Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Edher on February 28, 2005, 06:31:39 PM
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What [OH-] should be maintained in a solution if, after precipitation of Mg+2 as
Mg(OH)2, the remaining Mg+2 is to be at a level of 1 microgram Mg2+ / L?
Ksp = 1.8x10^-11.
I found the initial concentration of OH- to be
After I did stoicheometry I foud that 1 microgram Mg2+ / L is tantamount to 4.11x10^-8 moles.
What exactly do I do with this result? Am I supposed to add it to the initial [OH-]
that's what I did and I didn't get the right answer. According to the book, the answer should be 0.02M.
Edher
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You have the definition of Qsp:
Qsp = [Mg2+ ][OH- ]2
You know what final concentration (M) of Mg2+ is to be.
Simply rearrange above definition and you will find what concentration of OH- is necessary to keep Mg2+ concentration below given treshold.
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After I did stoicheometry I foud that 1 microgram Mg2+ / L is tantamount to 4.11x10^-8 moles.
Should be 4.17x10^-8
Correction added later - this number is for rounded mass of Mg to 24 - this causes error of 1.3 %.
Your result is correct!