A 1.0 M Na2SO4 solution is slowly added to 10.0 mL of a solution that is 0.20 M in Ca2+ and 0.30 M in Ag+. (a) Which compound will precipitate first: CaSO4 (Ksp = 2.4 * 10^-5) or Ag2SO4 (Ksp = 1.5 * 10^-5)?

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Calculate the initial concentrations of Ca^{2+} and Ag^+ ions in the solution before any Na2SO4 is added. Use the formula C_i = \frac{n}{V}, where n is the number of moles and V is the volume in liters.

Determine the ion product (Q) for each potential precipitate (CaSO4 and Ag2SO4) as Na2SO4 is added. The ion product is given by Q = [cation][anion].

For CaSO4, calculate Q = [Ca^{2+}][SO4^{2-}]. Initially, [Ca^{2+}] = 0.20 M and [SO4^{2-}] is initially 0, but increases as Na2SO4 is added.

For Ag2SO4, calculate Q = [Ag^+]^2[SO4^{2-}]. Initially, [Ag^+] = 0.30 M and [SO4^{2-}] is initially 0, but increases as Na2SO4 is added.

Compare the calculated Q values to the Ksp values for each compound. The compound for which Q first exceeds Ksp will precipitate first.

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 indicates the extent to which a compound can dissolve in water. A lower Ksp value means that the compound is less soluble, and thus, it will precipitate out of solution at lower concentrations of its ions.

Precipitation Reaction

A precipitation reaction occurs when two soluble salts react in solution to form an insoluble compound, or precipitate. The formation of a precipitate is driven by the exceeding of the solubility product (Ksp) of the compound. In this scenario, the concentrations of Ca2+ and Ag+ ions will determine which sulfate compound precipitates first as Na2SO4 is added.

Ion Concentration and Reaction Quotient (Q)

The reaction quotient (Q) is calculated using the current concentrations of the ions in solution and is compared to the Ksp to predict precipitation. If Q exceeds Ksp for a compound, precipitation occurs. In this case, the concentrations of Ca2+ and Ag+ ions will be used to calculate Q for both CaSO4 and Ag2SO4 to determine which will precipitate first as Na2SO4 is added.

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