Question

Aluminium reacts with NaOH and forms compound \(X\). If the coordination number of aluminium in \(X\) is \(6\), the correct formula of \(X\) is

• A. \([Al(H_2O)_4(OH)_2]^+\)
• B. \([Al(H_2O)_6(OH)_2]^+\)
• C. \([Al(H_2O)_2(OH)_4]^-\)
• D. \([Al(H_2O)_6](OH)_3\)

Hint:

If Al has coordination number 6 in aluminate species, it must have total 6 donor ligands: \(4OH^- + 2H_2O\).

Answer 1

The Correct Option is C

Step 1: Identify the reaction.
Al reacts with NaOH forming sodium aluminate in solution.
The aluminate ion is:
\[ [Al(OH)_4]^- \]
Step 2: Coordination number condition.
If coordination number is 6, aluminium must be surrounded by 6 ligands.
So two water molecules also coordinate along with 4 hydroxide ions:
\[ [Al(H_2O)_2(OH)_4]^- \]
Step 3: Confirm coordination number.
Number of ligands:
\[ 2 + 4 = 6 \]
So coordination number is 6.
Final Answer:
\[ \boxed{[Al(H_2O)_2(OH)_4]^-} \]