---
title: "A student titrates separate \\(25.00 \\text{ mL}\\) samples of a \\(0.100 \\text{ M}\\) weak monoprotic acid, \\(\\text{HA}\\) (\\(K_a = 1.0 \\times 10^{-5}\\) at \\(25^\\circ\\text{C}\\)), with \\(0.100 \\text{ M NaOH(aq)}\\). In each trial, a different acid-base indicator is used to identify the endpoint of the titration. The results and the pH transition ranges of the indicators are recorded in the table below.  | Indicator | pH transition range | Titrant volume at endpoint (mL) | Apparent calculated \\([\\text{HA}]\\) (M) | | :— | :— | :— | :— | | Methyl red | \\(4.4 – 6.2\\) | \\(18.50\\) | \\(0.0740\\) | | Bromothymol blue | \\(6.0 – 7.6\\) | \\(24.60\\) | \\(0.0984\\) | | Phenolphthalein | \\(8.2 – 10.0\\) | \\(25.00\\) | \\(0.100\\) | | Alizarin yellow R | \\(10.2 – 12.0\\) | \\(28.20\\) | \\(0.113\\) |  Based on the data and the principles of acid-base equilibria, which of the following claims and justifications best explains why phenolphthalein accurately determines \\([\\text{HA}]\\) whereas methyl red results in a substantial systematic error?"
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url: "https://nerd-notes.com/ubq/123675/"
date_modified: "2026-09-28T12:01:57+00:00"
---

# A student titrates separate \(25.00 \text{ mL}\) samples of a \(0.100 \text{ M}\) weak monoprotic acid, \(\text{HA}\) (\(K_a = 1.0 \times 10^{-5}\) at \(25^\circ\text{C}\)), with \(0.100 \text{ M NaOH(aq)}\). In each trial, a different acid-base indicator is used to identify the endpoint of the titration. The results and the pH transition ranges of the indicators are recorded in the table below.

| Indicator | pH transition range | Titrant volume at endpoint (mL) | Apparent calculated \([\text{HA}]\) (M) |
| :— | :— | :— | :— |
| Methyl red | \(4.4 – 6.2\) | \(18.50\) | \(0.0740\) |
| Bromothymol blue | \(6.0 – 7.6\) | \(24.60\) | \(0.0984\) |
| Phenolphthalein | \(8.2 – 10.0\) | \(25.00\) | \(0.100\) |
| Alizarin yellow R | \(10.2 – 12.0\) | \(28.20\) | \(0.113\) |

Based on the data and the principles of acid-base equilibria, which of the following claims and justifications best explains why phenolphthalein accurately determines \([\text{HA}]\) whereas methyl red results in a substantial systematic error?

A student titrates separate \(25.00 \text{ mL}\) samples of a \(0.100 \text{ M}\) weak monoprotic acid, \(\text{HA}\) (\(K_a = 1.0 \times 10^{-5}\) at \(25^\circ\text{C}\)), with \(0.100 \text{ M NaOH(aq)}\). In each trial, a different acid-base indicator is used to identify the endpoint of the titration. The results and the pH transition ranges of the indicators are recorded in the table below.

| Indicator | pH transition range | Titrant volume at endpoint (mL) | Apparent calculated \([\text{HA}]\) (M) |
| :--- | :--- | :--- | :--- |
| Methyl red | \(4.4 - 6.2\) | \(18.50\) | \(0.0740\) |
| Bromothymol blue | \(6.0 - 7.6\) | \(24.60\) | \(0.0984\) |
| Phenolphthalein | \(8.2 - 10.0\) | \(25.00\) | \(0.100\) |
| Alizarin yellow R | \(10.2 - 12.0\) | \(28.20\) | \(0.113\) |

Based on the data and the principles of acid-base equilibria, which of the following claims and justifications best explains why phenolphthalein accurately determines \([\text{HA}]\) whereas methyl red results in a substantial systematic error?

- **A.** Phenolphthalein is effective because its color change coincides with the steep pH surge containing the equivalence point (\(\text{pH} \approx 8.9\)) generated by the hydrolysis of \(\text{A}^-\text{(aq)}\); methyl red changes color prematurely within the buffer region while a significant amount of unneutralized \(\text{HA(aq)}\) remains in solution.
- **B.** Phenolphthalein is effective because an indicator must change color at \(\text{pH} = 7.00\) in any monoprotic acid-base titration; methyl red changes color prematurely because \(\text{Na}^+\text{(aq)}\) acts as a Lewis acid and consumes \(\text{OH}^-\text{(aq)}\) ions before neutralization is complete.
- **C.** Phenolphthalein is effective because its \(\text{p}K_a\) is identical to the \(\text{p}K_a\) of \(\text{HA}\), ensuring maximum buffer capacity; methyl red changes color after the true equivalence point because the indicator donates protons that delay the neutral pH transition.
- **D.** Phenolphthalein is effective because its transition range spans the half-equivalence point where \([\text{HA}] = [\text{A}^-]\); methyl red changes color prematurely because its acidic form reacts preferentially with \(\text{OH}^-\text{(aq)}\) instead of the analyte.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123675/*