science( chem)

Calculate the molar solubility of
Mg(OH)2 in water

I know
Mg(OH)2=> Mg2+ + 2OH-
2H2O=> H3O+ + OH-

Ksp= [Mg2+][OH]2
Kw= [H30+][OH-]

Mass balance
[Mg2+]=2[OH] So..
[OH-]=2[Mg2+]+ [H3O+]

Charge balance is where I get kind of confused
I know that the positive charge is equivalent to = 2[Mg2+]+[H3O+]
but on the negative side there are 2 sources of negative [OH-] ions...

[OH-]= ??

Isn't it supposed to be the molar concentration of the ion x charge of the ion?
I was thinking of 2[OH][OH] or 2[OH][H30+] but if it was mass charge I know it would be 2[Mg2+][H3O+] but
since it is a charge balance it can't have a + charge for the negative side right?
I need help on this...

Christina--I don't quite understand why you have approached the problem this way unless, perhaps, this was the manner in which the instructor wants the problem solved. I would calculate molar solubility this way.
Mg(OH)2 ==> Mg^+2 + 2OH^-
Let molar solubility of Mg(OH)2 be S, then (Mg^+2) = S
and (OH^-) = 2S
Then Ksp = (Mg^+2)(OH^-)^2
Ksp = (S)(2S)^2 = 4S^3
Look up Ksp and solve.
I know this doesn't answer any of your qustions about mass balance or charge balance but it solves the molar solubility of magnesium hydroxide, which it appears is the problem here.

um...ha..well that's how they approach it in the text...first we used this method that you described but then when we were doing the electrolyte effect on solubility we had to go through these steps and in this example I described which is by the way in chapter- [solving eq problems ]for complex systems" this is how they do the problem but I didn't get how they got the charge balance...The reason I'm going through all this even though it would be simpler to approach it that way is that I have to do more complicated problems which require the mass balance and charge balance so If I don't get 1 part of this simpler Q (they put the simpler q's first)then how am I supposed to get the more complicated ones? =/

Is ANYONE going to help me on the balancing ???

I really need help on that...
please?

I'm sorry I took so long. But here goes. I had too many things to do today including a lot of things I have placed on hold for the last two weeks.

Mass balance
[Mg2+]=2[OH] So.. I don't agree that (Mg^+2) = 2(OH^-). I think if Mg is x then OH is 2x; therefore, (OH^-)= 2(Mg^+).

[OH-]=2[Mg2+]+ [H3O+] This one is OK but it doesn't follow from your first equation.


Charge balance is where I get kind of confused
I know that the positive charge is equivalent to = 2[Mg2+]+[H3O+]
but on the negative side there are 2 sources of negative [OH-] ions...
[OH-]= ??

Isn't it supposed to be the molar concentration of the ion x charge of the ion?
I was thinking of 2[OH][OH] or 2[OH][H30+] but if it was mass charge I know it would be 2[Mg2+][H3O+] but
since it is a charge balance it can't have a + charge for the negative side right?

Yes, and yes. You DO have two sources for OH^- but that doesn't change anything. The positive ions are Mg^+2 and H3O^+. The negative charge is OH^- only. So set + = - and get
2(Mg^+2) + (H3O^+) = (OH^-)


Also, I understand what you are doing now. You are using the simple ones to help understand the complicated ones when you get there.

Thanks Dr.Bob =), and I appreciate it that you are spending your time to help even though you're busy

I'll probably find out if I get this concept on the Q's I'm doing now which are tougher...

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