Question

In qualitative analysis, \(NH_4Cl\) is added before \(NH_4OH\):

• A. to increase \([OH^-]\) concentration
• B. for making HCl
• C. to decrease \([OH^-]\) concentration
• D. statement is wrong

Hint:

Common ion effect: Adding \(NH_4Cl\) to \(NH_4OH\) suppresses \(OH^-\) concentration, enabling selective precipitation in qualitative analysis.

Answer 1

The Correct Option is C

Concept: Common ion effect — the suppression of ionization of a weak electrolyte by the addition of a strong electrolyte containing a common ion.

Step 1: Identify the weak electrolyte. \(NH_4OH\) (ammonium hydroxide) is a weak base that partially ionizes as: \[ NH_4OH \rightleftharpoons NH_4^+ + OH^- \]

Step 2: Role of \(NH_4Cl\). \(NH_4Cl\) is a strong electrolyte and dissociates completely: \[ NH_4Cl \rightarrow NH_4^+ + Cl^- \] It provides a high concentration of \(NH_4^+\) ions (common ion).

Step 3: Apply common ion effect. Due to the high concentration of \(NH_4^+\) from \(NH_4Cl\), the equilibrium of \(NH_4OH\) shifts backward (Le Chatelier's principle), suppressing its ionization.

Step 4: Effect on \([OH^-]\). Since ionization of \(NH_4OH\) is suppressed, fewer \(OH^-\) ions are produced. \[ [OH^-] \text{ decreases} \]

Step 5: Why add \(NH_4Cl\) before \(NH_4OH\) in qualitative analysis? This controlled low \([OH^-]\) allows selective precipitation of Group III cations (Al\(^{3+}\), Fe\(^{3+}\), Cr\(^{3+}\)) as hydroxides, while preventing precipitation of higher group cations like Mg\(^{2+}\), Zn\(^{2+}\), etc., which require higher \([OH^-]\).