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Difficulty: EasySalt Hydrolysis and Solution Acidity/Alkalinity

When solid ammonium chloride (NH4ClNH_4Cl) is dissolved in water, the resulting solution exhibits an acidic pH. Which species undergoes hydrolysis to cause this acidity?

  1. Ammonium ion (NH4+NH_4^+)Answer
  2. B
    Chloride ion (Cl−Cl^-)
  3. C
    Both ammonium (NH4+NH_4^+) and chloride (Cl−Cl^-) ions
  4. D
    Un-ionized ammonium chloride molecule (NH4ClNH_4Cl)

Answer

The ammonium ion (NH4+NH_4^+) undergoes cation hydrolysis to form hydronium ions (H3O+H_3O^+), making the solution acidic.
Ammonium chloride (NH4ClNH_4Cl) completely dissociates in water into NH4+NH_4^+ and Cl−Cl^-. Since NH4+NH_4^+ is the conjugate acid of the weak base NH3NH_3, it reacts with water molecules (cation hydrolysis) to donate a proton, producing excess H3O+H_3O^+ ions which lower the pH below 7.

Step-by-Step Solution

1
Identify the parent acid and base of the salt.
Ammonium chloride (NH4ClNH_4Cl) is formed from the weak base ammonia (NH3NH_3) and the strong acid hydrochloric acid (HClHCl).
Salts composed of a weak base and strong acid form acidic aqueous solutions due to cation hydrolysis.
2
Determine which ion hydrolyzes in aqueous solution.
The ammonium ion (NH4+NH_4^+) reacts with water: NH_4^+_{(aq)} + H_2O_{(l)} \rightleftharpoons NH_{3(aq)} + H_3O^+_{(aq)}.
Conjugate acids of weak bases are strong enough to donate protons to water molecules, generating hydronium ions.

Key Concept

Salt Hydrolysis of Weak Base and Strong Acid Salts
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Topics

Salt Hydrolysis and Solution Acidity/Alkalinity

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