---
title: "A student titrates a \\(50.0\\text{ mL}\\) sample of a weak monoprotic acid, \\(\\text{HA(aq)}\\), with \\(0.100\\text{ M }\\text{NaOH(aq)}\\). The titration curve for the process is shown below. Based on the titration curve, which of the following correctly identifies the approximate \\(K_a\\) of \\(\\text{HA}\\) and provides the correct justification?"
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url: "https://nerd-notes.com/ubq/119349/"
date_modified: "2026-08-19T12:39:54+00:00"
---

# A student titrates a \(50.0\text{ mL}\) sample of a weak monoprotic acid, \(\text{HA(aq)}\), with \(0.100\text{ M }\text{NaOH(aq)}\). The titration curve for the process is shown below. Based on the titration curve, which of the following correctly identifies the approximate \(K_a\) of \(\text{HA}\) and provides the correct justification?

A student titrates a \(50.0\text{ mL}\) sample of a weak monoprotic acid, \(\text{HA(aq)}\), with \(0.100\text{ M }\text{NaOH(aq)}\). The titration curve for the process is shown below. Based on the titration curve, which of the following correctly identifies the approximate \(K_a\) of \(\text{HA}\) and provides the correct justification?

![A 2D line graph titled 'Titration of HA with NaOH' on a Cartesian grid with dark gray gridlines spaced every 10 mL on the horizontal axis and every 2 pH units on the vertical axis. The horizontal axis is labeled 'Volume of NaOH(aq) Added (mL)' with numeric labels 0, 10, 20, 30, 40, and 50. The vertical axis is labeled 'pH' with numeric labels 0, 2, 4, 6, 8, 10, 12, and 14. A single solid black curve begins at coordinate (0, 3.1) and rises gently through the buffer region, passing through (10, 4.5), (20, 5.0), and (30, 5.5). The curve then rises sharply through a near-vertical inflection region centered at (40, 9.0), spanning from pH 7.0 to pH 11.0. Above pH 11.0, the curve levels off horizontally, passing through (50, 12.5). A filled black dot is plotted at coordinate (20, 5.0) and another filled black dot is plotted at coordinate (40, 9.0). No other lines, labels, text, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1787143194-G7MjIZ.jpg)

- **A.** \(1.0 \times 10^{-5}\), because at \(20.0\text{ mL}\) of added \(\text{NaOH(aq)}\), \([\text{HA}] = [\text{A}^-]\) and \(\text{pH} = p K_a = 5.0\).
- **B.** \(1.0 \times 10^{-5}\), because at \(40.0\text{ mL}\) of added \(\text{NaOH(aq)}\), \([\text{HA}] = [\text{A}^-]\) and \(\text{pH} = p K_a = 5.0\).
- **C.** \(1.0 \times 10^{-9}\), because at \(40.0\text{ mL}\) of added \(\text{NaOH(aq)}\), the equivalence point is reached where \(\text{pH} = p K_a = 9.0\).
- **D.** \(1.0 \times 10^{-3}\), because at \(0.0\text{ mL}\) of added \(\text{NaOH(aq)}\), the initial \(\text{pH}\) of the acid solution equals its \(p K_a\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119349/*