chemistry : class11 Hydroxide Ion Concentration : Common ion effect : Equilibrium MCQ Questions and Answers
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Hydroxide Ion Concentration
MCQ questions and answers.
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MCQ Questions
1. What happens to the hydroxide ion concentration of a weak base solution when a soluble hydroxide containing a common OH− ion is added?
A.
It increases because the equilibrium shifts toward ionization of the weak base
B.
It decreases to zero because the weak base becomes neutral
C.
The ionization of the weak base is suppressed, while the total OH− concentration generally increases
D.
It remains unchanged under all conditions
Correct Answer:
The ionization of the weak base is suppressed, while the total OH− concentration generally increases
Explanation:
The added OH− is a common ion for the weak-base equilibrium. It shifts the equilibrium toward the unionized base, reducing its ionization, although the added hydroxide increases the total OH− concentration.
2. For the equilibrium NH3 + H2O ⇌ NH4+ + OH−, which ion acts as the common ion when NaOH is added?
A.
Na+
B.
NH4+
C.
H2O
D.
OH−
Correct Answer:
OH−
Explanation:
OH− is already present in the ionization equilibrium of ammonia. Therefore, hydroxide ion from NaOH is the common ion.
3. A solution contains 0.10 M NaOH. What is the approximate hydroxide ion concentration of the solution, assuming complete dissociation of NaOH?
A.
0.010 M
B.
0.10 M
C.
1.0 M
D.
10.0 M
Correct Answer:
0.10 M
Explanation:
NaOH is a strong electrolyte and dissociates completely. Each mole of NaOH gives one mole of OH−, so [OH−] is approximately 0.10 M.
4. If 0.01 mol of NaOH is dissolved to make 1.0 L of solution, what is the hydroxide ion concentration?
A.
0.001 M
B.
0.010 M
C.
0.10 M
D.
1.0 M
Correct Answer:
0.010 M
Explanation:
The concentration of NaOH is moles divided by volume: 0.01 mol ÷ 1.0 L = 0.010 M. Complete dissociation gives the same OH− concentration.
5. For a weak base BOH that ionizes as BOH ⇌ B+ + OH−, what is the effect of adding a soluble hydroxide to the hydroxide ion concentration contributed by BOH?
A.
The contribution from BOH increases
B.
The contribution from BOH decreases
C.
The contribution from BOH becomes equal to the concentration of BOH
D.
The contribution from BOH becomes independent of equilibrium
Correct Answer:
The contribution from BOH decreases
Explanation:
The added OH− is a common ion. By Le Chatelier’s principle, the equilibrium shifts left, so less BOH ionizes and less additional OH− is produced by BOH.
6. A solution contains 0.20 M NaOH and a small amount of a weak base. Which value is closest to the total hydroxide ion concentration?
A.
Much less than 0.20 M
B.
Approximately 0.20 M
C.
Exactly equal to the weak base concentration
D.
Approximately 1.00 M
Correct Answer:
Approximately 0.20 M
Explanation:
NaOH dissociates completely and supplies 0.20 M OH−. The weak base contributes only a small additional amount because its ionization is suppressed by the common OH− ion.
7. Which expression represents the base-ionization constant for the equilibrium B + H2O ⇌ BH+ + OH−?
A.
Kb = [B][OH−]/[BH+]
B.
Kb = [BH+][OH−]/[B]
C.
Kb = [B]/([BH+][OH−])
D.
Kb = [BH+]/([B][OH−])
Correct Answer:
Kb = [BH+][OH−]/[B]
Explanation:
For a weak base equilibrium, the concentration of water is omitted from the equilibrium expression. Thus, Kb = [BH+][OH−]/[B].
8. If the hydroxide ion concentration in a solution changes from 1 × 10−3 M to 1 × 10−2 M, what happens to the hydroxide ion concentration?
A.
It decreases tenfold
B.
It increases tenfold
C.
It increases twofold
D.
It remains unchanged
Correct Answer:
It increases tenfold
Explanation:
The concentration changes from 0.001 M to 0.01 M. Since 0.01 is ten times 0.001, the hydroxide ion concentration increases tenfold.
9. When NaOH is added to an aqueous solution of NH3, what happens to the ionization of NH3?
A.
It increases because NaOH is a base
B.
It decreases because OH− is a common ion
C.
It becomes complete
D.
It changes into acid ionization
Correct Answer:
It decreases because OH− is a common ion
Explanation:
NH3 ionizes to produce OH−. Adding NaOH increases OH− concentration, so the equilibrium shifts backward and the ionization of NH3 decreases.
10. A 0.05 M solution of a strong hydroxide gives complete dissociation. What is its hydroxide ion concentration?
A.
0.005 M
B.
0.025 M
C.
0.05 M
D.
0.50 M
Correct Answer:
0.05 M
Explanation:
A strong hydroxide dissociates completely. If one mole of the compound produces one mole of OH−, a 0.05 M solution gives [OH−] = 0.05 M.
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