Clothing washed in water that has a manganese [Mn^2+ (aq)] concentration exceedi

Question

Clothing washed in water that has a manganese [Mn^2+ (aq)] concentration exceeding 0.1 mg/L (1.8 times 10^-6 M) may be stained by the manganese upon oxidation, but the amount of Mn^2+ in the water can be reduced by adding a base. If a person doing laundry wishes to add a buffer to keep the pH high enough to precipitate the manganese as the hydroxide, Mn(OH)_2, what pH is required to keep [Mn^2+] equal to 1.8 times 10^-6 M? Mn(OH)_2 (s) Mn^2+ (aq) + 2OH^- (aq) Ksp = 4.5 times 10^-14 A) 3.80 B) 7.60 C) 11.4 D) 5.60 E) 10.20

Explanation / Answer

Ksp = [Mn2+][OH-]2

4.5*10-14 = (1.8*10-6) [OH-]2

[OH-] = 1.58 * 10-4 M

pH+pOH = 14

pH = 14 - pOH

pH = 14 + Log[OH-]

pH = 14 + Log(1.58*10-4)

pH = 10.20

(E)

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