Question

When aluminium hydroxide is reacted with excess of sodium hydroxide, the soluble amphoteric compound is formed. Mark the correct formula.
A. ${\left[ {Al{{\left( {OH} \right)}_4}} \right]^ - }$
B. ${\left[ {Al{{\left( {OH} \right)}_5}} \right]^{2 - }}$
C. ${\left[ {Al{{\left( {OH} \right)}_6}} \right]^{3 - }}$
D. ${\left[ {Al{{\left( {OH} \right)}_7}} \right]^{4 - }}$

Answer 1

Hint: Amphoteric oxides are the compounds that can act as an acid or a base. The metallic oxides are generally basic and the non-metallic oxides are acidic. The metallic oxides are called amphoteric oxides which act as both acid and base. The elements that form amphoteric oxides have some non-metals and Metal characteristics as well. They react with acid and base to produce water and salt as main products. The above reaction is written below:

$ZnO\left( s \right) + 2HCl \to ZnC{l_2}\left( {aq} \right) + {H_2}O\left( l \right)$

$ZnO\left( s \right) + 2NaOH \to N{a_2}Zn{O_2}\left( {aq} \right) + {H_2}O\left( l \right)$

Complete step by step answer:
When aluminium hydroxide reacts with an excess of sodium hydroxide, it forms a soluble amphoteric compound which is known as sodium tetra hydroxoaluminate (III). The reaction involved in this process is written below:

$Al{\left( {OH} \right)_3} + NaOH \to N{a^ + }{\left[ {Al{{\left( {OH} \right)}_4}} \right]^ - }$
                                           (Sodium tetra hydroxoaluminate III)

Therefore the correct formula is ${\left[ {Al{{\left( {OH} \right)}_4}} \right]^ - }$.

So, the correct answer is Option A .

Additional Information:
1.Ampholytes are a type of amphoteric molecule that contain both basic and acidic groups and will exist mostly as zwitterions in a certain range of pH.
2.The pH at which the average charge is zero is called a molecule's isoelectric point.
3.It is used to establish a stable pH gradient for use in isoelectric focusing.
4.Another type of amphoteric species includes amphiprotic molecules that can accept or donate protons. For example-amino acids and proteins which have carboxylic acid groups and have self ionizable compounds such as water.

Note: Sodium tetra hydroxoaluminate does not occur in free state. They are formed at room temperature in a concentrated solution of sodium hydroxide. When we heat the composition of anion, it gets complicated. At crystallization the following compounds are formed - $N{a_4}\left[ {Al{{\left( {OH} \right)}_7}} \right]$, $N{a_6}\left[ {A{l_6}{O_4}{{\left( {OH} \right)}_{16}}} \right]$, $N{a_4}\left[ {A{l_4}{O_3}{{\left( {OH} \right)}_{10}}} \right]$. It gets decomposed upon dilution of solution with water and treatment with acids. It also reacts with ammonium carbonate, aluminium chloride.