[MightyMan]

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]

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.

[MightyMan]

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 [S^2-]=2[Ag⁺],
which is equal to the solubility of silver sulfide

[Borek]

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

You have missed the main point - it gives you [S^2-] 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.

[MightyMan]

Do you mean that [S^2-]=[H₂S]+[HS^-]+[S^2-]?
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:
[S^2-]=1x10^-14
[HS^-]=9.524x10^-8
[H₂S]=1x10^-7

Then the sum of these will equal the total S^2- 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]

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]

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.