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Difficulty: MediumAluminium: Extraction, Amphoteric Properties, Alloys, and Compounds

Which of the following observations is made when aqueous ammonia is added gradually until in excess to a solution containing aluminium ions, Al3+(aq)Al^{3+}(aq)?

  1. A
    A white gelatinous precipitate forms which dissolves in excess aqueous ammonia to yield a clear, colorless solution.
  2. A white gelatinous precipitate forms which remains insoluble in excess aqueous ammonia.Answer
  3. C
    No precipitate is formed because aluminium hydroxide is completely soluble in weak ammonium solutions.
  4. D
    A reddish-brown precipitate forms which turns white upon adding excess aqueous ammonia.

Answer

A white gelatinous precipitate of aluminium hydroxide forms, which remains insoluble in excess aqueous ammonia.
When aqueous ammonia is added to a solution containing Al3+Al^{3+} ions, hydroxide ions precipitate aluminium hydroxide, Al(OH)3Al(OH)_3, as a white gelatinous solid. Because aqueous ammonia is a weak base, it cannot provide the high concentration of OH−OH^- ions required to react with amphoteric Al(OH)3Al(OH)_3 to form a soluble complex ion. Consequently, the white precipitate remains insoluble in excess aqueous ammonia.

Step-by-Step Solution

1
Identify the initial precipitation reaction when aqueous ammonia is added to Al3+(aq)Al^{3+}(aq).
Aqueous ammonia provides hydroxide ions (OH−OH^-), reacting with Al3+Al^{3+} to form a white gelatinous precipitate of aluminium hydroxide, Al(OH)3(s)Al(OH)_3(s).
The precipitation ionic equation is Al3+(aq)+3OH−(aq)→Al(OH)3(s)Al^{3+}(aq) + 3OH^-(aq) \rightarrow Al(OH)_3(s).
2
Evaluate the effect of adding excess aqueous ammonia.
Aluminium hydroxide is amphoteric and dissolves in strong alkalis (like excess NaOHNaOH) to form aluminate complex ions [Al(OH)4]−[Al(OH)_4]^-, but it does NOT dissolve in weak alkalis like aqueous ammonia (NH3(aq)NH_3(aq)).
Aqueous ammonia does not supply a high enough hydroxide concentration to form the complex ion, nor does aluminium form a soluble ammine complex.

Key Concept

Distinct qualitative analysis reactions of Al3+Al^{3+} ions with aqueous ammonia versus sodium hydroxide
Estimated Time:1m 0s
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Topics

Aluminium: Extraction, Amphoteric Properties, Alloys, and Compounds

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