text{O}^+\text{(aq)}+\text{CH}_3\text{COO}^-\text{(aq)}\)

The student adds \(0.50\text{ mL}\) of \(0.100\text{ M}\) \(\text{HCl(aq)}\) to the solution. Which statement best predicts the change in pH and explains the buffer's response?

- **A.** The pH increases slightly because the added \(\text{H}_3\text{O}^+\) shifts the equilibrium toward the reactants, causing \([\text{H}_3\text{O}^+]\) to become lower than its initial value.
- **B.** The pH increases slightly because \(\text{CH}_3\text{COOH}\) acts as a Brønsted–Lowry base and consumes the added \(\text{H}_3\text{O}^+\).
- **C.** The pH decreases slightly because \(\text{HCl}\) ionizes only partially in the buffer, limiting the amount of \(\text{H}_3\text{O}^+\) produced.
- **D.** The pH decreases slightly because \(\text{CH}_3\text{COO}^-\) consumes most of the added \(\text{H}_3\text{O}^+\), forming \(\text{CH}_3\text{COOH}\) and \(\text{H}_2\text{O}\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119512/*