[o1ocups]

Consider a solution that is 2.3×10^-2 M in Fe2+ and 1.9×10^-2 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?

Is it Fe+ because FeCO3 has a smaller ksp value (less solubility)? But I am not sure 

[Borek]

Correct thinking

[o1ocups]

Cool thanks 

[o1ocups]

What would be the remaining concentration of Fe2+, when the other cation (Mg2+) just begins to precipitate? I have no idea where to start  Wouldn't it just be 0?

[Loyal]

Do remember that if the Cations are of different charge the K_sp may be lying to you, but in the case where both ions are the same charge the K_sp values should tell which one precipitates first.

[o1ocups]

Do remember that if the Cations are of different charge the K_sp may be lying to you, but in the case where both ions are the same charge the K_sp values should tell which one precipitates first.

Yeah, and you also have to look at the stoichiometry right? Like 3 molecules as opposed to 2 etc.
By the way would mind giving me a hint on how to calculate the "remaining concentration" of Fe2+ when Mg2+ just starts precipitating? 

[Borek]

Do remember that if the Cations are of different charge the K_sp may be lying to you, but in the case where both ions are the same charge the K_sp values should tell which one precipitates first.

Ksp won't be lying, they just can't be simply compared then, but they still can be used to calculate precipitation order. And the problem is not in the cation charge, but in the salt stoichiometry, you can't directly compare CaCO₃ and Ca₃(PO₄)₂ even if the cation charge is identical.

[Borek]

What would be the remaining concentration of Fe2+, when the other cation (Mg2+) just begins to precipitate? I have no idea where to start  Wouldn't it just be 0?

What will be concentration of the carbonate the moment Mg²⁺ starts to precipitate?

[Loyal]

Do remember that if the Cations are of different charge the K_sp may be lying to you, but in the case where both ions are the same charge the K_sp values should tell which one precipitates first.

Ksp won't be lying, they just can't be simply compared then, but they still can be used to calculate precipitation order. And the problem is not in the cation charge, but in the salt stoichiometry, you can't directly compare CaCO₃ and Ca₃(PO₄)₂ even if the cation charge is identical.

Actually you kind of restated my point. I think you misunderstood me a little bit.

In a problem where you are adding a single anion to solution to separate two or more cations, the different cation charges will result in different stiochiometry coefficients and subsequently different exponents in the actual equilibrium problem.  So yes the charge does matter when dealing with a single anion because the cations require different quantities of the anion to be eletroneutral.

It is a trap that gets many people who look at K_sp alone.  While one K_sp may be smaller, when you actually do the calculations you end up taking the cube root or greater of the K_sp compared to the square root in a 1 to 1 cation to anion which results vastly different number.

[Borek]

While your analysis is correct, your original statement, without "ifs" and "whens", is confusing. I am afraid this is one of those overgeneralisations that sound good at first, but do more harm than good.

[Loyal]

While your analysis is correct, your original statement, without "ifs" and "whens", is confusing. I am afraid this is one of those overgeneralisations that sound good at first, but do more harm than good.

Understandable.