What is the expression for the solubility product, Ksp, of Ca(NO3)2? A. Ksp = [Ca2+][NO3-] B. Ksp = [Ca2+][NO3-]^2 C. Ksp = [Ca2+][NO3-]^3 D. Ksp = [Ca2+][NO3-]^4 E. Ksp = [Ca2+][NO3-]^5
The Correct Answer and Explanation is:
Correct Answer: B
Explanation:
The solubility product constant, Ksp, is an equilibrium constant that describes the equilibrium between a solid ionic compound and its constituent ions in a saturated aqueous solution. The expression for Ksp is derived from the balanced chemical equation for the dissolution process.
- Write the Dissolution Equation: First, we must write the balanced chemical equation that represents the dissolution of calcium nitrate, Ca(NO₃)₂, in water. Calcium nitrate is an ionic salt that dissociates into its ions: one calcium cation (Ca²⁺) and two nitrate anions (NO₃⁻).Ca(NO₃)₂(s) ⇌ Ca²⁺(aq) + 2NO₃⁻(aq)This equation shows that for every one formula unit of solid Ca(NO₃)₂ that dissolves, one aqueous calcium ion and two aqueous nitrate ions are produced.
- Formulate the Ksp Expression: The expression for the solubility product constant (Ksp) is written by taking the product of the molar concentrations of the aqueous product ions, with each concentration raised to the power of its stoichiometric coefficient from the balanced dissolution equation. The solid reactant, Ca(NO₃)₂(s), is not included in the expression because the concentration (or more accurately, the activity) of a pure solid is considered constant and is incorporated into the Ksp value.Based on the dissolution equation:
- The product ions are Ca²⁺(aq) and NO₃⁻(aq).
- The stoichiometric coefficient of Ca²⁺ is 1.
- The stoichiometric coefficient of NO₃⁻ is 2.
- Compare with Options:
- Option A, Ksp = [Ca²⁺][NO₃⁻], is incorrect because it does not raise the nitrate ion concentration to the power of its stoichiometric coefficient, 2.
- Option B, Ksp = [Ca²⁺][NO₃⁻]², correctly represents the product of the ion concentrations, each raised to the power of its stoichiometric coefficient.
- Options C, D, and E are incorrect because they improperly include the concentration of the solid reactant, [Ca(NO₃)₂], or invert the expression. The Ksp expression only includes the concentrations of the dissolved aqueous ions.
