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 x 10-5), or Ag2SO4 (Ksp = 1.5 x 10-5)? (2 points) B) How much Na2SO4 solution must be added to initiate the precipitation?

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Answer:

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Explanation:

A) Since the [SO₄²⁻] required to precipitate CaSO₄ is smaller, CaSO₄ will precipitate first.

B) 1.2 * 10⁻⁶ L of Na₂SO₄ is required to initiate the precipitation of CaSO₄

From the data provided:

  • Concentration of Na₂SO₄ solution = 1.0 M
  • Volume of solution = 10.0 mL = 0.01 L
  • Concentration of Ca²⁺ = 0.2 M
  • Concentration of Ag²⁺ = 0.3 M.
  • Ksp of CaSO₄ = 2.4 x 10⁻⁵ M²
  • Ksp of Ag₂SO₄ = 1.5 x 10⁻⁵ M²

To determine which salt will precipitate first, the concentration of SO₄²⁻ in each salt is determined from the Ksp values of the salts:

Dissociation equations of CaSO₄ and Ag₂SO₄ is given below:

  • CaSO₄ (s)  Ca²⁺ (aq) + SO₄²⁻ (aq)
  • Ag₂SO₄ (s) ⇄ 2 Ag⁺ (aq) + SO₄²⁻ (aq)

For CaSO₄; Ksp = [Ca²⁺][SO₄²⁻]

[SO₄²⁻] = Ksp/[Ca²⁺]

[SO₄²⁻] = 2.4 x 10⁻⁵/ 0.2

[SO₄²⁻] = 1.2 * 10⁻⁴ M

For Ag₂SO₄; Ksp = [Ag⁺]²[SO₄²⁻]

[SO₄²⁻] = Ksp/[Ag⁺]

[SO₄²⁻] = 2.4 x 10⁻⁵/ [0.3]²

[SO₄²⁻] = 1.67 * 10⁻⁴ M

Therefore, since the [SO₄²⁻] required to precipitate CaSO₄ is smaller, CaSO₄ will precipitate first.

B) Volume of Na₂SO₄ required to initiate the precipitation is calculated using the formula:

  • Volume = number of moles / concentration of solution
  • concentration of Na₂SO₄ = 1.0 M

Number of moles SO₄²⁻ = concentration * volume

volume of initial solution = 0.01 L; concentration of SO₄²⁻ = 1.67 * 10⁻⁴ M

number of moles SO₄²⁻ = 1.2 * 10⁻⁴ M * 0.01 L = 1.2 * 10⁻⁶ moles

Volume of Na₂SO₄ required = 1.2 * 10⁻⁶ moles * 1.0 M

Volume of Na₂SO₄ required = 1.2 * 10⁻⁶ L

Therefore, 1.2 * 10⁻⁶ L of Na₂SO₄ is required to initiate the precipitation of CaSO₄

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