A solution containing sulfide ions is added to a solution of 0.036 M Cu2+ and 0.044 M Fe2+. At what concentration of sulfide ion will a precipitate begin to form? What is the identity of the precipitate? (a) 1.4 x 10^-16 M, FeS(b) 3.6 x 10^-35 M, CuS(c) 3.6 x 10^-35 M, FeS(d) 1.4 x 10^-16 M, C

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Identify the solubility product constants (Ksp) for CuS and FeS.

Write the equilibrium expressions for the formation of CuS and FeS: CuS(s) ⇌ Cu^{2+}(aq) + S^{2-}(aq) and FeS(s) ⇌ Fe^{2+}(aq) + S^{2-}(aq).

Use the Ksp expressions: Ksp(CuS) = [Cu^{2+}][S^{2-}] and Ksp(FeS) = [Fe^{2+}][S^{2-}].

Calculate the concentration of sulfide ion [S^{2-}] required to start precipitation for each metal ion using the given concentrations of Cu^{2+} and Fe^{2+}.

Compare the calculated [S^{2-}] values to determine which precipitate forms first, identifying the precipitate and the corresponding sulfide ion concentration.

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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 a numerical value that represents the equilibrium between a solid and its ions in a saturated solution. It is specific to a particular compound and is used to predict whether a precipitate will form when solutions are mixed. For sulfide compounds like CuS and FeS, Ksp values are essential for determining the concentration of sulfide ions needed to initiate precipitation.

Precipitation Reaction

A precipitation reaction occurs when two soluble salts react in solution to form an insoluble solid, known as a precipitate. This process is driven by the formation of a compound that has a low solubility in water. In this case, the sulfide ions react with Cu2+ and Fe2+ ions to form CuS and FeS, respectively, depending on the concentration of sulfide ions present.

Ion Product (Q)

The ion product (Q) is the product of the concentrations of the ions in a solution at any point, not necessarily at equilibrium. It is compared to the Ksp to determine if a precipitate will form: if Q exceeds Ksp, precipitation occurs. In this scenario, calculating Q for the sulfide ions with respect to Cu2+ and Fe2+ will help identify the concentration at which precipitation begins.

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