[numbersixman]

Trying to get a head start on the fall semester and I'm already stuck.

The question is:

At what pH will FeS begin to precipitate if the Fe^2+ ion concentration is 0.1 M and the H2S concentration is 0.01 M.

I thought I had it figured out, but solving the exact same question for CuS gave me an answer that didn't make sense.

Using the Ksp for FeS (4.9 x 10^-18), I find the amount of S^2- needed for precipitation (4.9 x 10^-17).

I then used the given value for the Ksp of H2S (1.3 x 10^-20), and try to solve for the hydronium ion concentration.

The lab manual states that you can solve for either the S^2- or H30+ concentration if either value is known. Since I know the amount of S^2- needed, I solve for H30+, using the equation given in the book:

[S^2-] = [H2S] (1.3 x 10^-20)/[H30+]^2

[S^2-] = 4.9 x 10^-17
[H2S] = 0.01 M

4.9 x 10^-17 = (.01) (1.3 x 10^-20)/ [H30+]^2


Solving for x:

1.3 x 10^-22/ 4.9 x 10^-17 = [H30]^2 = 2.65 x 10^-6

Squaring both sides = [H30+] = 1.63 x 10^-3                 

-log(1.63 x 10^-3) = pH 2.78


Any help on where I'm going wrong would be greatly appreciated. As I said before, I thought this was the right answer, but the exact same question for CuS is giving me a negative pH, so I must be doing something wrong.

[AWK]

pH about ~3 - 4 is a quite reliable  value.
CuS practically is precipitated at pH ~0.5. From calculations you will get negative pH value and this is OK since CuS cannot be solubilized even in concentrated HCl. 

[numbersixman]

Thanks, AWK. After some extensive digging, I found  a power point with similar problems, and several of the compounds had negative pH values, including CuS. I'd never ran into a problem with a negative pH, so I assumed I was doing something wrong.

Thanks again.