Consider a solution that is 2.1×10−2 M in Fe2+ and 1.6×10−2 M in Mg2+.
If potassium carbonate is used to selectively precipitate one of the cations while leaving the other cation in solution, which cation will precipitate first? ANSWER: Fe 2+
What minimum concentration of K2CO3 is required to cause the precipitation of the cation that precipitates first? ANSWER: [K2CO3] = 1.5×10−9 M
What is the remaining concentration of the cation that precipitates first, when the other cation just begins to precipitate? (ANSWER IS NOT .021 or 2.0E-6)
Fe2+ will precipitate first as solubility product of FeCO3 is lower than solubility product of MgCO3.
Fe2+ + CO32-
Ksp = [Fe2+][CO32-] = 3.07 x 10-11
3.07 x 10-11 = (2.1 x 10-2)[CO32-]
[CO32-] = 1.5 x 10-9 M to precipitate the Fe2+ ions
Mg2+ + CO32-
Ksp = [Mg2+][CO32-] = 3.07 x 10-11
6.82 x 10-6 = (1.6 x 10-2)[CO32-]
[CO32-] = 4.3 x 10-4 M to precipitate the Mg2+ ions
Ksp = [Fe2+][CO32-]
= 3.07 x 10-11
3.07 x 10-11 = [Fe2+](4.3 x 10-4)
[Fe2+] = 7.1 x 10-8 M when the Mg2+ ions precipitates
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