---
title: "A student titrates a \\(25.0\\text{ mL}\\) sample of an unknown weak monoprotic acid, \\(\\text{HA}\\), with a \\(0.10\\text{ M}\\) solution of \\(\\text{NaOH}\\) at \\(25\\,^{\\circ}\\text{C}\\). The resulting titration curve is shown below.  Based on the titration curve, which of the following correctly identifies the \\(\\text{p}K_a\\) of \\(\\text{HA}\\) and provides the correct chemical justification for why the \\(\\text{pH}\\) at the equivalence point is greater than \\(7.0\\)?"
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url: "https://nerd-notes.com/ubq/119780/"
date_modified: "2026-08-21T08:12:20+00:00"
---

# A student titrates a \(25.0\text{ mL}\) sample of an unknown weak monoprotic acid, \(\text{HA}\), with a \(0.10\text{ M}\) solution of \(\text{NaOH}\) at \(25\,^{\circ}\text{C}\). The resulting titration curve is shown below.

Based on the titration curve, which of the following correctly identifies the \(\text{p}K_a\) of \(\text{HA}\) and provides the correct chemical justification for why the \(\text{pH}\) at the equivalence point is greater than \(7.0\)?

A student titrates a \(25.0\text{ mL}\) sample of an unknown weak monoprotic acid, \(\text{HA}\), with a \(0.10\text{ M}\) solution of \(\text{NaOH}\) at \(25\,^{\circ}\text{C}\). The resulting titration curve is shown below.

Based on the titration curve, which of the following correctly identifies the \(\text{p}K_a\) of \(\text{HA}\) and provides the correct chemical justification for why the \(\text{pH}\) at the equivalence point is greater than \(7.0\)?

![A 2D line graph plotted on a Cartesian coordinate system with light gray gridlines. The horizontal x-axis is labeled 'Volume of 0.10 M NaOH added (mL)' and ranges from 0 to 40 with major tick marks labeled every 5 units: 0, 5, 10, 15, 20, 25, 30, 35, 40. The vertical y-axis is labeled 'pH' and ranges from 0 to 14 with major tick marks labeled every 2 units: 0, 2, 4, 6, 8, 10, 12, 14. A single solid black curve begins at (0, 3.0), rises gradually through the buffer region passing directly through the half-equivalence point at coordinate (12.5, 4.8), exhibits a steep vertical inflection centered at coordinate (25.0, 8.8) where the curve rises sharply from pH 6.5 to 11.0, and levels off asymptotically toward pH 12.5 at 40 mL. No other curves, points, labels, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1787299940-skfImp.jpg)

- **A.** \(\text{p}K_a \approx 3.0\), because \(\text{p}K_a\) is equal to the initial \(\text{pH}\) of the weak acid solution before any \(\text{NaOH}\) is added.
- **B.** \(\text{p}K_a \approx 4.8\), because at the equivalence point, the conjugate base \(\text{A}^-\text{(aq)}\) hydrolyzes with \(\text{H}_2\text{O(l)}\) to produce \(\text{OH}^-\text{(aq)}\).
- **C.** \(\text{p}K_a \approx 4.8\), because at the equivalence point, excess unreacted \(\text{OH}^-\text{(aq)}\) from the titrant remains in the solution.
- **D.** \(\text{p}K_a \approx 8.8\), because \([\text{HA}] = [\text{A}^-]\) at the equivalence point of the titration.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119780/*