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title: "A student performs a titration of a \\(25.0 \\text{ mL}\\) sample of \\(0.10 \\text{ M } \\text{CH}_3\\text{COOH(aq)}\\) (\\(K_a = 1.8 \\times 10^{-5}\\)) with \\(0.10 \\text{ M } \\text{NaOH(aq)}\\). The student must select an appropriate acid-base indicator from a laboratory chart that lists the \\(pK_a\\) and color-transition range for several candidate indicators. Which of the following pieces of information provides the most direct evidence needed to select the indicator that will change color closest to the stoichiometric equivalence point?"
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url: "https://nerd-notes.com/ubq/119737/"
date_modified: "2026-08-21T08:12:11+00:00"
---

# A student performs a titration of a \(25.0 \text{ mL}\) sample of \(0.10 \text{ M } \text{CH}_3\text{COOH(aq)}\) (\(K_a = 1.8 \times 10^{-5}\)) with \(0.10 \text{ M } \text{NaOH(aq)}\). The student must select an appropriate acid-base indicator from a laboratory chart that lists the \(pK_a\) and color-transition range for several candidate indicators. Which of the following pieces of information provides the most direct evidence needed to select the indicator that will change color closest to the stoichiometric equivalence point?

A student performs a titration of a \(25.0 \text{ mL}\) sample of \(0.10 \text{ M } \text{CH}_3\text{COOH(aq)}\) (\(K_a = 1.8 \times 10^{-5}\)) with \(0.10 \text{ M } \text{NaOH(aq)}\). The student must select an appropriate acid-base indicator from a laboratory chart that lists the \(pK_a\) and color-transition range for several candidate indicators. Which of the following pieces of information provides the most direct evidence needed to select the indicator that will change color closest to the stoichiometric equivalence point?

- **A.** The measured pH of the \(0.10 \text{ M } \text{CH}_3\text{COOH(aq)}\) solution before any base is added, because the indicator must remain entirely in its acid form at the start of the titration.
- **B.** The pH of the reaction mixture at the half-equivalence point where \(\text{pH} = pK_a\), because the indicator must begin its color transition in the buffer region of the titration.
- **C.** The calculated pH of the solution at the stoichiometric equivalence point governed by \(\text{CH}_3\text{COO}^-\text{(aq)}\), because the indicator \(pK_a\) should closely match that basic pH value.
- **D.** The assumption that the titration mixture reaches neutrality at \(\text{pH} = 7.00\), because mixing stoichiometric amounts of acid and base always yields a completely neutral solution.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119737/*