Given below are two statements :
Statement (I) : Aqueous solution of ammonium carbonate is basic.
Statement (II) : Acidic/basic nature of salt solution of a salt of weak acid and weak base depends on Kₐ and K_b value of acid and the base forming it.
In the light of the above statements, choose the most appropriate answer from the options given below :

Solution

Chemistry ma’am · 4 steps

Both ions hydrolyse here, so compare their strengths: carbonate pulls far harder than ammonium does.

1. Weak acid, weak base. For a salt of a weak acid and a weak base, both ions react with water. The cation makes H₃O⁺ and the anion makes OH⁻, so the pH depends on which reaction goes further.

   pH = 7 + 1/2(pKₐ - pK_b)

2. Statement II. The formula above contains only Kₐ and K_b. If Kₐ > K_b the solution is acidic, if K_b > Kₐ it is basic, and if they are equal it is neutral. So Statement II is correct.
3. Statement I. CO₃²⁻ comes from the very weak acid HCO₃⁻ (Kₐ = 4.7 × 10⁻¹¹), so its hydrolysis constant is K_w/Kₐ ≈ 2.1 × 10⁻⁴. NH₄⁺ comes from NH₃ (K_b = 1.8 × 10⁻⁵), so its hydrolysis constant is only K_w/K_b ≈ 5.6 × 10⁻¹⁰. Carbonate makes far more OH⁻ than ammonium makes H₃O⁺, so the solution is basic and Statement I is correct.
4. So. Both statements are correct, and the answer is (a).

The idea to keep. For a weak acid plus weak base salt, ignore concentration and compare strengths. The weaker partner hydrolyses more: K_b > Kₐ makes the solution basic, Kₐ > K_b makes it acidic.

Answer from the NTA final answer key. Solution by PrepWiser.