---
title: "A student prepares an aqueous solution of lactic acid, \\(\\text{HC}_3\\text{H}_5\\text{O}_3\\text{(aq)}\\) (\\(p K_a = 3.86\\)), and adjusts its pH to \\(5.20\\) by adding a small amount of \\(\\text{NaOH(aq)}\\). Which species, \\(\\text{HC}_3\\text{H}_5\\text{O}_3\\text{(aq)}\\) or \\(\\text{C}_3\\text{H}_5\\text{O}_3^-\\text{(aq)}\\), is present at a higher concentration in the solution, and why?"
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url: "https://nerd-notes.com/ubq/119273/"
date_modified: "2026-08-19T12:39:36+00:00"
---

# A student prepares an aqueous solution of lactic acid, \(\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}\) (\(p K_a = 3.86\)), and adjusts its pH to \(5.20\) by adding a small amount of \(\text{NaOH(aq)}\). Which species, \(\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}\) or \(\text{C}_3\text{H}_5\text{O}_3^-\text{(aq)}\), is present at a higher concentration in the solution, and why?

A student prepares an aqueous solution of lactic acid, \(\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}\) (\(p K_a = 3.86\)), and adjusts its pH to \(5.20\) by adding a small amount of \(\text{NaOH(aq)}\). Which species, \(\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}\) or \(\text{C}_3\text{H}_5\text{O}_3^-\text{(aq)}\), is present at a higher concentration in the solution, and why?

- **A.** \(\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}\), because the solution pH is higher than the \(p K_a\), indicating that protonation of the acid is favored.
- **B.** \(\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}\), because lactic acid is a weak acid that only partially dissociates, so the protonated form remains predominant at any pH.
- **C.** \(\text{C}_3\text{H}_5\text{O}_3^-\text{(aq)}\), because the solution pH is greater than the \(p K_a\), indicating that the deprotonated form predominated.
- **D.** \(\text{C}_3\text{H}_5\text{O}_3^-\text{(aq)}\), because the solution pH is greater than the \(p K_a\), which means that \([\text{H}^+] > K_a\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119273/*