Ionisation of Weak Acids and Bases - UNSOLVED PRACTICE SET
Chapter: Equilibrium | Topic: Ionisation of Weak Acids and Bases
IONISATION OF WEAK ACIDS AND BASES - UNSOLVED PRACTICE SET
Topic: Ionisation of Weak Acids and Bases
Multiple Choice Questions
Q1. The degree of ionisation (α) of a weak electrolyte is given by Ostwald's dilution law as:
- α = √(Ka/V)
- α = Ka/V
- α = Ka × V
- α = 1/√(Ka×V)
Q2. For a weak acid HA with Ka = 1.8 × 10⁻⁵, the pH of 0.1 M solution is approximately:
- 1
- 2.87
- 5
- 7
Q3. The ionisation constant of water (Kw) at 25°C is:
- 1.0 × 10⁻⁷
- 1.0 × 10⁻¹⁴
- 1.0 × 10⁻¹⁰
- 1.0 × 10⁻¹²
Q4. For a weak base BOH with Kb = 1.8 × 10⁻⁵, the pOH of 0.1 M solution is approximately:
- 1
- 2.87
- 5
- 11.13
Q5. The degree of ionisation of a weak electrolyte increases with:
- Increase in concentration
- Decrease in concentration (dilution)
- Addition of a strong electrolyte with a common ion
- Decrease in temperature
Q6. For a weak acid HA, if the concentration is doubled, the H⁺ ion concentration:
- Becomes double
- Becomes half
- Increases by a factor of √2
- Remains unchanged
Short Answer Questions
Q7. State Ostwald's dilution law for a weak electrolyte. What are the assumptions made in deriving this law
Q8. The degree of ionisation of acetic acid (CH₃COOH) is found to be 0.013 in 0.1 M solution. Calculate the ionisation constant Ka of acetic acid.
Q9. Explain why the degree of ionisation of a weak acid increases upon dilution, even though the concentration of H⁺ ions decreases.
Q10. Derive the relationship between Ka, Kb, and Kw for a conjugate acid-base pair.
Q11. A 0.05 M solution of a weak acid has a pH of 3.0. Calculate the Ka of the acid and its degree of ionisation.
Q12. Why is the ionisation of weak electrolytes considered an equilibrium process? Write the equilibrium expression for the ionisation of a weak acid HA.
Long Answer Questions
Q13. (a) Derive Ostwald's dilution law for a weak acid HA: Ka = Cα²/(1–α). State the conditions under which this simplifies to Ka = Cα².
(b) Calculate the degree of ionisation and pH of 0.05 M acetic acid solution. (Ka for CH₃COOH = 1.8 × 10⁻⁵)
(c) What happens to the degree of ionisation if the solution is diluted 10 times? Calculate the new degree of ionisation.
Q14. (a) Explain the ionisation of a weak base BOH in water. Derive the expression for Kb and relate it to pOH.
(b) Calculate the pH of 0.1 M NH₄OH solution. (Kb for NH₄OH = 1.8 × 10⁻⁵)
(c) A student adds NH₄Cl to the above solution. Predict what happens to the pH and explain using Le Chatelier's Principle.
Q15. (a) For a weak acid HA, derive the relationship between pH, pKa, and the degree of ionisation.
(b) The pKa of benzoic acid is 4.20. Calculate the pH of 0.01 M benzoic acid solution.
(c) Explain why polyprotic acids like H₂SO₄ and H₂CO₃ have multiple ionisation constants (Ka₁, Ka₂, etc.) and why Ka₁ >> Ka₂.
Numerical / Application-Based Problems
Q16. Calculate the pH of the following solutions:
(a) 0.01 M CH₃COOH (Ka = 1.8 × 10⁻⁵)
(b) 0.1 M NH₄OH (Kb = 1.8 × 10⁻⁵)
(c) 0.001 M HCN (Ka = 4.9 × 10⁻¹⁰)
(d) 0.05 M formic acid (Ka = 1.8 × 10⁻⁴)
For each case, also calculate the degree of ionisation (α).
Q17. A 0.10 M solution of a weak monoprotic acid has a pH of 2.87.
(a) Calculate the H⁺ ion concentration.
(b) Calculate the degree of ionisation (α) of the acid.
(c) Calculate the ionisation constant Ka of the acid.
(d) If the solution is diluted to 0.01 M, what will be the new pH? (Assume α << 1)
Q18. In India, many traditional remedies use weak acids and bases:
(a) Tamarind (contains tartaric acid, a weak acid with Ka ≈ 1.0 × 10⁻³) is used in cooking. Calculate the pH of a 0.05 M solution of tartaric acid.
(b) Buttermilk contains lactic acid (Ka = 1.4 × 10⁻⁴). If the pH of buttermilk is 3.8, calculate the concentration of lactic acid in the buttermilk.
(c) A student prepares a solution by mixing 25 mL of 0.1 M acetic acid with 25 mL of water. Calculate the pH of the diluted solution. (Ka for CH₃COOH = 1.8 × 10⁻⁵)
(d) Explain why vinegar (acetic acid solution) is used as a preservative in Indian pickles. How does the weak acidity of vinegar help in preservation?