Question

In the analysis of III group basic radicals of salts, the purpose of adding NH₄Cl (solid) to NH₄OH is:

• A. to increase the concentration of OH⁻ ions
• B. to precipitate the radicals of group IV and V
• C. to suppress the dissociation of NH₄OH
• D. to introduce Cl⁻ ions

Hint:

NH₄Cl suppresses NH₄OH dissociation via the common ion effect, controlling OH⁻ concentration for selective group III radical precipitation.

Answer 1

The Correct Option is C

In the context of analyzing III group basic radicals of salts, it is essential to understand the role of each component added during the process. Here, we will explore why NH₄Cl is added to NH₄OH in this specific procedure.

The primary reason for adding solid NH₄Cl to NH₄OH is to suppress the dissociation of NH₄OH. This can be explained through the concept of common ion effect and equilibrium dynamics:

1. **Dissociation of NH₄OH**:
   • NH₄OH is a weak base that partially dissociates in water to produce NH₄⁺ and OH⁻ ions:
   • \(NH₄OH \rightleftharpoons NH₄^+ + OH^−\) 
2. **Addition of NH₄Cl**:
   • NH₄Cl dissociates completely to NH₄⁺ and Cl⁻ ions in solution:
   • \(NH₄Cl \rightarrow NH₄^+ + Cl^−\)
3. **Common Ion Effect**:
   • The presence of a common ion, NH₄⁺ in this case, from NH₄Cl increases the concentration of NH₄⁺ ions in solution.
   • This results in the suppression of NH₄OH dissociation due to Le Chatelier's principle (shifting the equilibrium towards the reactant side), reducing the production of OH⁻ ions.

This suppression of dissociation prevented by the common ion effect helps maintain a controlled concentration of OH⁻ ions, essential for selective precipitation in qualitative inorganic analysis, particularly to avoid premature precipitation of III group radicals.

Now, let’s evaluate the given options:

• To increase the concentration of OH⁻ ions: This option is incorrect because the addition of NH₄Cl actually leads to a decrease in the dissociation of NH₄OH, therefore reducing the OH⁻ ion concentration due to the common ion effect.
• To precipitate the radicals of group IV and V: This option is also incorrect as the purpose of adding NH₄Cl is not to precipitate later group radicals, but specifically to control the environment for III group radicals.
• To introduce Cl⁻ ions: While Cl⁻ ions are introduced, it is not the primary purpose. The introduction of NH₄⁺ ions is more pertinent to controlling the dissociation of NH₄OH.
• To suppress the dissociation of NH₄OH: This is the correct answer, as explained above.

Thus, the correct answer is: To suppress the dissociation of NH₄OH.