Solubility Product Problem. I am REALLY stuck on this one :(?

Question

What sulfide ion concentration is required to just initiate precipitation of CoS from a solution that is initially 0.10 M in Co++. Ksp for CoS is 4E-21. Assume all sulfide comes from CoS (no common ion effect)

2010-07-01T13:25:22Z

Thank you for any help you can give me :)

GeorgeSiO2 · Accepted Answer

CoS(s)  ⇄ Co^2+(aq)  +  S^2-(aq)
At equilibrium
Ksp = [Co^2+][S^2-]  = 4× 10^-21
[Co^2+] = 0.10 M hence [0.10][S^2-]  = 4× 10^-21 and [S^2-] = 4× 10^-20 M (you can multiply by 32 to get it into g L^-1 but that is not required).  Any concentration greater than this value will cause pptn of CoS (if you like: Le Chatelier's: addn of S^2-)
cheers.

Solubility Product Problem. I am REALLY stuck on this one :(?

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