---
title: "A student titrates a \\(25.0\\text{ mL}\\) sample of an unknown monoprotic weak acid, \\(\\text{HA}\\), with \\(0.100\\text{ M }\\text{NaOH(aq)}\\). The titration curve obtained is shown below.  Based on the curve, which of the following correctly estimates the \\(pK_a\\) of the acid and provides the correct justification?"
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url: "https://nerd-notes.com/ubq/121423/"
date_modified: "2026-08-23T05:02:12+00:00"
---

# A student titrates a \(25.0\text{ mL}\) sample of an unknown monoprotic weak acid, \(\text{HA}\), with \(0.100\text{ M }\text{NaOH(aq)}\). The titration curve obtained is shown below.

Based on the curve, which of the following correctly estimates the \(pK_a\) of the acid and provides the correct justification?

A student titrates a \(25.0\text{ mL}\) sample of an unknown monoprotic weak acid, \(\text{HA}\), with \(0.100\text{ M }\text{NaOH(aq)}\). The titration curve obtained is shown below.

Based on the curve, which of the following correctly estimates the \(pK_a\) of the acid and provides the correct justification?

![A titration curve plotted on a Cartesian coordinate system with light gray gridlines. The horizontal x-axis is labeled 'Volume of 0.100 M NaOH Added (mL)' with major tick marks labeled from 0.0 to 50.0 in increments of 10.0 mL, with minor gridlines every 5.0 mL. The vertical y-axis is labeled 'pH' with tick marks labeled from 0 to 14 in increments of 2 pH units, with minor gridlines every 1 pH unit. A single smooth, solid black curve begins at (0.0, 2.9), rises gradually through a buffer region passing through (20.0, 4.8), and enters a steep vertical inflection centered at an equivalence point of (40.0, 8.8). Past the equivalence point, the curve levels off smoothly toward a pH of approximately 12.5 at 50.0 mL. No other curves, labels, symbols, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1787461331-cjn9pL.jpg)

- **A.** \(pK_a \approx 4.8\), because at \(20.0\text{ mL}\) of added \(\text{NaOH(aq)}\), half of the initial \(\text{HA}\) has been converted to \(\text{A}^-\), resulting in \([\text{HA}] = [\text{A}^-]\).
- **B.** \(pK_a \approx 4.8\), because at \(40.0\text{ mL}\) of added \(\text{NaOH(aq)}\), the titration has reached the equivalence point where \([\text{HA}] = [\text{A}^-]\).
- **C.** \(pK_a \approx 8.8\), because at \(40.0\text{ mL}\) of added \(\text{NaOH(aq)}\), the acid is completely neutralized and the solution reaches the equivalence point.
- **D.** \(pK_a \approx 8.8\), because at \(20.0\text{ mL}\) of added \(\text{NaOH(aq)}\), the buffering capacity of the solution is completely exhausted.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/121423/*