Question

Aqueous ammonia is used as a precipitating reagent for $Al^{3 +}$ ions as $Al\left(\right. OH \left.\right)_{3}$ rather than aqueous $NaOH$ , because

• A. $NH_{4}^{+}$ is a weak base
• B. $NaOH$ is a very strong base
• C. $NaOH$ forms $\left(\left[\right. Al \left(\right. OH \left.\right)_{4} ]\right)^{-}$ ions
• D. $NaOH$ forms $\left(\left[\right. Al \left(\right. OH \left.\right)_{2} ]\right)^{+}$ ions.

Answer 1

Answer: (C)

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