AP Chemistry Practice Quiz: Weak Acid and Base Equilibria

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 14 questions to check your progress.

Question 1 of 14

A 0.1 M solution of a monoprotic weak acid, HA, is prepared. Which statement correctly describes the concentration of hydronium ions, [H₃O⁺], in the solution?

A)[H₃O⁺] will be much less than 0.1 M.B)[H₃O⁺] will be equal to 0.1 M.C)[H₃O⁺] will be greater than 0.1 M.D)[H₃O⁺] will be equal to the concentration of HA at equilibrium.

All Questions (14)

A 0.1 M solution of a monoprotic weak acid, HA, is prepared. Which statement correctly describes the concentration of hydronium ions, [H₃O⁺], in the solution?

A) [H₃O⁺] will be much less than 0.1 M.

B) [H₃O⁺] will be equal to 0.1 M.

C) [H₃O⁺] will be greater than 0.1 M.

D) [H₃O⁺] will be equal to the concentration of HA at equilibrium.

Correct Answer: A

Content point 2 states that for a weak acid, only a small percentage of molecules ionize. Therefore, the equilibrium concentration of hydronium ions, [H₃O⁺], will be significantly less than the initial concentration of the acid.

A solution of a weak base, B, is at equilibrium with water according to the equation: B(aq) + H₂O(l) ⇌ BH⁺(aq) + OH⁻(aq). If the initial concentration of B was 0.5 M, which of the following is true at equilibrium?

A) [OH⁻] is approximately 0.5 M.

B) [B] is approximately 0 M.

C) [OH⁻] is much less than 0.5 M.

D) [BH⁺] is greater than [B].

Correct Answer: C

Content point 4 explains that weak bases only partially ionize in water. This means the equilibrium lies to the left, and the resulting hydroxide ion concentration, [OH⁻], is significantly lower than the initial concentration of the weak base.

The equilibrium constant for a weak acid, HA, is represented by Ka. A larger Ka value indicates which of the following?

A) The equilibrium favors the un-ionized acid, HA.

B) The pKa of the acid is also large.

C) A greater percentage of the acid molecules have ionized.

D) The pH of the solution is higher than for an acid with a smaller Ka.

Correct Answer: C

According to content point 3, the equilibrium constant Ka represents the extent of ionization. A larger Ka value signifies a stronger weak acid, meaning the equilibrium HA ⇌ H⁺ + A⁻ lies further to the right, and a greater fraction (percentage) of the acid molecules have ionized.

A student compares two weak bases, Base X and Base Y, both at 0.1 M concentration. Base X has a pKb of 4.0 and Base Y has a pKb of 6.0. Which statement is correct?

A) The solution of Base Y will have a higher pH than the solution of Base X.

B) The solution of Base X will have a higher concentration of hydroxide ions, [OH⁻].

C) Base Y is a stronger base than Base X.

D) Both solutions will have the same pH because their initial concentrations are the same.

Correct Answer: B

Based on content point 5, pKb = -log Kb. A lower pKb value (like 4.0 for Base X) corresponds to a larger Kb value, which indicates a stronger base. A stronger base will ionize to a greater extent, producing a higher concentration of hydroxide ions and therefore a higher pH.

In a 0.1 M solution of acetic acid, CH₃COOH (a weak acid), which of the following correctly ranks the concentrations of the species present at equilibrium?

A) [H₃O⁺] = [CH₃COO⁻] > [CH₃COOH]

B) [CH₃COOH] > [H₃O⁺] ≈ [CH₃COO⁻]

C) [CH₃COO⁻] > [CH₃COOH] > [H₃O⁺]

D) [CH₃COOH] > [CH₃COO⁻] > [H₃O⁺]

Correct Answer: B

Based on content points 1, 2, and 3, a weak acid solution involves an equilibrium where the un-ionized acid is the major species. The acid ionizes to produce hydronium ions and its conjugate base in roughly equal, but small, amounts. Therefore, the concentration of the un-ionized acid, [CH₃COOH], is the highest.

The weak acid, hydrofluoric acid (HF), has a Ka of 6.8 × 10⁻⁴ at 25°C. What is the value of the base dissociation constant, Kb, for its conjugate base, the fluoride ion (F⁻)? (Note: Kw = 1.0 × 10⁻¹⁴)

A) 6.8 × 10⁻¹⁸

B) 1.5 × 10⁻¹¹

C) 1.0 × 10⁻¹⁴

D) 6.8 × 10⁻⁴

Correct Answer: B

Content point 7 states the relationship for any conjugate acid-base pair is Kw = Ka × Kb. To find Kb for the conjugate base F⁻, the formula is rearranged to Kb = Kw / Ka. Calculation: Kb = (1.0 × 10⁻¹⁴) / (6.8 × 10⁻⁴) ≈ 1.5 × 10⁻¹¹.

The percent ionization of a 0.1 M weak acid solution is found to be 1.3%. If the initial concentration of the same acid is increased to 1.0 M, the percent ionization will...

A) increase.

B) decrease.

C) remain the same.

D) become 13%.

