Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: kdbmvp on February 23, 2019, 05:29:25 AM
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Hello,
I am a bit confused re. solubility calculations.
Calculate the solubility of Pb(OH)2 at pH 10. Setting up the expression for Ksp:
Ksp = [Pb2+][OH-]2 = 8* 10^-17 (Ksp value from SI chemical data)
pOH = 14 - 10 = 4, i.e. [OH-] = 10-4.
8*10-17 = [Pb2+][OH-]2. Solving for [Pb2+] we get that [Pb2+] = 8*10-9.
My question is: why couldn't we use the relationship between the concentrations of Pb2+ and OH- like we do with ICE tables, to get the result? I mean, couldn't we just divide the concentration of OH- by 2 and get the concentration of Pb2+..?
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This would be possible, if lead hydroxide would be dissolved like sodium hydroxide. But it doesn't. You need the soloubility product, because the soloubility is poor.
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Thanks. Is it possible to deduce that from the Ksp value?
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How you want to do it?
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I just wondered if you could from the Ksp value see that lead hydroxide has poor solubility, and that it wouldn't be possible to use the relationship between the concentrations.
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Of course you can. Check the formula between soloubility, concentration and soloubility product
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You mean the equation for Ksp?
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Yes and No
Read here: https://chemistry.stackexchange.com/questions/75592/for-the-solubility-of-ab2-ksp-ab2-x2x2-why-is-2x2-not-considered-d