JEE Main · 2024 · Shift-ImediumIEQ-020

Given below are two statements: Statement (I): Aqueous solution of ammonium carbonate is basic. Statement (II):…

Ionic Equilibrium · Class 11 · JEE Main Previous Year Question

Question

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 KaK_a and KbK_b values of acid and the base forming it.

Choose the most appropriate answer:

Options
  1. a

    Both Statement I and Statement II are correct

    ✓
  2. b

    Statement I is correct but Statement II is incorrect

  3. c

    Both Statement I and Statement II are incorrect

  4. d

    Statement I is incorrect but Statement II is correct

Correct Answera

Both Statement I and Statement II are correct

Detailed Solution

Step 1 — Evaluate Statement I: (NH4)2CO3\mathrm{(NH_4)_2CO_3} is a salt of weak acid (H2CO3\mathrm{H_2CO_3}, Ka1=4.3×10−7K_{a1} = 4.3 \times 10^{-7}) and weak base (NH4OH\mathrm{NH_4OH}, Kb=1.8×10−5K_b = 1.8 \times 10^{-5}).

For a salt of weak acid + weak base: pH=7+12(pKa−pKb)pKa(H2CO3)≈6.37,pKb(NH4OH)≈4.74pH=7+12(6.37−4.74)=7+0.815=7.815>7→\mathrm{pH} = 7 + \frac{1}{2}(\mathrm{pK_a} - \mathrm{pK_b}) \mathrm{pK_a}(\mathrm{H_2CO_3}) \approx 6.37, \quad \mathrm{pK_b}(\mathrm{NH_4OH}) \approx 4.74 \mathrm{pH} = 7 + \frac{1}{2}(6.37 - 4.74) = 7 + 0.815 = 7.815 > 7 \rightarrow Basic ✓ Statement I is correct

Step 2 — Evaluate Statement II: For a salt of weak acid + weak base, the pH depends on the relative strengths:

  • If Ka>KbK_a > K_b: acidic
  • If Ka<KbK_a < K_b: basic
  • If Ka=KbK_a = K_b: neutral Statement II is correct ✓

Answer: Option (1) — Both correct

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