ZnCO3, a slightly soluble substance, is least soluble in which one of the following solutions / solvents?
pure water,
0.2M ZnCl2,
0.1M Na2CO3 or
0.1M HCl
The Correct Answer and Explanation is :
The least soluble form of ZnCO₃ (zinc carbonate), which is slightly soluble in water, would be in the solution that promotes the common ion effect. The common ion effect refers to the decrease in solubility of a salt when a solution contains an ion that is already a part of the salt.
Let’s evaluate each option:
- Pure Water: In pure water, ZnCO₃ will dissolve according to its solubility product (Ksp), but without any ions present to affect its solubility, it will dissolve to some extent. Its solubility will be determined solely by its own solubility equilibrium.
- 0.2M ZnCl₂: This solution contains Zn²⁺ ions, which are also part of ZnCO₃. The presence of the common ion Zn²⁺ will shift the solubility equilibrium of ZnCO₃ to the left, meaning less ZnCO₃ will dissolve in this solution due to the common ion effect. Thus, ZnCO₃ will be least soluble in this solution.
- 0.1M Na₂CO₃: This solution provides CO₃²⁻ ions, which are part of ZnCO₃. The presence of a common ion (CO₃²⁻) would also reduce the solubility of ZnCO₃ by shifting the equilibrium to the left. However, it might still be more soluble than in the ZnCl₂ solution since Zn²⁺ ions might have a stronger effect on solubility compared to CO₃²⁻ ions.
- 0.1M HCl: HCl is a strong acid, and it will dissociate completely to produce H⁺ ions. The H⁺ ions will react with CO₃²⁻ ions from ZnCO₃ to form CO₂ and water, driving the dissolution of ZnCO₃. This would result in ZnCO₃ being more soluble in the presence of HCl.
Correct Answer: 0.2M ZnCl₂. The common ion effect from the Zn²⁺ ions in ZnCl₂ will make ZnCO₃ the least soluble in this solution.
Explanation Summary:
- In pure water, the solubility is governed by the salt’s own Ksp.
- The 0.2M ZnCl₂ solution provides Zn²⁺, a common ion that decreases the solubility of ZnCO₃.
- The 0.1M Na₂CO₃ introduces CO₃²⁻, another common ion, which also reduces solubility but likely less effectively than Zn²⁺.
- The 0.1M HCl dissociates to produce H⁺, which neutralizes CO₃²⁻ and increases ZnCO₃ solubility.
Let me know if you’d like me to generate the image showing the common ion effect and solubility equilibrium!