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title: "A student prepares an aqueous buffer solution by mixing \\(100.\\text{ mL}\\) of \\(0.20\\text{ M }\\text{HC}_2\\text{H}_3\\text{O}_2\\text{(aq)}\\) with \\(100.\\text{ mL}\\) of \\(0.80\\text{ M }\\text{NaC}_2\\text{H}_3\\text{O}_2\\text{(aq)}\\) at \\(25\\ ^\\circ\\text{C}\\).  The student then divides the resulting mixture into two equal \\(100.\\text{ mL}\\) samples. To Sample 1, the student adds \\(0.020\\text{ mol}\\) of \\(0.020\\text{ mol}\\) \\(\\text{HCl}\\). To Sample 2, the student adds \\(0.020\\text{ mol}\\) of \\(\\text{NaOH}\\). Assuming the volume changes are negligible, which of the following best predicts the effect of these additions on the \\(\\text{pH}\\) of the two samples?"
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url: "https://nerd-notes.com/ubq/119977/"
date_modified: "2026-08-21T08:31:26+00:00"
---

# A student prepares an aqueous buffer solution by mixing \(100.\text{ mL}\) of \(0.20\text{ M }\text{HC}_2\text{H}_3\text{O}_2\text{(aq)}\) with \(100.\text{ mL}\) of \(0.80\text{ M }\text{NaC}_2\text{H}_3\text{O}_2\text{(aq)}\) at \(25\ ^\circ\text{C}\).

The student then divides the resulting mixture into two equal \(100.\text{ mL}\) samples. To Sample 1, the student adds \(0.020\text{ mol}\) of \(0.020\text{ mol}\) \(\text{HCl}\). To Sample 2, the student adds \(0.020\text{ mol}\) of \(\text{NaOH}\). Assuming the volume changes are negligible, which of the following best predicts the effect of these additions on the \(\text{pH}\) of the two samples?

A student prepares an aqueous buffer solution by mixing \(100.\text{ mL}\) of \(0.20\text{ M }\text{HC}_2\text{H}_3\text{O}_2\text{(aq)}\) with \(100.\text{ mL}\) of \(0.80\text{ M }\text{NaC}_2\text{H}_3\text{O}_2\text{(aq)}\) at \(25\ ^\circ\text{C}\).

The student then divides the resulting mixture into two equal \(100.\text{ mL}\) samples. To Sample 1, the student adds \(0.020\text{ mol}\) of \(0.020\text{ mol}\) \(\text{HCl}\). To Sample 2, the student adds \(0.020\text{ mol}\) of \(\text{NaOH}\). Assuming the volume changes are negligible, which of the following best predicts the effect of these additions on the \(\text{pH}\) of the two samples?

- **A.** Sample 1 remains an effective buffer because the moles of \(\text{C}_2\text{H}_3\text{O}_2^-\text{(aq)}\) exceed the moles of added \(\text{H}^+\text{(aq)}\), whereas Sample 2 experiences a large increase in \(\text{pH}\) because the moles of added \(\text{OH}^-\text{(aq)}\) exceed the moles of \(\text{HC}_2\text{H}_3\text{O}_2\text{(aq)}\) present.
- **B.** Sample 2 remains an effective buffer because the added \(\text{OH}^-\text{(aq)}\) is consumed by \(\text{C}_2\text{H}_3\text{O}_2^-\text{(aq)}\), whereas Sample 1 experiences a large decrease in \(\text{pH}\) because all \(\text{HC}_2\text{H}_3\text{O}_2\text{(aq)}\) is consumed.
- **C.** Both samples maintain a nearly constant \(\text{pH}\) because the total concentration of buffer components is greater than the concentration of added strong acid or strong base.
- **D.** Sample 1 experiences a larger change in \(\text{pH}\) than Sample 2 because \(\text{HCl}\) is a stronger electrolyte than \(\text{NaOH}\), causing it to ionize more completely and overwhelm the buffer.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119977/*