[DM7000]

Hey everyone! So I have my first inorganic lab this week (Chemistry and Qualitative Analysis of Cations) and two of the questions in the prelab are throwing me for a loop. There is a description of how to do the problems in the lab manual but it never says where the numbers come from so I'm completely lost. The problem is:

At what pH will FeS begin to precipitate if the Fe2+ ion concentration is 0.1M and the H2S concentration is 0.01M? Show all calculations to support your answer.

I know that I have to do a [k1a][k2a]. I have Ka1 = [H30+][HS-]/[H2S]. The manual gives me the answer as 1.0x10-7. There are no numbers though or anything so I'm really confused as how it gets the number. I have Ka2 = [H30+][S2-]/[HS-2]. 

As I said before I think I have the right approach to this but I have no idea where the numbers would come from, if anyone could shed a little insight, it would be wonderful.

[solution101]

  I really don't know. If I had to guess, I would think the numbers are atomic or molecular weights.

[Borek]

You have to find what concentration of S^2- is needed for the precipitate to form - use Ksp for that.

Then you need to find out at what pH ([H⁺]) 0.01M solution of H₂S contains this concentration of S^2- - and you will need Ka1 and Ka2 for that.