Questions on Self-Ionization of Water (Kw)
20 Multiple-Choice Questions on Self-Ionization of Water (Kw)
1. What is the self-ionization of water?
A) The reaction of water with salt
B) The dissociation of water into Na⁺ and Cl⁻
C) The reaction of water with itself to form H₃O⁺ and OH⁻
D) The boiling of water into steam
E) The reaction of water with acids
2. Which equation correctly represents the self-ionization of water?
A) H₂O → H₂ + O₂
B) H₂O + H₂O ⇌ H₃O⁺ + OH⁻
C) H₂O + NaCl ⇌ Na⁺ + OH⁻
D) H₂O + CO₂ → H₂CO₃
E) H₂O → H⁺ + H₂
3. What is the expression for the ion-product constant of water (Kw)?
A) [H₃O⁺]/[OH⁻]
B) [H₃O⁺] - [OH⁻]
C) [H⁺] + [OH⁻]
D) [H⁺][OH⁻]
E) [OH⁻]/[H⁺]
4. At 25°C, what is the value of Kw?
A) 1.0 × 10⁻⁷
B) 1.0 × 10⁻¹⁴
C) 1.0 × 10⁻⁶
D) 7.0 × 10⁻¹⁴
E) 1.0 × 10⁻⁴
5. In pure water at 25°C, what is the concentration of H⁺ ions?
A) 1.0 M
B) 1.0 × 10⁻⁷ M
C) 1.0 × 10⁻¹⁴ M
D) 0 M
E) 7.0 M
6. Which of the following conditions increases the value of Kw?
A) Lowering temperature
B) Adding salt
C) Increasing temperature
D) Decreasing pressure
E) Freezing water
7. If [H⁺] = 1.0 × 10⁻⁶ M, what is [OH⁻] at 25°C?
A) 1.0 M
B) 1.0 × 10⁻⁶ M
C) 1.0 × 10⁻⁷ M
D) 1.0 × 10⁻⁸ M
E) 1.0 × 10⁻¹⁴ M
8. What is the pH of pure water at 25°C?
A) 1
B) 5
C) 7
D) 9
E) 14
9. What happens to Kw when the temperature increases?
A) It increases
B) It decreases
C) It stays the same
D) It becomes zero
E) It becomes negative
10. In neutral water at 25°C, the concentrations of H⁺ and OH⁻ are:
A) Equal and very high
B) Equal and very low
C) Unequal but balanced
D) Not measurable
E) Both 1 M
11. The Kw value is derived from which type of equilibrium?
A) Strong acid dissociation
B) Weak acid-base equilibrium
C) Water’s autoionization equilibrium
D) Precipitation reactions
E) Buffer solutions
12. Which is a product of water’s self-ionization?
A) CO₂
B) Na⁺
C) OH⁻
D) Cl⁻
E) H₂
13. What is the effect of adding a strong acid to water on [OH⁻]?
A) Increases it
B) Decreases it
C) Doubles it
D) Doesn’t affect it
E) Eliminates it
14. Which of the following best describes Kw?
A) A solubility constant
B) A strong base
C) A rate constant
D) An equilibrium constant
E) A reaction quotient
15. The self-ionization of water is an example of:
A) Oxidation-reduction reaction
B) Ion-exchange
C) Neutralization
D) Amphoteric behavior
E) Combustion
16. Why is Kw temperature-dependent?
A) Water freezes faster
B) Ion mobility changes
C) Ionization of water is exothermic
D) The density of water changes
E) Water becomes acidic
17. What happens to pH as Kw increases?
A) pH increases
B) pH decreases
C) pH remains constant
D) pH becomes negative
E) pH becomes 0
18. In a neutral solution at 50°C, how do [H⁺] and [OH⁻] compare?
A) [H⁺] > [OH⁻]
B) [OH⁻] > [H⁺]
C) [H⁺] = [OH⁻]
D) [H⁺] = 0
E) Not predictable
19. Which of the following is true for Kw at 0°C?
A) Greater than 1.0 × 10⁻¹⁴
B) Equal to 1.0 × 10⁻¹⁴
C) Less than 1.0 × 10⁻¹⁴
D) Cannot be measured
E) Depends on pressure only
20. What is the relationship between [H⁺] and pH?
A) pH = [H⁺]
B) pH = log[H⁺]
C) pH = -log[H⁺]
D) pH = Kw / [H⁺]
E) pH = [OH⁻] – [H⁺]
- Questions on Arrhenius Bases
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Answer Key with Explanations
1. C – Water self-ionizes by reacting with itself to form H₃O⁺ and OH⁻.
2. B – The correct ionization reaction: H₂O + H₂O ⇌ H₃O⁺ + OH⁻.
3. D – Kw = [H⁺][OH⁻].
4. B – At 25°C, Kw = 1.0 × 10⁻¹⁴.
5. B – In pure water, [H⁺] = 1.0 × 10⁻⁷ M.
6. C – Increasing temperature increases ionization, hence Kw increases.
7. D – Kw = [H⁺][OH⁻], so [OH⁻] = Kw / [H⁺] = 1.0 × 10⁻⁸ M.
8. C – In pure water, pH = 7.
9. A – Higher temperature promotes more ionization = higher Kw.
10. B – [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ M = low but equal.
11. C – Kw comes from water's autoionization equilibrium.
12. C – OH⁻ is one of the products.
13. B – More H⁺ shifts equilibrium left, decreasing OH⁻.
14. D – Kw is an equilibrium constant specific to water.
15. D – Water acts both as acid and base = amphoteric.
16. C – Ionization is exothermic; thus, Kw increases with temperature.
17. B – If Kw increases, [H⁺] increases → pH decreases.
18. C – Neutral solution always has [H⁺] = [OH⁻].
19. C – Kw is smaller at lower temperatures.
20. C – pH = –log[H⁺].
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