Multiple Choice
What is the pH at the equivalence point for a titration involving 25 mL of 0.1 M acetic acid and 0.2 M KOH, given that the acid dissociation constant (Ka) for acetic acid is 1.8 × 10^-5?
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18. Aqueous Equilibrium
Titrations: Weak Acid-Strong Base
Multiple Choice
During the titration of vinegar (acetic acid, CH3COOH) with a strong base such as NaOH, which of the following best describes the pH at the equivalence point?
A
The pH is exactly 7 because acetic acid is neutralized by NaOH
B
The pH is greater than 7 because the conjugate base (CH3COO-) hydrolyzes to produce OH-
C
The pH is unpredictable and depends only on the initial concentration of acetic acid
D
The pH is less than 7 because acetic acid is a weak acid
Verified step by step guidance1
Understand that during the titration of a weak acid (acetic acid, CH3COOH) with a strong base (NaOH), the equivalence point is reached when all the acetic acid has been neutralized to form its conjugate base (acetate ion, CH3COO-).
Recognize that at the equivalence point, the solution contains mainly the conjugate base (CH3COO-) and water, with no excess strong acid or base present.
Recall that the conjugate base of a weak acid is typically a weak base, which can react with water in a process called hydrolysis, producing hydroxide ions (OH-) and increasing the pH.
Write the hydrolysis reaction of the acetate ion: \(\mathrm{CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^-}\), which shows the generation of OH- ions that make the solution basic.
Conclude that because of this hydrolysis, the pH at the equivalence point will be greater than 7, indicating a basic solution rather than neutral.
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