Question

What is the solubility of Mg(OH)2 in an aqueous solution buffered to a pH of 13.13? The Ksp of Mg(OH)2 is 1.8 x 10-11. Hint: Determine the [OH-] from the pH then deal with solubility with common ion.

Determine if adding each of the following to a saturated Ca₃(PO₄)₂ solution will increase, decrease, or have little to no effect on it ssolubility.

-       A.       B.       C.    HCl       -       A.       B.       C.    CaCl2       -       A.       B.       C.    NaCl       -       A.       B.       C.    Na3PO4

A. increase B. Decrease C. Little to no effect

Answer 1

Solubility of Mg(OH)2 at pH 13.13

pOH = 14 - pH

         = 14 - 13.13 = 0.87

pOH = -log[OH-] = 0.87

[OH-] = 0.135 M

Ksp for Mg(OH)2 = 1.8 x 10^-11

solubility = [Mg2+] = Ksp/[OH-]^2

                              = 1.8 x 10^-11/(0.135)^2

                              = 9.88 x 10^-10 M

Addition of following on the solubility of Ca3(PO4)2

HCl : A

Added H+ reacts with PO4^3- in solution and reduce its concentration. According to Lechatellier's Principle more of salt would dissolve until concentration is reestablished to equilibrium state.

CaCl2 : B

Decrease and more of Ca2+ in solution would shift equilibrium towards reactant end.

NaCl : C

No effect. NaCl gives neutral pH

Na3PO4 : B

Increase as common ion PO4^3- in solution would force more of salt to form.