Question

[Mg2+] = 0.059M, [Ca2+] = 0.011M are in a solution, and they can be seperated by selective precipitation of KOH (strong base). When the [OH-] reaches 1.9E-6M, Mg(OH)2 begins to precip out of sol'n. As you continue adding KOH, Mg(OH)2 cont. to precipitate. However, at some pt the [OH-] becomes larger enough to begin to precipitate the Ca2+ ions as well. What is the [Mg2+] when Ca2+ begins to precipitate out?

Answer 1

My Attempt: (PLEASE SEE ATTACHMENT FOR THE "BOXES" I AM REFFERING TO) ONE SEC
---------------------------------------------------------------
It wants the [Mg2+] when Ca2+ begins to precipitate. I understand up to the second box: I know that you have to set Q = Ksp for the Ca(OH)2 expression if you want to find out what is the concentration of [OH-] that triggers the Ca2+ to precipitate out. However, for the third box, I do not understand why they equated the Qsp for Mg(OH)2 with the Ksp of Mg(OH)2.If we want to figure out the Mg2+ at the time that Ca2+ started to precipitate, why do we not do:
Qsp of Mg(OH)2 = Ksp of Ca(OH)2
^- ?
In short, I do not understand why they would do Qsp of Mg(OH)2 = Ksp of Mg(OH)2, because to me this looks like getting the concentration for when Mg2+ will begin to precip out.
----------------------------------------------------------------

Answer 2

you have to set Q = Ksp for the Ca(OH)2 expression if you want to find out what is the concentration of [OH-] that triggers the Ca2+ to precipitate out
that is the concetration of OH- at that time
but the same conc.. is in balance with Mg(OH)2 (what has been left of it because it has allready been precipitating...

Answer 3

there is no need to take it like you set Q equal to Ksp
you can take it like "at the same time Ksp of Wg(OH)2 is .........................(a word meaning that it stands, it is in effect, is true...)

Answer 4

I do not see how that makes sense though..

Answer 5

you add OH
Mg starts to precipitate....
some Mg and OH goes down(precipitate) and the rest complay to Ksp
more OH
more Mg precipitate...
still rest complay to Ksp

at some time [OH] rises to such value that Ca starts to conce...
at that time concetration of ion validate both Ksp...

Answer 6

Ksp1=[Ca]*[OH]
Ksp2=[Mg]*[OH]
same OH to both of them....