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Topic: solubility  (Read 5310 times)

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Offline WHITECHIC9

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solubility
« on: November 08, 2008, 03:31:04 PM »
I was just wanting to make sure if my answer was right...
The question: Discuss how the solubility of Ca(OH)2 changes with increasing OH concentration.

I think that the solubility would increase with increasing OH, is that correct? I am pretty sure if you add an acid solubility goes up but i don't know about bases. Can you help?

Offline macman104

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Re: solubility
« Reply #1 on: November 08, 2008, 03:45:07 PM »
Don't think in terms of acid and base, think about this from the point of view of Le Chatlier's Principle.

Offline WHITECHIC9

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Re: solubility
« Reply #2 on: November 08, 2008, 04:04:12 PM »
OK soo since you're adding the OH wouldn't it go in a forward direction causing increase in solubility?

Offline macman104

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Re: solubility
« Reply #3 on: November 08, 2008, 04:06:21 PM »
OK soo since you're adding the OH wouldn't it go in a forward direction causing increase in solubility?
Write the chemical equation for the dissociation of Ca(OH)2 in water.  If you add OH- (maybe by adding KOH), which side does that add to?  Why way will that push the equilibrium?

Offline WHITECHIC9

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Re: solubility
« Reply #4 on: November 08, 2008, 04:11:43 PM »
So if you have Ca(OH)2 + KOH= Ca+2  + OH-
I fyou add more OH, then you'd need more Ca(OH)2 correct? Shifting it in reverse direction?

Offline macman104

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Re: solubility
« Reply #5 on: November 08, 2008, 04:20:14 PM »
Yes, it will shift to the left, but I didn't mean to confuse you with the KOH.  I was more saying, that is a possible source of OH-.

You have

Ca(OH)2 + H2O :rarrow: Ca2+ + 2OH-.

Think of this like a balance, if the system is at equilibrium, then the balance is level.
If you were to add OH-, which way will it tip the balance?  So, now which way does the system (the balance) have to shift, in order for it to be level again?

Offline WHITECHIC9

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Re: solubility
« Reply #6 on: November 08, 2008, 04:24:50 PM »
OK i get the equation now, so if you added more OH-, then the equation will shift left to create a balance. Right? Which will increase solubility of Ca(OH)2?

Offline macman104

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Re: solubility
« Reply #7 on: November 08, 2008, 04:34:08 PM »
Close.  It will decrease solubility, because when the equilibrium shifts to the left, dissolved Ca(OH)2 (which are actually ions, like on the right side of the equation), become undissolved Ca(OH)2, like on the left.

Offline WHITECHIC9

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Re: solubility
« Reply #8 on: November 08, 2008, 04:38:45 PM »
ok, does it matter if there's a common ion ?

Offline macman104

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Re: solubility
« Reply #9 on: November 08, 2008, 05:52:08 PM »
That is exactly why you observe the equilibrium shift.  Because there is a common ion.

Offline WHITECHIC9

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Re: solubility
« Reply #10 on: November 08, 2008, 09:08:46 PM »
oh ok. thank you!

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