A) 0.86 B) 1.21 C)2.00 D) 1.12 E) 2.86 Clothing washed in water that has a manga

Question

A) 0.86 B) 1.21 C)2.00 D) 1.12 E) 2.86 Clothing washed in water that has a manganese [Mn^2+(aq)] concentration exceeding 0.1 mg/L (1.8 times 10^-8 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^-(ag) Ksp = 4.5 times 10^-14 A) 3.80 B) 7.60 C) 11.4 D) 5.60 E) 10.20

Explanation / Answer

The answer is E) 10.20

Explanation

Mn(OH)2 (s) <------> Mn2+(aq) + 2OH-(aq)

Ksp = [Mn2+] [ OH- ] ^2 = 4.5×10^-14M^3

[Mn2+] = 1.8 × 10^-6M

4.5 × 10^-14 M^3 = (1.8×10^-6) × [ OH- ]^2

[OH- ]^2= 4.5×10^-14M^3/1.8×10^-6M

= 2.5 × 10^-8M^2

[OH-] = 1.58 ×10^-4M

pOH = 3.80

pH + pOH = 14

Therefore,

pH = 14- 3.80

= 10.20

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