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### Topic: Solubility Question.. or is it?  (Read 16320 times)

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#### MightyMan

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##### Re: Solubility Question.. or is it?
« Reply #15 on: October 07, 2009, 04:07:32 PM »
I know Borek..
However I spoke to my professor and that is the assumption that he wants us to make..
I still cant figure out why my variables are cancelling!

#### Borek

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##### Re: Solubility Question.. or is it?
« Reply #16 on: October 07, 2009, 05:58:47 PM »
My guess is that if they cancel out you are 'oversubstituting'  - that is a good way of proving 1=1. It works almost always

Take a look at equation 9.13:

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-acid-base

it may come handy. Note, that assumption you are asked to make means pH=7.00.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### MightyMan

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##### Re: Solubility Question.. or is it?
« Reply #17 on: October 07, 2009, 06:23:14 PM »
I'm not too sure about what you mean by over-substituting,
well, i do know what you mean, but are my equations over-substituted?
i mean, there are two variables and i have two equations

I know that those equations can be used to solve the question,
I've been told that they will work...

I took a look at the equation you suggested,
it tells me that [S2-]=2[Ag+],
which is equal to the solubility of silver sulfide

#### Borek

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##### Re: Solubility Question.. or is it?
« Reply #18 on: October 07, 2009, 06:52:19 PM »
I took a look at the equation you suggested,
it tells me that [S2-]=2[Ag+],
which is equal to the solubility of silver sulfide

You have missed the main point - it gives you [S2-] as a function of TOTAL concentration of all forms of dissolved hydrogen sulfide. If pH is 7 everything else is known. But I won't tell anything more, as at this moment it is 30 sec work.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### MightyMan

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##### Re: Solubility Question.. or is it?
« Reply #19 on: October 07, 2009, 07:38:59 PM »
Do you mean that [S2-]=[H2S]+[HS-]+[S2-]?
I'm still uncertain on how to calculate the concentration of the different forms of hydrogen sulfide.
Does pH=7 allow me to assume that [HS-]=[H2S]=1x10-7?

I can then find the concentration of the reactant:
[S2-]=1x10-14
[HS-]=9.524x10-8
[H2S]=1x10-7

Then the sum of these will equal the total S2- in the solution,
and therefore the solubility of silver sulfide=1.952x10-7?

I apologize if it seems as if I have no background knowledge on this subject,
it is an extra-work question and it seems to be a tad advanced, but i'd still like to understand how to solve it..

#### MightyMan

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##### Re: Solubility Question.. or is it?
« Reply #20 on: October 07, 2009, 08:23:43 PM »
Nevermind Borek,

I figured out the answer using my original method.
I understood what you meant by over-substituting....

Thanks again for all your help,
I really appreciate it.

Just out of curiousity,
should the mass balance be 0.5[Ag+]?
as there are 2 mols of silver per mole of sulfide

#### inispree

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##### Re: Solubility Question.. or is it?
« Reply #21 on: October 08, 2009, 12:02:10 AM »
Hello!
I am trying to solve this problem as well.
I'm still very lost.
How did you go about solving this?

I got till here and I'm stuck..
Ag2S --> 2Ag+ + S2-   Ksp 6x10^-50, where unknown x = [S2-]
S2- + H2O --> OH- + HS-    Kb = 1
I assumed all other Ka and Kb don't play a big role since their values are less than 10^-4.