Ksp = 1.8 x 10-11 for magnesium hydroxide at 25°C. C) What are the molar concent
ID: 500918 • Letter: K
Question
Ksp = 1.8 x 10-11 for magnesium hydroxide at 25°C.
C) What are the molar concentrations of magnesium ion and hydroxide ion in an aqueous solution prepared by dissolving as much solid magnesium hydroxide as possible in 0.10 M KOH?
D) What are the molar concentrations of magnesium ion and hydroxide ion in an aqueous solution prepared by dissolving as much solid magnesium hydroxide as possible in 0.10 M Mg(NO3)2?
E) Which solvent – 0.10 M KOH, 0.10 M Mg(NO3)2, neither, or cannot be determined – has a greater effect on the solubility of magnesium hydroxide? Offer an explanation why.
F) What happens (decrease, increase, no change, or cannot be determined) to the molar concentration of magnesium ion in the solution described in Part b above if some 6.0 M HCl is added to the saturated aqueous solution? Justify your choice.
Explanation / Answer
c) Ksp for magnesium hydroxide, Mg(OH)2 is 1.8*10-11.
Write down the dissociation of Mg(OH)2 as
Mg(OH)2 (s) <=====> Mg2+ (aq) + 2 OH- (aq)
Ksp = [Mg2+][OH-]2
Given we have 0.10 M KOH, we must have [OH-] = 0.10 M (ans).
Plug in the value of [OH-] in the expression for Ksp and obtain
Ksp = [Mg2+](0.10)2
===> 1.8*10-11 = [Mg2+](0.10)2
===> [Mg2+] = 1.8*10-11/(0.10)2 = 1.8*10-11/0.01 = 1.8*10-9
The concentrations of Mg2+ and OH- in 0.10 M KOH solution are [Mg2+] = 1.8*10-9 M and [OH-] = 0.10 M (ans).
d) We next have a 0.10 M Mg(NO3)2 solution so that [Mg2+] = 0.10 M (ans).
Use the expression for Ksp to deduce the concentration of OH-; therefore,
Ksp = [Mg2+][OH-]2
===> 1.8*10-11 = (0.10).[OH-]2
===> [OH-]2 = 1.8*10-11/(0.10) = 1.8*10-10
===> [OH-] = 1.8*10-10
===> [OH-] = 1.3416*10-5 1.34*10-5
Therefore, the concentrations of Mg2+ and OH- in a 0.10 M solution of Mg(NO3)2 are [Mg2+] = 0.10 M and [OH-] = 1.34*10-5 M (ans).
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