Solveeit Logo
Login

Question

Chemistry Question on Qualitative Analysis

In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to :

A

Prevent interference by phosphate ions

B

Decrease concentration of OH−\text{OH}^- ions

C

Increase concentration of Cl−\text{Cl}^- ions

D

Increase concentration of NH4+\text{NH}_4^+ ions

Answer

Decrease concentration of OH−\text{OH}^- ions

Explanation

Solution

Role of Ammonium Chloride in Precipitation of Group III Cations:

In qualitative analysis, group III cations (such as Fe3+,Cr3+,Al3+\text{Fe}^{3+}, \text{Cr}^{3+}, \text{Al}^{3+}) are precipitated as hydroxides by adding ammonium hydroxide NH4OH\text{NH}_4\text{OH}.

Effect of Adding Ammonium Chloride:

Ammonium chloride NH4Cl\text{NH}_4\text{Cl} is added before ammonium hydroxide to control the concentration of OH−\text{OH}^- ions. This is achieved through the common ion effect:

NH4OH↔NH4++OH−\text{NH}_4\text{OH} \leftrightarrow \text{NH}_4^+ + \text{OH}^-

Adding NH4Cl\text{NH}_4\text{Cl} increases the concentration of NH4+\text{NH}_4^+ ions, which shifts the equilibrium to the left, decreasing the concentration of OH−\text{OH}^- ions.

Importance of Controlled OH−\text{OH}^- Concentration:

By reducing OH−\text{OH}^- concentration, we avoid the formation of precipitates from cations of higher groups (such as Group IV and V cations), ensuring selective precipitation of Group III cations only.

Conclusion:

The addition of ammonium chloride decreases the concentration of OH−\text{OH}^- ions through the common ion effect, which corresponds to Option (2).