Consider the acid-base nature of the salt NH4[C6H5COO], when it is dissolved in water. (1) What are the acid-base properties of the cation, NH4+? (2) What are the acid-base properties of the anion, C6H5COO? (3) Would an aqueous solution of NH4[C6H5COO] be acidic, basic or neutral?
The Correct Answer and Explanation is:
To determine the acid-base nature of the salt NH₄[C₆H₅COO] (ammonium benzoate) in water, we analyze the acid-base properties of its cation (NH₄⁺) and anion (C₆H₅COO⁻):
(1) Acid-base properties of the cation, NH₄⁺:
Ammonium ion (NH₄⁺) is the conjugate acid of ammonia (NH₃), a weak base. Since NH₃ does not completely accept protons in water, its conjugate acid (NH₄⁺) is a weak acid. It can donate a proton to water: NH₄⁺+H₂O⇌NH₃+H₃O⁺\text{NH₄⁺} + \text{H₂O} \rightleftharpoons \text{NH₃} + \text{H₃O⁺}
Thus, NH₄⁺ is weakly acidic.
(2) Acid-base properties of the anion, C₆H₅COO⁻:
Benzoate ion (C₆H₅COO⁻) is the conjugate base of benzoic acid (C₆H₅COOH), which is a weak acid. Since weak acids do not fully donate their protons, their conjugate bases can accept protons from water: C₆H₅COO⁻+H₂O⇌C₆H₅COOH+OH⁻\text{C₆H₅COO⁻} + \text{H₂O} \rightleftharpoons \text{C₆H₅COOH} + \text{OH⁻}
Thus, C₆H₅COO⁻ is weakly basic.
(3) Would the solution be acidic, basic, or neutral?
Now, we compare the relative strengths of the weak acid (NH₄⁺) and the weak base (C₆H₅COO⁻). This determines the pH nature of the solution.
- NH₄⁺ has a Ka ≈ 5.6 × 10⁻¹⁰
- C₆H₅COOH has a Ka ≈ 6.3 × 10⁻⁵, so its conjugate base C₆H₅COO⁻ has a Kb ≈ Kw/Ka ≈ 1.6 × 10⁻¹⁰
Since Ka (NH₄⁺) ≈ 5.6 × 10⁻¹⁰ is slightly greater than Kb of C₆H₅COO⁻, the acidic nature of NH₄⁺ dominates slightly.
✅ Final Answer: The solution of NH₄[C₆H₅COO] in water is slightly acidic.
Summary:
- NH₄⁺ is a weak acid.
- C₆H₅COO⁻ is a weak base.
- Because the weak acid is slightly stronger than the weak base, the net effect in water is the release of more H₃O⁺ than OH⁻.
- Therefore, the solution is slightly acidic.
