The molar solubility of zinc sulfide (ZnS) in pure water is 1.6 x 10⁻¹² M. This exceptionally low value indicates that ZnS is highly insoluble in water.
Understanding Molar Solubility
Molar solubility refers to the number of moles of a solute (a substance that dissolves) that can dissolve in one liter of a solvent (usually water) to form a saturated solution. It is a crucial measure in chemistry, indicating how soluble a compound is under specific conditions. For sparingly soluble ionic compounds like zinc sulfide, molar solubility provides insight into the equilibrium between the undissolved solid and its dissolved ions in solution.
- Significance: Understanding molar solubility is vital in various fields, including environmental science (predicting pollutant dispersion), material science (designing new compounds), and geochemistry (explaining mineral formation).
The Molar Solubility of Zinc Sulfide (ZnS)
Zinc sulfide (ZnS) is a compound known for its very low solubility in water. Its dissolution in an aqueous solution can be represented by the following equilibrium reaction:
ZnS(s) ⇌ Zn²⁺(aq) + S²⁻(aq)
In this equilibrium, solid zinc sulfide (ZnS) is in balance with its constituent ions, zinc ions (Zn²⁺) and sulfide ions (S²⁻), dissolved in water. The molar solubility (often denoted as 's') corresponds to the concentration of Zn²⁺ or S²⁻ ions at equilibrium in a saturated solution.
For ZnS, which dissociates into a 1:1 ratio of ions, the solubility product constant (Ksp) is directly related to its molar solubility. The Ksp expression for ZnS is:
Ksp = [Zn²⁺][S²⁻]
Given that the molar solubility (s) represents the concentration of both [Zn²⁺] and [S²⁻] at equilibrium, we can express Ksp as:
Ksp = (s)(s) = s²
Using the molar solubility of 1.6 x 10⁻¹² M for ZnS in pure water, we can calculate its Ksp:
Ksp = (1.6 x 10⁻¹² M)² = 2.56 x 10⁻²⁴
This extremely small Ksp value further confirms that ZnS is indeed very insoluble, meaning only a tiny fraction of the solid dissolves to form ions in solution.
Key Solubility Data for Zinc Sulfide
| Property | Value | Notes |
|---|---|---|
| Molar Solubility (s) | 1.6 x 10⁻¹² M | In pure water at standard conditions. |
| Solubility Product (Ksp) | 2.56 x 10⁻²⁴ | Calculated from molar solubility. |
| Solubility Classification | Highly Insoluble | Due to very low molar solubility and Ksp. |
Factors Influencing ZnS Solubility
While the inherent solubility of ZnS is low, several factors can influence its actual concentration in solution:
- Temperature: Like most solids, the solubility of ZnS generally increases with increasing temperature, as higher kinetic energy allows more ions to break away from the solid lattice.
- Common Ion Effect: The presence of a common ion (either Zn²⁺ or S²⁻) from another source in the solution will shift the equilibrium to the left, further reducing the molar solubility of ZnS. For example, adding zinc chloride (ZnCl₂) to a ZnS solution would decrease ZnS solubility.
- pH of the Solution: The solubility of metal sulfides like ZnS is significantly affected by pH. Sulfide ions (S²⁻) are the conjugate base of a weak acid (H₂S) and are prone to hydrolysis in water:
S²⁻(aq) + H₂O(l) ⇌ HS⁻(aq) + OH⁻(aq)
In acidic solutions (low pH), H⁺ ions react with S²⁻ or HS⁻ to form H₂S gas, effectively removing sulfide ions from the solution and shifting the ZnS dissolution equilibrium to the right, thus increasing its solubility. This is why many metal sulfides are more soluble in acidic conditions.
Applications and Importance
The low solubility of zinc sulfide makes it useful in various applications. For instance, its insolubility is a key property in certain analytical chemistry procedures for separating metal ions. In environmental contexts, understanding its solubility helps in assessing the mobility and bioavailability of zinc in sulfide-rich environments. ZnS is also widely used as a pigment, in luminescent materials, and in optical coatings, where its stable, insoluble nature is beneficial.
