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

The question is about the role of \( \text{NH}_4\text{Cl} \) in qualitative analysis, specifically when added before \(\text{NH}_4\text{OH}\). Let's break this down:

1. Concept of Common Ion Effect:
   • \( \text{NH}_4\text{Cl} \) dissociates in solution to form \(\text{NH}_4^+\) and \(\text{Cl}^-\) ions.
   • \( \text{NH}_4\text{OH} \) is a weak base and dissociates slightly to form \(\text{NH}_4^+\) and \(\text{OH}^-\).
2. Effect of Adding \( \text{NH}_4\text{Cl} \):
   • By introducing \( \text{NH}_4^+ \) ions through \( \text{NH}_4\text{Cl} \), we increase the concentration of \( \text{NH}_4^+ \) in the solution.
   • This shift, according to Le Chatelier's Principle, results in a reduction in the ionization of \( \text{NH}_4\text{OH} \) because the equilibrium will shift to reduce the disturbance caused by the added \( \text{NH}_4^+ \).
   • As a consequence, the concentration of \( \text{OH}^- \) ions, produced by the dissociation of \( \text{NH}_4\text{OH} \), decreases.
3. Conclusion:
   • Hence, the addition of \( \text{NH}_4\text{Cl} \) before \( \text{NH}_4\text{OH} \) is to decrease \([OH^-]\) concentration.

So, the correct option is: to decrease \([OH^-]\) concentration.