Question

The reagent \(NH_4Cl\) and aqueous \(NH_3\) will precipitate:

• A. Ca\(^{2+}\)
• B. Al\(^{3+}\)
• C. Mg\(^{2+}\)
• D. Zn\(^{2+}\)

Hint:

\(NH_4Cl + NH_3\) buffer → selectively precipitates Group III cations (Al\(^{3+}\), Fe\(^{3+}\), Cr\(^{3+}\)).

Answer 1

The Correct Option is B

Concept: \(NH_4Cl + NH_3\) forms an ammoniacal buffer that maintains low but controlled \(OH^-\) concentration.

Step 1: Role of buffer. \[ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^- \] \(NH_4Cl\) suppresses ionization of \(NH_3\), keeping \(OH^-\) low.

Step 2: Precipitation condition. Only ions forming highly insoluble hydroxides at low \(OH^-\) will precipitate. \[ Al^{3+} + 3OH^- \rightarrow Al(OH)_3 \downarrow \]

Step 3: Why others do not precipitate.
• Zn\(^{2+}\) → forms soluble complex \([Zn(NH_3)_4]^{2+}\)
• Mg\(^{2+}\), Ca\(^{2+}\) → require higher \(OH^-\) concentration