Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: kdbmvp on February 23, 2019, 05:29:25 AM

Title: Solubility calculations
Post by: kdbmvp on February 23, 2019, 05:29:25 AM
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+..?
Title: Re: Solubility calculations
Post by: chenbeier on February 23, 2019, 06:19:14 AM
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.
Title: Re: Solubility calculations
Post by: kdbmvp on February 23, 2019, 06:22:06 AM
Thanks. Is it possible to deduce that from the Ksp value?
Title: Re: Solubility calculations
Post by: chenbeier on February 23, 2019, 09:39:13 AM
How you want to do it?
Title: Re: Solubility calculations
Post by: kdbmvp on February 23, 2019, 11:19:06 AM
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.
Title: Re: Solubility calculations
Post by: chenbeier on February 23, 2019, 01:58:49 PM
Of course you can.  Check the formula between soloubility, concentration and soloubility product
Title: Re: Solubility calculations
Post by: kdbmvp on February 24, 2019, 04:53:39 AM
You mean the equation for Ksp?
Title: Re: Solubility calculations
Post by: chenbeier on February 24, 2019, 07:12:25 AM
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