Will FeS precipitate in a solution that is 0.10 M in Fe(NO3)2, 0.4 M in HCl, and 0.10 M in H2S? Will FeS precipitate if the pH of the solution is adjusted to pH 8 with an NH4+ - NH3 buffer? Kspa = 6 x 10^2 for FeS.

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Identify the relevant chemical reaction: FeS(s) ⇌ Fe²⁺(aq) + S²⁻(aq).

Determine the solubility product expression: K_{sp} = [Fe^{2+}][S^{2-}].

Calculate the concentration of S²⁻ in the initial solution using the given concentration of H₂S and the fact that H₂S is a weak acid that dissociates in two steps: H₂S ⇌ H⁺ + HS⁻ and HS⁻ ⇌ H⁺ + S²⁻.

Use the given pH to find the concentration of H⁺ ions and calculate the concentration of S²⁻ at pH 8 using the NH₄⁺ - NH₃ buffer.

Compare the ion product [Fe²⁺][S²⁻] with the K_{sp} value to determine if precipitation occurs in both scenarios.

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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 represents the product of the molar concentrations of the ions in a saturated solution, each raised to the power of their coefficients in the balanced equation. For FeS, Ksp = [Fe^2+][S^2-], and a comparison of the ion product (Q) to Ksp determines whether a precipitate will form.

Ion Product (Q)

The ion product (Q) is calculated using the concentrations of the ions in solution at any given moment. If Q exceeds Ksp, precipitation occurs; if Q is less than Ksp, the solution remains unsaturated. In this case, calculating Q for FeS using the concentrations of Fe^2+ and S^2- will help determine if FeS will precipitate under the given conditions.

Effect of pH on Solubility

The pH of a solution can significantly affect the solubility of metal sulfides like FeS. At lower pH levels, the concentration of H2S is higher, which can suppress the formation of S^2- ions, thus reducing the likelihood of precipitation. Conversely, increasing the pH to 8 with an NH4+-NH3 buffer will increase the concentration of S^2- ions, potentially leading to precipitation of FeS if the ion product exceeds Ksp.