September 29, 2020, 10:00:30 PM
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### Topic: How many grams of Mg(OH)2 can dissolve in a pH=5 solution  (Read 884 times)

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#### sharbeldam

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##### Re: How many grams of Mg(OH)2 can dissolve in a pH=5 solution
« Reply #15 on: December 17, 2019, 01:50:49 AM »
Yes I checked that s is much smaller after I solved.
I dont mean to be close minded but I think the only logical way I can get this, is that if they put in the question " Assuming that Mg(OH)2 dissociates completely "

if that assumption is not made, we cant say that the moles of OH- is exactly double its moles, since simply the moles of OH- is " 2s " and the moles of Mg(OH)2 if it does NOT dissociate completely is XXX-s, if it did dissociate completely then it is " s ".

I really hope my point is logical a bit
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#### AWK

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##### Re: How many grams of Mg(OH)2 can dissolve in a pH=5 solution
« Reply #16 on: December 17, 2019, 02:07:18 AM »
Ksp is used only for ionic compounds.
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#### sharbeldam

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##### Re: How many grams of Mg(OH)2 can dissolve in a pH=5 solution
« Reply #17 on: December 17, 2019, 02:30:21 AM »
Just so it can be final, can you answer with a yes or no (I know you like to make people think more and not give direct answer AWK )

Despite it being equilibrium do we assume that the ionic compounds dissociate completely?
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#### AWK

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##### Re: How many grams of Mg(OH)2 can dissolve in a pH=5 solution
« Reply #18 on: December 17, 2019, 02:41:20 AM »
If not then the stability constant of complext is used. Eg Al(OH)3 in water is simply ionic (very good approximation) - use Ksp, in 0.3 M NaOH mainly covalent Al(OH)63- is present.
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#### Borek

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##### Re: How many grams of Mg(OH)2 can dissolve in a pH=5 solution
« Reply #19 on: December 17, 2019, 03:09:53 AM »
A   +   B <--------> 2C
2M      2M               0
-x       -x                 +2x
2-x      2-x               2x

2-x doesn't equal 2x in a usual equilbrium problem.

It doesn't. But first - -x and +2x come from a simple stoichiometry like the one you should follow here, second reaction you wrote is different from the one at hand.

Quote
Mg(OH)2 <-----> Mg+2  + 2OH-

That's not the reaction you are dealing with. Mg(OH)2 is a solid, so it it is not present in a dissolved from in the solution:

Mg(OH)2(s) Mg2+(aq) + 2OH-(aq)
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