The reaction of divalent manganese with oxygen in aqueous solution is given as follows: Mn2+ + ½ O2 (aqueous) + H2O ⇌ Mn

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The reaction of divalent manganese with oxygen in aqueous
solution is given as follows:
Mn2+ + ½ O2 (aqueous) + H2O ⇌
MnO2 (solid) + 2H+
The equilibrium constant (K) for this reaction is 23.7. It has
been found that a lake water sample that contains no oxygen at
25°C, pH = 8.5, originally contained 0.6 mg/L of Mn2+.
The sample was aerated (atmospheric conditions so the dissolved
oxygen concentration is 9.2 mg/L) and after 10 days of contact with
atmospheric oxygen, the Mn2+ concentration was 0.4
mg/L.
a) Assuming that the pH remains constant during aeration,
will the precipitate continue to form after the measurement on the
tenth day? Assume ideal conditions.
b) What should the Mn2+ concentration be
(in moles/L) at equilibrium, assuming that pH and presence of
dissolved oxygen are the same as in part “A”? Assume ideal
conditions.
c) What should the Mn2+ concentration (in
moles/L) be at equilibrium if 2x10-3 moles/liter of
NaCl are added to the solution and the pH is adjusted to 2? (Hint:
This is an electrolyte solution, thus ionic strength is
important.)