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title: "A student titrates \\(25.0 \\text{ mL}\\) of \\(0.10 \\text{ M NH}_3\\text{(aq)}\\) with \\(0.10 \\text{ M HCl(aq)}\\) at \\(25^\\circ\\text{C}\\). The net ionic equation for the titration reaction is \\[ \\text{NH}_3\\text{(aq)} + \\text{H}_3\\text{O}^+\\text{(aq)} \\rightarrow \\text{NH}_4^+\\text{(aq)} + \\text{H}_2\\text{O(l)} \\] The student considers two acid-base indicators: methyl red (\\(pK_a = 5.0\\), color transition range \\(\\text{pH } 4.4\\text{ to } 6.2\\)) and phenolphthalein (\\(pK_a = 9.4\\), color transition range \\(\\text{pH } 8.2\\text{ to } 10.0\\)). Which of the following claims and justifications correctly identifies the more suitable indicator for detecting the equivalence point of this titration?"
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url: "https://nerd-notes.com/ubq/123784/"
date_modified: "2026-09-28T12:30:17+00:00"
---

# A student titrates \(25.0 \text{ mL}\) of \(0.10 \text{ M NH}_3\text{(aq)}\) with \(0.10 \text{ M HCl(aq)}\) at \(25^\circ\text{C}\). The net ionic equation for the titration reaction is
\[ \text{NH}_3\text{(aq)} + \text{H}_3\text{O}^+\text{(aq)} \rightarrow \text{NH}_4^+\text{(aq)} + \text{H}_2\text{O(l)} \]
The student considers two acid-base indicators: methyl red (\(pK_a = 5.0\), color transition range \(\text{pH } 4.4\text{ to } 6.2\)) and phenolphthalein (\(pK_a = 9.4\), color transition range \(\text{pH } 8.2\text{ to } 10.0\)). Which of the following claims and justifications correctly identifies the more suitable indicator for detecting the equivalence point of this titration?

A student titrates \(25.0 \text{ mL}\) of \(0.10 \text{ M NH}_3\text{(aq)}\) with \(0.10 \text{ M HCl(aq)}\) at \(25^\circ\text{C}\). The net ionic equation for the titration reaction is
\[ \text{NH}_3\text{(aq)} + \text{H}_3\text{O}^+\text{(aq)} \rightarrow \text{NH}_4^+\text{(aq)} + \text{H}_2\text{O(l)} \]
The student considers two acid-base indicators: methyl red (\(pK_a = 5.0\), color transition range \(\text{pH } 4.4\text{ to } 6.2\)) and phenolphthalein (\(pK_a = 9.4\), color transition range \(\text{pH } 8.2\text{ to } 10.0\)). Which of the following claims and justifications correctly identifies the more suitable indicator for detecting the equivalence point of this titration?

- **A.** Phenolphthalein is more suitable because neutralizing a basic solution always requires an indicator whose color transition occurs in the basic \(\text{pH}\) region above \(7.0\).
- **B.** Phenolphthalein is more suitable because its \(pK_a\) of \(9.4\) is closest to the initial \(\text{pH}\) of the unneutralized \(0.10 \text{ M NH}_3\text{(aq)}\) solution.
- **C.** Methyl red is more suitable because the salt solution formed at the equivalence point contains \(\text{NH}_4^+\), which hydrolyzes to create an acidic solution with \(\text{pH} < 7.0\) that falls within the transition range of methyl red.
- **D.** Methyl red is more suitable because it donates protons to consume unreacted \(\text{NH}_3\text{(aq)}\), establishing a neutral equivalence point at \(\text{pH} = 7.0\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123784/*