chemistry : class11 Weak Base Ionization : Common ion effect : Equilibrium MCQ Questions and Answers
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Weak Base Ionization
MCQ questions and answers.
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MCQ Questions
1. Which equation represents the ionization of the weak base ammonia in water?
A.
NH3 + H2O ⇌ NH4+ + OH−
B.
NH3 ⇌ N3− + 3H+
C.
NH3 + H+ ⇌ NH2−
D.
NH4+ + OH− ⇌ NH3 + H2O
Correct Answer:
NH3 + H2O ⇌ NH4+ + OH−
Explanation:
Ammonia is a weak base that accepts a proton from water, producing NH4+ and OH− in a reversible equilibrium.
2. What is the expression for the base ionization constant, Kb, of ammonia?
A.
Kb = [NH4+][OH−]/[NH3]
B.
Kb = [NH3]/[NH4+][OH−]
C.
Kb = [NH4+][NH3]/[OH−]
D.
Kb = [OH−]/[NH4+][NH3]
Correct Answer:
Kb = [NH4+][OH−]/[NH3]
Explanation:
For NH3 + H2O ⇌ NH4+ + OH−, liquid water is omitted, so Kb is the product of the ion concentrations divided by the concentration of un-ionized NH3.
3. What happens to the ionization of ammonia when ammonium chloride is added to an ammonia solution?
A.
It increases because NH4+ is added
B.
It decreases because NH4+ is a common ion
C.
It remains completely unchanged
D.
It stops because chloride ions react with ammonia
Correct Answer:
It decreases because NH4+ is a common ion
Explanation:
NH4Cl provides NH4+, which is already a product of ammonia ionization. The common ion effect shifts the equilibrium toward NH3, reducing its ionization.
4. In the equilibrium NH3 + H2O ⇌ NH4+ + OH−, which ion acts as the common ion when NH4Cl is added?
A.
NH3
B.
H2O
C.
NH4+
D.
Cl−
Correct Answer:
NH4+
Explanation:
NH4+ is a product of weak base ionization and is also supplied by NH4Cl. Therefore, NH4+ is the common ion.
5. The addition of NH4Cl to an ammonia solution generally causes the concentration of OH− ions to:
A.
Increase greatly
B.
Decrease
C.
Remain exactly zero
D.
Change into H2 gas
Correct Answer:
Decrease
Explanation:
Adding NH4+ shifts the ammonia ionization equilibrium to the left, so fewer OH− ions are formed and their concentration decreases.
6. Which substance can suppress the ionization of the weak base NH3 by providing a common ion?
A.
NH4Cl
B.
NaCl
C.
KNO3
D.
CaCO3
Correct Answer:
NH4Cl
Explanation:
NH4Cl dissociates to give NH4+, which is one of the ions produced when NH3 ionizes in water. Thus, it suppresses NH3 ionization by the common ion effect.
7. When the ionization of a weak base is suppressed, the concentration of the un-ionized weak base generally:
A.
Decreases
B.
Increases
C.
Becomes zero
D.
Changes into a strong acid
Correct Answer:
Increases
Explanation:
Suppression of ionization shifts the equilibrium toward the un-ionized base. Therefore, the concentration of NH3 increases when NH4+ is added.
8. For the equilibrium NH3 + H2O ⇌ NH4+ + OH−, adding NH4+ shifts the equilibrium in which direction?
A.
To the right
B.
To the left
C.
In both directions equally
D.
It removes the equilibrium completely
Correct Answer:
To the left
Explanation:
According to the common ion effect, adding a product ion such as NH4+ shifts the equilibrium toward the reactants, so the equilibrium moves to the left.
9. Which statement is correct about adding NH4Cl to a solution of weak base NH3?
A.
The degree of ionization of NH3 increases
B.
The degree of ionization of NH3 decreases
C.
NH3 becomes a strong base
D.
NH4Cl removes all NH3 molecules
Correct Answer:
The degree of ionization of NH3 decreases
Explanation:
NH4Cl supplies NH4+, a common ion. Its presence shifts the ionization equilibrium of NH3 backward, decreasing the degree of ionization.
10. Why does the ionization of a weak base decrease in the presence of its conjugate acid salt?
A.
The conjugate acid provides a common ion that shifts equilibrium backward
B.
The salt converts the weak base directly into water
C.
The salt removes the solvent from the solution
D.
The conjugate acid increases the amount of unreacted water only
Correct Answer:
The conjugate acid provides a common ion that shifts equilibrium backward
Explanation:
A conjugate acid salt such as NH4Cl provides NH4+, which is a product of weak base ionization. The added common ion shifts the equilibrium toward the un-ionized base.
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