chemistry : class11 Weak Acid Ionization : Common ion effect : Equilibrium MCQ Questions and Answers
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Weak Acid Ionization
MCQ questions and answers.
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MCQ Questions
1. For the ionization of a weak acid HA in water, which equilibrium is established?
A.
HA ⇌ H⁺ + A⁻
B.
HA ⇌ H⁻ + A⁺
C.
HA → H⁺ + A⁻ completely
D.
HA + H⁺ ⇌ A⁻
Correct Answer:
HA ⇌ H⁺ + A⁻
Explanation:
A weak acid ionizes only partially in water, establishing the equilibrium HA ⇌ H⁺ + A⁻.
2. What happens to the ionization of acetic acid when sodium acetate is added to its solution?
A.
It increases
B.
It decreases
C.
It becomes complete
D.
It remains exactly unchanged
Correct Answer:
It decreases
Explanation:
Sodium acetate provides acetate ions, which are common ions. Their increased concentration shifts the acetic acid ionization equilibrium to the left, decreasing ionization.
3. Which ion is common to the ionization of both acetic acid and sodium acetate in solution?
A.
Na⁺
B.
H⁺
C.
CH₃COO⁻
D.
OH⁻
Correct Answer:
CH₃COO⁻
Explanation:
Acetic acid produces CH₃COO⁻ on ionization, and sodium acetate dissociates to give CH₃COO⁻. Therefore, acetate ion is the common ion.
4. For the weak acid equilibrium HA ⇌ H⁺ + A⁻, what is the effect of adding a soluble salt containing A⁻?
A.
The equilibrium shifts to the right
B.
The equilibrium shifts to the left
C.
The acid changes into a strong acid
D.
The acid completely disappears
Correct Answer:
The equilibrium shifts to the left
Explanation:
Adding A⁻ increases the concentration of a product. According to Le Chatelier’s principle, the equilibrium shifts left, reducing weak acid ionization.
5. Which solution would suppress the ionization of benzoic acid most effectively?
A.
A solution containing sodium benzoate
B.
A solution containing sodium chloride
C.
Pure water
D.
A solution containing potassium nitrate
Correct Answer:
A solution containing sodium benzoate
Explanation:
Sodium benzoate supplies benzoate ions, which are common ions with benzoic acid. This suppresses the ionization of benzoic acid.
6. When the common ion effect suppresses the ionization of a weak acid, the concentration of unionized acid generally:
A.
Increases
B.
Decreases to zero
C.
Remains zero
D.
Changes into a strong acid
Correct Answer:
Increases
Explanation:
The common ion shifts the equilibrium toward the unionized weak acid, so the concentration of HA increases.
7. For the equilibrium HF ⇌ H⁺ + F⁻, which substance provides a common ion when added to the solution?
A.
NaF
B.
NaCl
C.
KNO₃
D.
NH₄Cl
Correct Answer:
NaF
Explanation:
NaF dissociates to produce F⁻, which is already present from the ionization of HF. Thus, F⁻ is the common ion.
8. What happens to the concentration of H⁺ ions when a common ion is added to a weak acid solution?
A.
It generally decreases
B.
It always increases greatly
C.
It becomes equal to the acid concentration
D.
It becomes zero immediately
Correct Answer:
It generally decreases
Explanation:
The common ion shifts the weak acid ionization equilibrium to the left, reducing the formation of H⁺ ions.
9. Which expression represents the ionization constant of the weak acid HA?
A.
Kₐ = [H⁺][A⁻]/[HA]
B.
Kₐ = [HA]/[H⁺][A⁻]
C.
Kₐ = [H⁺][HA]/[A⁻]
D.
Kₐ = [A⁻]/[H⁺][HA]
Correct Answer:
Kₐ = [H⁺][A⁻]/[HA]
Explanation:
For HA ⇌ H⁺ + A⁻, the acid ionization constant is the product of ion concentrations divided by the concentration of unionized HA.
10. A weak acid is partially ionized in water. Adding its sodium salt usually makes the acid:
A.
Less ionized
B.
Fully ionized
C.
Stronger in its intrinsic acid strength
D.
Unable to form an equilibrium
Correct Answer:
Less ionized
Explanation:
The sodium salt supplies the conjugate base ion, creating a common ion. This shifts the ionization equilibrium toward the unionized acid and reduces its ionization.
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