Question

Why only \(\Lambda \mathrm{s}^{3+}\) gets precipitated as \(\Lambda \mathrm{s}_{2} \mathrm{~S}_{3}\) not \(\mathrm{Zn}^{2-}\) as \(\mathrm{ZnS}\) when \(\mathrm{II}_{2} \mathrm{~S}\) is passed through an acidic solution containing \(\Lambda s^{3-}\) and \(Z n^{2-}\) ? (1) Solubility product of \(\mathrm{As}_{2} \mathrm{~S}_{3}\) is less than that of \(\mathrm{ZnS}\) (2) Enough As s' are present in the acidic medium (3) Zinc salt does not ionize in the acidic medium (4) Solubility product changes in the presence of an acid

Short answer

The solubility product of \(\text{As}_2\text{S}_3\) is less than that of \(\text{ZnS}\).

Step-by-step solution

Understand the Problem

Identify why only \(\text{As}^{3+}\) precipitates as \(\text{As}_2\text{S}_3\) and not \(\text{Zn}^{2+}\) as \(\text{ZnS}\) in the given conditions.

Solubility Product (Ksp) Concept

Recall that a compound precipitates when the product of the ion concentrations exceeds the solubility product (Ksp) of the compound.

Compare Solubility Products

Recognize that \(\text{As}_2\text{S}_3\) has a lower solubility product than \(\text{ZnS}\). This means \(\text{As}_2\text{S}_3\) is less soluble in water than \(\text{ZnS}\). The lower the Ksp, the more likely the compound will precipitate first.

Acidic Medium Influence

Note the medium of the solution is acidic. In acidic conditions, the solubility product of compounds can change, but the significant factor here is the difference in Ksp values.

Conclude the Right Answer

Based on the above points, the primary reason for \(\text{As}_2\text{S}_3\) precipitating over \(\text{ZnS}\) in acidic conditions is that \(\text{As}_2\text{S}_3\) has a much lower solubility product than \(\text{ZnS}\). Therefore, the correct choice is option (1).

Key concepts

Precipitation Reactions

Precipitation reactions occur when the concentration of ions in a solution exceeds the solubility product, or Ksp, of an insoluble compound. As a result, a solid precipitate forms from the solution. Understanding these reactions is essential in predicting which compounds will form solids under various conditions. For example, if we mix two aqueous solutions containing different ions, a precipitate will form if the product of the ion concentrations exceeds the Ksp value for that particular compound.

Solubility Product Constant (Ksp)

The Solubility Product Constant, commonly known as Ksp, is a crucial concept when studying solubility and precipitation. It provides a measure of the maximum concentration of a salt that can dissolve in water before it precipitates. The lower the Ksp value, the less soluble the compound is in aqueous solutions.
For instance, in our exercise, \(As_2S_3\) has a Ksp value significantly lower than that of \(ZnS\). This means \(As_2S_3\) is much less soluble and will precipitate first when both \(As^{3+}\) and \(Zn^{2+}\) ions are present in the solution.

Acidic Medium Influence

Acidic conditions can affect the solubility of compounds significantly. In an acidic medium, the solubility products of certain salts can either increase or decrease based on the nature of the ions involved. For \(As_2S_3\), the presence of excess protons (\text{H}^+) helps keep the sulfide ions (\text{S}^{2-}) in solution, reducing the formation of \(ZnS\). As such, only \(As_2S_3\) precipitates out.
However, the solubility of \(ZnS\) remains relatively unaffected, and it does not precipitate because its Ksp is relatively higher and the available \(S^{2-}\) ions are insufficient to exceed this threshold under the acidic conditions.