Aqueous ammonia is used as a precipitating reagent for ${\rm{A}}{{\rm{l}}^{3 + }}$ ions as ${\rm{AI}}{\left( {{\rm{OH}}} \right)_3}$ rather than aqueous NaOH, because:

NaOH forms ${\left[ {{\rm{Al}}{{\left( {{\rm{OH}}} \right)}_4}} \right]^ - }$ ions

${\rm{NH}}_4^ + $ is a weak base

NaOH forms ${\left[ {{\rm{Al}}{{\left( {{\rm{OH}}} \right)}_2}} \right]^ + }$ ions

JEE CHEMISTRY MCQ: 13 TH GROUP ELEMENTS (P-BLOCK)

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Understanding +4 | -1

Aqueous ammonia is used as a precipitating reagent for

\mathrm{A}{\mathrm{l}^{3 + }}

ions as

\mathrm{AI}{\left( {\mathrm{OH}} \right)_3}

rather than aqueous NaOH, because:

View answer and explanation

Correct option: C

NaOH forms

{\left[ {\mathrm{Al}{{\left( {\mathrm{OH}} \right)}_4}} \right]^ - }

ions

Option C (NaOH forms

{\left[ {\mathrm{Al}{{\left( {\mathrm{OH}} \right)}_4}} \right]^ - }

ions) is correct because NaOH can dissolve

Al(OH)_3

forming a soluble complex, while ammonia does not.

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