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AP Chemistry Practice Quiz: Acid-Base Reactions and Buffers

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

Test your understanding with short quizzes. This quiz has 11 questions to check your progress.

Question 1 of 11

When 50 mL of 0.1 M HCl (a strong acid) is mixed with 100 mL of 0.1 M NaOH (a strong base), the reaction H+(aq) + OH−(aq) → H2O(l) occurs. What primarily determines the pH of the final solution?

All Questions (11)

When 50 mL of 0.1 M HCl (a strong acid) is mixed with 100 mL of 0.1 M NaOH (a strong base), the reaction H+(aq) + OH−(aq) → H2O(l) occurs. What primarily determines the pH of the final solution?

A) The concentration of the remaining weak acid.

B) The concentration of the excess H+ ions.

C) The concentration of the excess OH− ions.

D) The solution will be neutral with a pH of 7.

Correct Answer: C

The reaction involves 0.005 moles of H+ and 0.010 moles of OH-. The strong base is in excess. According to the provided content, the final pH is determined by the concentration of the excess reactant, which in this case is OH−.

A solution of a weak acid (HA) is titrated with a strong base (NaOH). What is the chemical justification for the solution being slightly basic at the equivalence point?

A) The excess strong base determines the pH.

B) The conjugate base (A−) reacts with water to produce OH− ions.

C) The weak acid (HA) does not fully react with the strong base.

D) Water autoionization is the only factor determining the pH.

Correct Answer: B

At the equivalence point of a weak acid-strong base titration, all the HA has reacted to form its conjugate base, A−. The content states that at this point, the solution is slightly basic due to A- hydrolysis (A−(aq) + H2O(l) ⇌ HA(aq) + OH−(aq)).

A chemist wants to create a buffer solution by mixing a weak base, B, with a strong acid, HCl. According to the reaction B(aq) + H3O+(aq) ⇌ HB+(aq) + H2O(l), which of the following conditions will result in a buffer?

A) The moles of strong acid added are greater than the initial moles of the weak base.

B) The moles of strong acid added are equal to the initial moles of the weak base.

C) The initial moles of the weak base are greater than the moles of strong acid added.

D) The reaction is performed with a weak acid instead of a strong acid.

Correct Answer: C

The provided content states that when a weak base reacts with a strong acid, the resulting solution can be a buffer if the weak base is in excess. This means there will be significant amounts of both the weak base (B) and its conjugate acid (HB+) in the solution.

In an aqueous solution containing both a strong acid like HCl and a weak acid like CH3COOH, what is the relationship between the major species?

A) Both acids dissociate completely, contributing equally to the H+ concentration.

B) The weak acid dissociates completely, while the strong acid remains mostly intact.

C) The strong acid dissociates completely, significantly suppressing the dissociation of the weak acid.

D) The weak acid neutralizes the strong acid, forming a buffer solution.

Correct Answer: C

In a mixture of a strong and weak acid, the strong acid fully dissociates, providing a large initial concentration of H+. This high concentration of H+ shifts the weak acid's dissociation equilibrium to the left (Le Châtelier's principle), suppressing its dissociation. Therefore, the major species are H+ and the anion from the strong acid, and the undissociated weak acid.

Which statement best describes the reaction that occurs when a weak acid (HA) is mixed with a weak base (B)?

A) The reaction runs to completion, forming the conjugate base (A−) and conjugate acid (HB+).

B) No reaction occurs because both reactants are weak.

C) The reaction proceeds to an equilibrium state and does not run to completion.

D) The reaction only occurs in the presence of a strong acid catalyst.

Correct Answer: C

The provided content explicitly states that when a weak acid and a weak base are mixed, they react to an equilibrium state represented by HA(aq) + B(aq) ⇌ A−(aq) + HB+(aq). Unlike reactions involving at least one strong species, this reaction does not go to completion.

A solution is prepared by mixing a weak base, NH3, with a strong acid, HCl. At the equivalence point, what is the nature of the resulting solution?

A) Neutral, because the acid and base have completely neutralized each other.

B) Slightly acidic, due to the hydrolysis of the conjugate acid (NH4+).

C) Slightly basic, due to the presence of the unreacted weak base.

D) Strongly acidic, because a strong acid was used in the titration.

Correct Answer: B

According to the content, for a weak base and strong acid reaction, the solution at the equivalence point is slightly acidic. This is because the only significant species present (besides water and the spectator ion Cl-) is the conjugate acid (HB+, or NH4+ in this case), which undergoes hydrolysis to produce H3O+.

A student titrates a sample of a weak acid, HA, with a strong base, NaOH. If the student adds an amount of NaOH that is more than the stoichiometric amount required for neutralization, the pH of the resulting solution is primarily determined by:

A) The concentration of the remaining weak acid, HA.

B) The concentration of the conjugate base, A−, formed during the reaction.

C) The equilibrium constant (Ka) of the weak acid.

D) The concentration of the excess strong base, OH−.

Correct Answer: D

The content states that for a weak acid and strong base reaction, if the strong base is in excess, the resulting solution is basic. The contribution of OH- from the hydrolysis of the conjugate base A- is negligible compared to the concentration of the excess strong base. Therefore, the pH is determined by the concentration of excess OH-.

Which of the following reaction types is explicitly described in the content as running to completion?

A) Weak acid + Weak base

B) Weak base + Strong acid

C) Dissociation of a weak acid in water

D) Hydrolysis of a conjugate base

Correct Answer: B

The provided content states that strong acid + strong base, weak acid + strong base, and weak base + strong acid reactions run to completion. The reaction between a weak acid and a weak base reaches equilibrium. Therefore, 'Weak base + Strong acid' is a correct choice.

Consider the reaction between a weak acid (HA) and a strong base (OH−). Under which condition is the resulting solution a buffer?

A) When the initial moles of HA are equal to the moles of OH− added.

B) When the moles of OH− added are greater than the initial moles of HA.

C) When the initial moles of HA are greater than the moles of OH− added.

D) When a strong acid is used instead of a weak acid.

Correct Answer: C

The content specifies that for a weak acid and strong base reaction, the resulting solution can be a buffer if the weak acid is in excess. This means that after the reaction, there are significant amounts of both unreacted weak acid (HA) and its newly formed conjugate base (A-), which is the definition of a buffer.

When equimolar amounts of a strong acid and a strong base are mixed in an aqueous solution, what is the expected outcome?

A) A buffer solution is formed.

B) The resulting solution is acidic due to hydrolysis.

C) The resulting solution is basic due to hydrolysis.

D) The reactants are consumed completely, and the pH is determined by water.

Correct Answer: D

The reaction H+(aq) + OH−(aq) → H2O(l) runs to completion. If equimolar amounts are mixed, there is no excess reactant. The resulting solution contains only water and spectator ions, so its pH is neutral (around 7 at 25°C), determined by the autoionization of water.

A solution is created by mixing a weak base (B) and a strong acid (H3O+), with the weak base in molar excess. The resulting buffer solution contains both B and its conjugate acid, HB+. What is the primary role of the HB+ species in this buffer?

A) To react with and neutralize any added base (OH−).

B) To react with and neutralize any added acid (H3O+).

C) To lower the initial pH of the weak base solution.

D) To increase the solubility of the weak base.

Correct Answer: A

A buffer solution must be able to resist pH changes from the addition of both acid and base. In a buffer composed of a weak base (B) and its conjugate acid (HB+), the weak base (B) neutralizes added acid. The conjugate acid (HB+) is present to neutralize any added base via the reaction HB+(aq) + OH-(aq) -> B(aq) + H2O(l). This is an application of the principles outlined in the provided content.