[Legato]

Here's the question:

At what pH will iron II sulfide begin to precipitate if the iron II ion concentration is .1 M, and the dihydrogen sulfide concentration is .01 M? Show all calculations.

What's given:
Solubility product constants of iron II sulfide  (4.9e-18)
and dihydrogen sulfide  (1.3e-20)
minimum sulfide ion concentration to precipitatee .1 M cations   (4.9e-17)
a table of pH's and sulfide ion concentrations at specific dihydrogen sulfide concentrations

I know this seems simple, but I've been out of chemistry for awhile and I'm not sure how to calculate what concentrations there are of certain ions without more information. Also, would the sulfide ion concentration simply be the minimum that is given? I don't need a numerical answer, just a general formula or method for solving this. Thank you for the help.

Edit: I tried it out and got 2.8 for the pH. Is this correct?

[Borek]

What do you mean by solubility product for H₂S?

How can we even try to check if you are in a correct ballpark not seeing the most important part of the data?

[Legato]

I'm sorry, I guess I didn't realize it was unclear. The way it's given in the problem is that [S2-]([H+]^2)/[H2S] = 1.3x10^20. I hope that helps.

[Borek]

OK, that's just second overall dissociation constant.

Can you use it to calculate concentration of S^2- in the solution as a function of pH?

If Ka1,2 is given - what is the table for?