chemistry : class11 Common Ion from Strong Electrolytes : Common ion effect : Equilibrium MCQ Questions and Answers
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Common Ion from Strong Electrolytes
MCQ questions and answers.
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MCQ Questions
1. What happens to the ionization of a weak electrolyte when a strong electrolyte containing a common ion is added?
A.
It increases greatly
B.
It decreases
C.
It remains exactly the same
D.
It becomes complete
Correct Answer:
It decreases
Explanation:
The strong electrolyte supplies an ion already present in the weak electrolyte equilibrium. The increased concentration of this common ion shifts the equilibrium backward and decreases ionization.
2. Which strong electrolyte provides a common ion to acetic acid, CH3COOH?
A.
NaCl
B.
KNO3
C.
CH3COONa
D.
HCl
Correct Answer:
CH3COONa
Explanation:
Sodium acetate dissociates completely as CH3COONa → CH3COO− + Na+. The acetate ion, CH3COO−, is also produced by the ionization of acetic acid.
3. The addition of sodium acetate to an acetic acid solution suppresses the ionization of acetic acid because sodium acetate supplies:
A.
H+ ions
B.
CH3COO− ions
C.
NaOH molecules
D.
Cl− ions
Correct Answer:
CH3COO− ions
Explanation:
Sodium acetate is a strong electrolyte and dissociates into Na+ and CH3COO− ions. The acetate ion is common to the ionization equilibrium of acetic acid, so its concentration suppresses ionization.
4. Which strong electrolyte provides the common ion NH4+ to an aqueous solution of NH4OH?
A.
NH4Cl
B.
NaCl
C.
KOH
D.
HNO3
Correct Answer:
NH4Cl
Explanation:
NH4Cl completely dissociates into NH4+ and Cl− ions. NH4+ is also produced in the ionization equilibrium of NH4OH, so it acts as the common ion.
5. When NH4Cl is added to NH4OH, the concentration of OH− ions generally:
A.
Increases
B.
Decreases
C.
Becomes zero immediately
D.
Remains unchanged because NH4Cl is neutral
Correct Answer:
Decreases
Explanation:
NH4Cl supplies NH4+, a common ion in the ionization equilibrium of NH4OH. This shifts the equilibrium toward unionized NH4OH and decreases the concentration of OH− ions.
6. The common ion effect is mainly explained by which principle?
A.
Le Chatelier's principle
B.
Boyle's law
C.
Charles's law
D.
Law of conservation of mass only
Correct Answer:
Le Chatelier's principle
Explanation:
Adding a common ion increases the concentration of one product or reactant in an ionization equilibrium. According to Le Chatelier's principle, the equilibrium shifts to oppose this change.
7. Which pair illustrates the common ion effect involving a weak acid and a strong electrolyte?
A.
HCl and NaCl
B.
CH3COOH and CH3COONa
C.
NaOH and KOH
D.
HNO3 and KNO3
Correct Answer:
CH3COOH and CH3COONa
Explanation:
Acetic acid is a weak acid, while sodium acetate is a strong electrolyte that supplies CH3COO−, the ion common to the acetic acid ionization equilibrium.
8. In the equilibrium CH3COOH ⇌ H+ + CH3COO−, addition of a strong electrolyte containing CH3COO− shifts the equilibrium:
A.
To the right
B.
To the left
C.
In neither direction
D.
First right and then permanently stops
Correct Answer:
To the left
Explanation:
The added strong electrolyte increases the concentration of CH3COO−. The equilibrium shifts to the left to reduce the effect of the added common ion.
9. Which ion is common to the equilibria of NH4OH and NH4Cl?
A.
OH−
B.
Cl−
C.
NH4+
D.
H+
Correct Answer:
NH4+
Explanation:
NH4OH ionizes to form NH4+ and OH−, while NH4Cl dissociates to form NH4+ and Cl−. Therefore, NH4+ is the common ion.
10. Adding a soluble salt containing a common ion to a weak electrolyte generally causes its degree of ionization to:
A.
Increase
B.
Decrease
C.
Become 100 percent
D.
Change only if the salt is insoluble
Correct Answer:
Decrease
Explanation:
A soluble strong electrolyte dissociates completely and adds the common ion to the solution. The common ion shifts the weak electrolyte equilibrium toward the unionized form, reducing its degree of ionization.
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