Correct Answer: B

According to content point 6, percent ionization depends on the initial concentration. For a weak acid equilibrium, increasing the initial concentration shifts the equilibrium to the right, producing more ions. However, the ratio of ionized acid to the total initial acid (the percent ionization) decreases as concentration increases, a consequence of Le Chatelier's principle.

The pKa of a certain monoprotic weak acid is 5.0. To determine the pH of a 0.2 M solution of this acid, which of the following expressions would be used to solve for [H₃O⁺] (represented by x)?

A) 10⁻⁵.⁰ = x² / (0.2)

B) 10⁻⁵.⁰ = x² / (0.2 - x)

C) 5.0 = x² / (0.2 - x)

D) 10⁻⁵.⁰ = (0.2 - x) / x²

Correct Answer: B

Content point 3 describes the weak acid equilibrium. The equilibrium constant is Ka = [H₃O⁺][A⁻]/[HA]. Given pKa = 5.0, Ka = 10⁻⁵.⁰. If the initial acid concentration is 0.2 M and we let x = [H₃O⁺] at equilibrium, then [A⁻] = x and [HA] = 0.2 - x. Substituting these into the Ka expression gives Ka = x² / (0.2 - x).

A weak acid, HA, has a conjugate base, A⁻. Which of the following statements is true regarding the relationship between pKa and pKb for this pair at 25°C?

A) If HA is a relatively strong weak acid (low pKa), then A⁻ will be a relatively strong weak base (low pKb).

B) The sum of their pKa and pKb values is always 7.0.

C) If HA is a relatively weak weak acid (high pKa), then A⁻ will be a relatively strong weak base (low pKb).

D) The pKa of the acid is always greater than the pKb of its conjugate base.

Correct Answer: C

Content point 7 states that pKw = pKa + pKb, which equals 14.0 at 25°C. This establishes an inverse relationship between the strength of an acid and its conjugate base. A weaker acid (higher pKa) must have a stronger conjugate base (lower pKb) for their sum to equal 14.

A chemist has two weak bases, Base A (pKb = 5.0) and Base B (pKb = 9.0). If 0.1 M solutions of each are prepared, which statement is accurate?

A) The solution of Base B will have a higher pH.

B) The solution of Base A will have a higher pOH.

C) The percent ionization of Base A will be greater than that of Base B.

D) The concentration of the conjugate acid, [BH⁺], will be greater in the solution of Base B.

Correct Answer: C

According to content points 5 and 6, a lower pKb indicates a stronger base (since pKb = -log Kb). Base A is stronger than Base B. A stronger base will react more with water, leading to a greater percent ionization and a higher concentration of its products ([BH⁺] and [OH⁻]).

The pKa for the ammonium ion (NH₄⁺) is 9.25. What is the pKb for its conjugate base, ammonia (NH₃), at 25°C?

A) 9.25

B) 1.8 x 10⁻⁵

C) 4.75

D) 14.00

Correct Answer: C

Content point 7 provides the direct relationship for a conjugate acid-base pair: pKw = pKa + pKb. At 25°C, pKw = 14.00. Therefore, pKb = pKw - pKa = 14.00 - 9.25 = 4.75.

In a 0.1 M solution of weak acid HX, the [H₃O⁺] is 10⁻⁴ M. In a 0.1 M solution of weak acid HY, the [H₃O⁺] is 10⁻⁵ M. Which statement is correct?

A) HX is a weaker acid than HY.

B) The percent ionization of HY is greater than that of HX.

C) The Ka for HX is larger than the Ka for HY.

D) The pKa for HX is larger than the pKa for HY.

Correct Answer: C

Based on content points 2, 3, and 6, the concentration of H₃O⁺ at equilibrium is a direct measure of acid strength for solutions of the same initial concentration. Since the HX solution has a higher [H₃O⁺], it is the stronger acid. A stronger acid has a larger Ka value and a smaller pKa value.

For a 0.1 M solution of the weak base ammonia, NH₃, which of the following is a correct statement about the concentrations of species at equilibrium?

A) [NH₃] < [NH₄⁺]

B) [OH⁻] = 0.1 M

C) [NH₄⁺] is approximately equal to [OH⁻].

D) [H₃O⁺] > [OH⁻]

Correct Answer: C

According to content points 1, 4, and 5, a weak base reacts with water in an equilibrium: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The reaction produces the conjugate acid (NH₄⁺) and hydroxide ions (OH⁻) in a 1:1 molar ratio. Therefore, their concentrations will be approximately equal, and much smaller than the initial concentration of the weak base.

A student observes that the percent ionization of a 0.01 M solution of a weak acid is greater than the percent ionization of a 0.1 M solution of the same acid. This observation is best explained by the fact that...

A) the value of Ka increases as the solution is diluted.

B) diluting the solution shifts the equilibrium towards the side with more moles of aqueous species.

C) the concentration of water is greater in the more dilute solution.

D) the pKa of the acid decreases as concentration decreases.

Correct Answer: B

This is an application of Le Chatelier's principle to the equilibrium HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A⁻(aq). According to content point 6, percent ionization is dependent on concentration. When the solution is diluted (concentration decreases), the equilibrium shifts to counteract the stress. It shifts to the right, favoring the side with a greater number of moles of dissolved particles (one mole of HA vs. two moles of ions), which results in a higher percentage of the acid molecules being ionized.