In qualitative inorganic analysis, why is ammonium chloride (NH4Cl) added to the solution prior to adding ammonium hydroxide (NH4OH) when precipitating Group III cations (Fe3+, Al3+, Cr3+)?

Answer and explanation

Correct answer: C. Common ion NH4+ suppresses NH4OH dissociation and lowers [OH-] so only low-Ksp Group III hydroxides precipitate

  1. Step 1: Group III cations () are precipitated as insoluble hydroxides () by adding .
  2. Step 2: These Group III hydroxides have extraordinarily small solubility products ( for ).
  3. Step 3: However, downstream Group IV and Group V cations () also form hydroxides that could precipitate if is too high.
  4. Step 4: Adding solid (strong electrolyte) introduces a high concentration of ions, which by the common ion effect strongly suppresses the dissociation of the weak base ().
  5. Step 5: The depressed is sufficient to exceed the of Group III hydroxides only, ensuring clean group separation.
Answer
The common ion NH4+ suppresses the ionization of weak base NH4OH, lowering [OH-] so that only Group III hydroxides with very low Ksp precipitate, avoiding precipitation of Group IV and V cations
Why not the others: 'Neutralizes all ferric ions into inert iron gas' is unscientific; 'Prevents formation of coordinate complexes...' is false; 'Reduces aluminum to aluminum foil...' is impossible in aqueous ammonium solutions.

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