H2S Dissociation Problem

[chromedragon88]

I am taking an engineering design class after not having chemistry for 15 years. I am having problems solving what appears to be a simple dissociation problem:

The pH of an acid mine drainage through which hydrogen sulfide bubbles continuously is measured and found to have a value of pH = 3.7. Estimate the Concentration of sulfide ion (S^2-) in this water.

Answer: 3 x 10-15 mol/L

Can anyone give me a hand so I can see the process for solving this?

[Xen]

I am taking an engineering design class after not having chemistry for 15 years. I am having problems solving what appears to be a simple dissociation problem:

The pH of an acid mine drainage through which hydrogen sulfide bubbles continuously is measured and found to have a value of pH = 3.7. Estimate the Concentration of sulfide ion (S^2-) in this water.

Answer: 3 x 10-15 mol/L

Can anyone give me a hand so I can see the process for solving this?

If the major source of was [H+] was H₂S dissociation (H₂S = H⁺ + HS^-, then:
[H+]=[HS-].
By the definition second acidity constant is:
Ka₂ = [H+][S₂-]/[HS-], since [H+]=[HS-], we have [S₂-] = Ka₂