Question

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

• A. $N{H}_4^{+}$ is a weak base
• B. $NaOH$ is a very strong base
• C. $NaOH$ forms ${\left([Al(OH{)}_4]\right)}^{-}$ ions
• D. $NaOH$ forms ${\left([Al(OH{)}_2]\right)}^{+}$ ions.

Answer 1

Answer: (C)

Al(OH)₃ formed with NaOH dissolves in excess of NaOH to form aluminate ion.
which is stable.
${\left(\text{AlCl}\right)}_3+3\text{NaOH}\to 2\text{Al}{\left(\text{OH}\right)}_3\downarrow +3\text{NaCl}$
$$\begin{gathered} \text{Al}{\left(\text{OH}\right)}_{\text{3}}+{\left(\text{OH}\right)}^{-}\to {\left(\left[\text{Al}{\left(\text{OH}\right)}_{\text{4}}\right]\right)}^{-} \\ \text{Soluble} \end{gathered}$$