AP Chemistry Practice Quiz: pH and pKa
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Test your understanding with short quizzes. This quiz has 10 questions to check your progress.
Question 1 of 10
All Questions (10)
A) The acid form (HA)
B) The base form (A⁻)
C) Equal concentrations of the acid and base forms
D) Neither form will dominate
Correct Answer: A
According to the provided content, if the solution pH is less than the pKa of the acid (3.5 < 4.76), the protonated acid form (HA) will be the predominant species in the solution.
A) The indicator reacts with the titrant to form a precipitate.
B) The indicator's protonation state, and thus its properties like color, changes with the solution pH.
C) The indicator is always a strong acid that changes color upon neutralization.
D) The indicator's solubility changes dramatically at a specific pH.
Correct Answer: B
The content states that acid-base indicators are substances that change properties, like color, depending on their protonation state, which is determined by the solution pH.
A) Methyl orange (pKa = 3.7)
B) Bromocresol green (pKa = 4.7)
C) Phenol red (pKa = 7.9)
D) Phenolphthalein (pKa = 9.3)
Correct Answer: D
To ensure accurate titration results, the pKa of the selected acid-base indicator should be as close as possible to the pH at the equivalence point. Phenolphthalein, with a pKa of 9.3, is the closest match to the equivalence point pH of 9.2.
A) pH < pKa
B) pH > pKa
C) pH = pKa
D) pH = 7.0
Correct Answer: B
The content specifies that if the pH is greater than the pKa (pH > pKa), the deprotonated base form (A⁻) will dominate in the solution.
A) To ensure the indicator completely dissolves in the solution.
B) To make the color change as vibrant as possible.
C) To ensure the indicator changes color precisely when the moles of acid and base are stoichiometrically equivalent, leading to accurate results.
D) To ensure the indicator does not react with the acid or base being titrated.
Correct Answer: C
The content states that indicators should have a pKa close to the equivalence point pH 'to ensure accurate titration results.' This accuracy is achieved because the color change, which signals the endpoint, occurs at the same pH as the equivalence point, where the reactants have been completely consumed.
A) The acid form (HIn) would dominate, and the solution would be yellow.
B) The base form (In⁻) would dominate, and the solution would be blue.
C) Both forms would be in equal concentration, and the solution would be green.
D) The indicator would be colorless.
Correct Answer: B
The pH of the solution (8.5) is greater than the pKa of the indicator (7.1). According to the rule 'If pH > pKa, the base form dominates,' the deprotonated base form (In⁻) will be the predominant species. Therefore, the solution will appear blue.
A) The base form (NH₃), because the solution is basic.
B) The acid form (NH₄⁺), because the pH is less than the pKa.
C) The base form (NH₃), because the pH is greater than 7.0.
D) Both forms will be present in equal amounts.
Correct Answer: B
The question concerns the conjugate acid/base pair NH₄⁺/NH₃. The relevant pKa is that of the conjugate acid, NH₄⁺, which is 9.25. Since the solution pH (7.4) is less than the pKa (9.25), the protonated acid form (NH₄⁺) will be the dominant species.
A) The endpoint volume will be greater than the equivalence point volume.
B) The endpoint volume will be less than the equivalence point volume.
C) The endpoint volume will be equal to the equivalence point volume, but the color change will be faint.
D) The indicator will not change color at all during the titration.
Correct Answer: B
The titration starts at a high pH and ends at a low pH. The indicator (pKa = 8.2) will change color when the pH is around 8.2. The true equivalence point is at pH 5.0. Therefore, the student will stop the titration prematurely, when the pH is still 8.2, well before enough acid has been added to reach the actual equivalence point at pH 5.0. This results in a measured titrant volume that is less than the true volume required.
A) The temperature of the solution.
B) The initial concentration of the weak acid.
C) The comparison between the solution pH and the acid's pKa.
D) The identity of the solvent used.
Correct Answer: C
The provided content explicitly states that 'The protonation state (relative concentrations of HA and A−) can be predicted by comparing solution pH to the acid's pKa.'
A) The concentration of HA will be greater than the concentration of A⁻.
B) The concentration of A⁻ will be greater than the concentration of HA.
C) The concentrations of HA and A⁻ will be equal.
D) The acid will be fully deprotonated.
Correct Answer: B
The rule provided is that if pH > pKa, the base form (A⁻) dominates. In this case, the pH (6.5) is greater than the pKa (6.0), so the concentration of the conjugate base, A⁻, will be greater than the concentration of the weak acid, HA.