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Topic: How do you know which cation will precipitate first?  (Read 32486 times)

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Offline o1ocups

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How do you know which cation will precipitate first?
« on: March 17, 2009, 06:04:29 PM »
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  ???

Offline Borek

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Re: How do you know which cation will precipitate first?
« Reply #1 on: March 17, 2009, 06:21:22 PM »
Correct thinking :)
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Offline o1ocups

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Re: How do you know which cation will precipitate first?
« Reply #2 on: March 17, 2009, 06:27:24 PM »
Cool thanks  ;D

Offline o1ocups

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Re: How do you know which cation will precipitate first?
« Reply #3 on: March 17, 2009, 10:24:15 PM »
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?

Offline Loyal

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Re: How do you know which cation will precipitate first?
« Reply #4 on: March 17, 2009, 10:44:15 PM »
Do remember that if the Cations are of different charge the Ksp may be lying to you, but in the case where both ions are the same charge the Ksp values should tell which one precipitates first.
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Offline o1ocups

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Re: How do you know which cation will precipitate first?
« Reply #5 on: March 17, 2009, 10:47:56 PM »
Do remember that if the Cations are of different charge the Ksp may be lying to you, but in the case where both ions are the same charge the Ksp 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?  ???

Offline Borek

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Re: How do you know which cation will precipitate first?
« Reply #6 on: March 18, 2009, 04:20:32 AM »
Do remember that if the Cations are of different charge the Ksp may be lying to you, but in the case where both ions are the same charge the Ksp 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.
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Offline Borek

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Re: How do you know which cation will precipitate first?
« Reply #7 on: March 18, 2009, 04:21:17 AM »
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?
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Offline Loyal

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Re: How do you know which cation will precipitate first?
« Reply #8 on: March 18, 2009, 07:04:43 AM »
Do remember that if the Cations are of different charge the Ksp may be lying to you, but in the case where both ions are the same charge the Ksp 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 Ksp alone.  While one Ksp may be smaller, when you actually do the calculations you end up taking the cube root or greater of the Ksp compared to the square root in a 1 to 1 cation to anion which results vastly different number.
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Offline Borek

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Re: How do you know which cation will precipitate first?
« Reply #9 on: March 18, 2009, 09:25:27 AM »
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.
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Offline Loyal

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Re: How do you know which cation will precipitate first?
« Reply #10 on: March 19, 2009, 02:02:54 AM »
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. 
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