CAPF AC 2025 Q105: Science & Technology – Chemistry
Consider the following statements :
1. Potassium carbonate is a salt of a weak acid and a weak base.
2. Ammonium nitrate is a salt of a strong acid and a weak base.
3. Ammonium acetate is a salt of a weak acid and a weak base.
Which of the statements given above are correct?
- (a) 1 and 3 only
- (b) 2 and 3 only
- (c) 1 and 2 only
- (d) 1, 2 and 3
Answer: (b) 2 and 3 only
Explanation
Salts and the acids and bases they come from
How to think: work out the acid and base that make each salt.
| Salt | Acid | Base | Type of salt |
|---|---|---|---|
| 1. Potassium carbonate (K₂CO₃) | Carbonic acid (weak) | Potassium hydroxide (strong) | Strong base + weak acid: not weak + weak |
| 2. Ammonium nitrate (NH₄NO₃) | Nitric acid (strong) | Ammonium hydroxide (weak) | Strong acid + weak base: correct |
| 3. Ammonium acetate (CH₃COONH₄) | Acetic acid (weak) | Ammonium hydroxide (weak) | Weak acid + weak base: correct |
Statements 2 and 3 are correct.
Bank key corrected: the stored key was (c) 1 and 2 only; the right answer is (b).
Answer: (b) 2 and 3 only.
Why each option is right or wrong
- (a) Incorrect. Statement 1 is wrong: potassium hydroxide is a strong base.
- (b) CORRECT. Statements 2 and 3 are correct.
- (c) Incorrect. Statement 1 is wrong, and statement 3 is left out.
- (d) Incorrect. Statement 1 is wrong.
Key point
K₂CO₃: strong base (KOH) + weak acid (H₂CO₃), basic solution. NH₄NO₃: strong acid (HNO₃) + weak base (NH₄OH), acidic solution. CH₃COONH₄: weak acid + weak base, nearly neutral.
Remember
K₂CO₃: strong base + weak acid. NH₄NO₃: strong acid + weak base. CH₃COONH₄: weak + weak.
Common mistakes
- Treating KOH as a weak base because it is a hydroxide of a metal.
- Calling ammonium nitrate a salt of a strong base.
- Forgetting that nitric acid is a strong acid.
Exam tip
Educated guess: all potassium compounds come from the strong base KOH, so statement 1 (weak base) cannot be true. That leaves (b).
Sources
- NCERT Class XI Chemistry, Part II, Chapter 7 (Equilibrium: hydrolysis of salts)
- NCERT Class X Science, Chapter 2 (Acids, Bases and Salts: family of salts and pH of salts)