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Ch.17 - Additional Aspects of Aqueous Equilibria
Chapter 17, Problem 55

A 1.00-L solution saturated at 25 C with calcium oxalate 1CaC2O42 contains 0.0061 g of CaC2O4. Calculate the solubility-product constant for this salt at 25 C.

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Step 1: Identify the balanced chemical equation for the dissolution of calcium oxalate in water. It is CaC2O4(s) ⇌ Ca2+(aq) + C2O42-(aq).
Step 2: Calculate the molar mass of calcium oxalate (CaC2O4). You can do this by adding up the molar masses of each element in the compound.
Step 3: Convert the mass of CaC2O4 in the solution to moles using the molar mass. This will give you the molarity of CaC2O4, which is the concentration of Ca2+ and C2O42- ions in the solution (since one formula unit of CaC2O4 produces one Ca2+ ion and one C2O42- ion).
Step 4: Write the expression for the solubility product constant (Ksp) of CaC2O4. According to the balanced chemical equation, Ksp = [Ca2+][C2O42-].
Step 5: Substitute the concentrations of Ca2+ and C2O42- ions (which are equal to the molarity of CaC2O4) into the Ksp expression to calculate the solubility product constant.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is an equilibrium constant that applies to the solubility of sparingly soluble ionic compounds. It is defined as the product of the molar concentrations of the ions, each raised to the power of their coefficients in the balanced equation. For calcium oxalate (CaC2O4), Ksp can be expressed as Ksp = [Ca^2+][C2O4^2-], where the concentrations are determined at saturation.
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Saturation and Concentration

A saturated solution is one in which the maximum amount of solute has been dissolved at a given temperature, resulting in a dynamic equilibrium between the dissolved ions and the undissolved solid. The concentration of the ions in a saturated solution can be calculated from the mass of solute present and the volume of the solution, which is essential for determining Ksp.
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Stoichiometry of Dissolution

Stoichiometry in the context of dissolution refers to the ratio of ions produced when a compound dissolves in water. For calcium oxalate, the dissolution can be represented as CaC2O4(s) ⇌ Ca^2+(aq) + C2O4^2-(aq). Understanding this stoichiometric relationship is crucial for calculating the concentrations of the ions needed to find the Ksp value.
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