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AP Chemistry Practice Quiz: Calculating Equilibrium Concentrations

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

Test your understanding with short quizzes. This quiz has 9 questions to check your progress.

Question 1 of 9

To predict the specific concentrations of all chemical species at equilibrium, which of the following sets of information is required?

All Questions (9)

To predict the specific concentrations of all chemical species at equilibrium, which of the following sets of information is required?

A) The balanced reaction and the temperature.

B) The initial concentrations of all species and the balanced reaction.

C) The balanced reaction, the initial concentrations of all species, and the equilibrium constant (K).

D) The equilibrium constant (K) and the rate law for the reaction.

Correct Answer: C

According to the provided content, the concentrations or partial pressures of species at equilibrium can be predicted given the balanced reaction, initial concentrations, and the appropriate K.

For a reversible reaction, if the reaction quotient (Q) is calculated and found to be less than the equilibrium constant (K), which of the following statements is true?

A) The system is at dynamic equilibrium.

B) The reaction will proceed, consuming products and forming reactants.

C) The reaction will proceed, consuming reactants and forming products.

D) The equilibrium constant K will decrease until it is equal to Q.

Correct Answer: C

The provided content states that when Q < K, the reaction proceeds in the forward direction, consuming reactants and forming products, until equilibrium is reached.

A chemical system is analyzed, and its reaction quotient (Q) is found to be greater than its equilibrium constant (K). To reach equilibrium, what change must occur in the concentrations of the species?

A) The concentrations of reactants will decrease, and the concentrations of products will increase.

B) The concentrations of reactants will increase, and the concentrations of products will decrease.

C) The concentrations of all species will increase.

D) The concentrations of all species will remain the same.

Correct Answer: B

The provided content states that when Q > K, the reaction proceeds in the reverse direction, consuming products and forming reactants until Q equals K.

For the reaction N2O4(g) <=> 2NO2(g), a flask is initially filled with 1.0 M N2O4 and 0 M NO2. If the change in concentration of N2O4 required to reach equilibrium is defined as 'x', what is the expression for the equilibrium concentration of NO2?

A) x

B) 2x

C) 1.0 - x

D) 1.0 + 2x

Correct Answer: B

Based on the stoichiometry of the balanced reaction, for every 'x' moles/L of N2O4 that is consumed, 2'x' moles/L of NO2 are formed. Since the initial concentration of NO2 is 0, the equilibrium concentration will be equal to the amount formed, which is 2x.

Which statement accurately describes a chemical system where the reaction quotient (Q) is equal to the equilibrium constant (K)?

A) The rate of the forward reaction is zero.

B) The concentrations of reactants and products are equal.

C) The system is at dynamic equilibrium.

D) The reaction has stopped completely.

Correct Answer: C

The content explicitly states that when Q = K, the system is at dynamic equilibrium. This means the forward and reverse reactions are occurring at the same rate, resulting in no net change in concentrations.

Consider the synthesis of ammonia: N2(g) + 3H2(g) <=> 2NH3(g). If a reaction vessel is initially charged with 1.0 M N2 and 2.0 M H2, and 'x' represents the change in concentration of N2 as the system approaches equilibrium, what is the equilibrium concentration of H2?

A) 2.0 - x

B) 2.0 + 3x

C) 3x

D) 2.0 - 3x

Correct Answer: D

According to the balanced reaction, for every 1 mole of N2 that reacts (a change of -x), 3 moles of H2 must react. Therefore, the change in H2 concentration is -3x. The equilibrium concentration is the initial concentration minus the change, which is 2.0 - 3x.

A reaction is initiated with only reactants present. For this system, it is determined that Q < K. Which of the following correctly describes the concentration of the reactants once the system reaches equilibrium?

A) The reactant concentrations will be higher than their initial concentrations.

B) The reactant concentrations will be lower than their initial concentrations.

C) The reactant concentrations will be equal to their initial concentrations.

D) The reactant concentrations will be zero.

Correct Answer: B

When a reaction starts with only reactants, Q is initially 0, so Q < K. The content states that when Q < K, the reaction proceeds by consuming reactants and forming products. Therefore, at equilibrium, the concentrations of the reactants must be lower than their initial concentrations.

Consider the reaction: 2A(g) <=> B(g). Initially, the partial pressure of A is 2.0 atm and the partial pressure of B is 0.5 atm. If the equilibrium constant, Kp, is 0.25, how will the system proceed?

A) The system is already at equilibrium.

B) The reaction will shift to the right, forming more B.

C) The reaction will shift to the left, forming more A.

D) The reaction will shift to the right, as there are always more reactants than products initially.

Correct Answer: B

First, calculate the reaction quotient, Qp = P(B) / [P(A)]^2 = 0.5 / (2.0)^2 = 0.5 / 4 = 0.125. Since Qp (0.125) < Kp (0.25), the reaction will proceed to the right, consuming reactants (A) and forming products (B) to reach equilibrium.

A system described by A(g) + B(g) <=> C(g) is at equilibrium. If additional A(g) is suddenly added to the container, what is the immediate effect on the reaction quotient (Q) and the subsequent direction of the reaction shift?

A) Q becomes greater than K, and the reaction shifts to the left.

B) Q becomes less than K, and the reaction shifts to the right.

C) Q becomes less than K, and the reaction shifts to the left.

D) Q becomes greater than K, and the reaction shifts to the right.

Correct Answer: B

The reaction quotient is Q = [C]/([A][B]). At equilibrium, Q = K. Adding more reactant A increases the value of the denominator, causing the value of Q to immediately become less than K. The provided content states that when Q < K, the reaction proceeds to consume reactants and form products, so it will shift to the right to re-establish equilibrium.