Question

A solution of Na₂SO₄ is added dropwise to a solution that is 0.010 M in Ba2+ and 0.010 M in Sr2+

What concentration of SO₄ 2- is necessary to begin precipitation? (Neglect volume changes. BaSO₄ Ksp = 1.1 x 10-10; SrSO₄: Ksp = 3.2 x 10⁻⁷)

Answer 1

To begin the precipitation of BaSO4 from the solution containing Ba2+ and Sr2+ ions, a SO4 2- ion concentration of 1.1 x 10-8 M is required; for SrSO4, higher concentration of 3.2 x 10-5 M is needed, indicating BaSO4 will precipitate first.

Step-by-step explanation:

To determine the concentration of sulfate ions (SO42-) needed to begin the precipitation of either BaSO4 or SrSO4, we apply the concept of the solubility product constant (Ksp). For the precipitation to start, the product of the ion concentrations must exceed the respective Ksp values. We already know the concentration of Ba2+ and Sr2+ is 0.010 M in the solution.

Using the formula Q = [Ba2+][SO42-] and the known Ksp values, we can solve for the sulfate concentration required for precipitation to occur. For BaSO4, the Ksp is 1.1 x 10-10. Hence, Q must equal to Ksp when precipitation begins:

[SO42-] = Ksp / [Ba2+] = 1.1 x 10-10 / 0.010 M = 1.1 x 10-8 M

For SrSO4, the Ksp value is 3.2 x 10-7, so:

[SO42-] = Ksp / [Sr2+] = 3.2 x 10-7 / 0.010 M = 3.2 x 10-5 M

Comparing the two, BaSO4 will precipitate first as it requires a lower concentration of SO42- to reach its Ksp level. This means that as Na2SO4 is added, BaSO4 will precipitate before SrSO4 does when the SO42- concentration in the solution reaches 1.1 x 10-8 M.