AP Chemistry 7.7 Calculating Equilibrium Concentrations - MCQs - Exam Style Questions
▶️ Answer/Explanation
1. Analyze the Stress:
An inert gas, \(Ar(g)\), is added to the container. The key condition is that this is done at constant volume.
2. Effect on Partial Pressures:
The partial pressure of a gas is defined by the Ideal Gas Law as \(P = \frac{nRT}{V}\). The equilibrium expression \(K_p\) depends only on the partial pressures of the reacting gases (\(NO_2\) and \(N_2O_4\)).
When \(Ar\) is added at constant volume, the moles (\(n\)), temperature (\(T\)), and volume (\(V\)) for \(NO_2\) and \(N_2O_4\) do not change. Therefore, their partial pressures do not change.
3. Effect on Equilibrium:
Since the partial pressures of \(NO_2\) and \(N_2O_4\) remain unchanged, the reaction quotient (\(Q_p\)) remains equal to the equilibrium constant (\(K_p\)). The system is still at equilibrium, and no shift occurs.
(Note: Adding an inert gas does increase the total pressure, but total pressure is irrelevant here. Only partial pressures of reacting gases matter.)
✅ Answer: (D)
Question
\(CO(l)(g)+2H_{2}(g)\rightleftharpoons CH_{3}OH(g) \) \(\Delta H<0\)
The synthesis of \(CH_3\)OH(g) from CO(g) and \(H_2\)(g) is represented by the equation above. The value of \(K_{c}\) for the reaction at 483 K is 14.5.
A mixture of CO(g) and\( H_{2}\)(g) is pumped into a previously evacuated 2.0 L reaction vessel. The total pressure of the reaction system is 1.2 atm at equilibrium. What will be the total pressure of the system if the volume of the reaction vessel is reduced to 1.0 L at constant temperature?
(A) Less than 1.2 atm
(B) Greater than 1.2 atm but less than 2.4 atm
(C) 2.4 atm
(D) Greater than 2.4 atm
▶️Answer/Explanation
Ans:B
Question
\(CO(g)+H_{2}O(g)\rightleftharpoons CO_{2}(g)+H_{2}(g)\) \( k_{c}=1.5\times 10^{3}\)
A 2.0 mol sample of CO(g) and a 2.0 mol sample of \(H_{2}\)O(g) are introduced into a previously evacuated 100. L rigid container, and the temperature is held constant as the reaction represented above reaches equilibrium. Which of the following is true at equilibrium?
(A) \([H_{2}O] > [CO] ~and ~[CO2] > [H2]\)
(B) \( [H_{2}O] > [H_{2}]\)
(C) \([CO_{2}] > [CO]\)
(D) \( [CO] = [H_{2}O]\) = \([CO_{2}]\) = \([H_{2}]\)
▶️Answer/Explanation
Ans:B
Question
\(2H_{2}S(g)+CH_{4}(g)\rightleftharpoons CS_{2}+4H_{2}(g)\) \( k_{c}=3.4\times 10^{-4}\)
A 0.10 mol sample of each of the four species in the reaction represented above is injected into a rigid, previously evacuated 1.0 L container. Which of the following species will have the highest concentration when the system reaches equilibrium?
(A)\( H_{2}\)S(g)
(B)\( CH_{4}\)(g)
(C) \(CS_{2}\)(g)
(D) \(H_{2}\)(g)
▶️Answer/Explanation
Ans:A