Calculate the solubility of FeS in water if the Ksp = 1 x 10–22.?



Answer:
FeS dissolves if the product of the concentration of Fe and the concentration of S is less than or equal to the Ksp.

In this case, it's easy because you start with no ions in solution, and all the Fe and S come from a common source. There is always 1 Fe[2+] ion formed in solution when 1 S[2-] ion is formed. So therefore you know that at any time when you dissolve more FeS, including at the limit of solubility, (Fe[2+]) = (S[2-]).

The items in square brackets are not figures to calculate with, but the charge of the ions written as superscripts.

So at saturation, Fe[2+])(S[2-]) = (Fe[2+])^2 = (S[2-])^2 = 1 x 10^-22.

So (Fe[2+]) = (S[2-]) = 1 x 10^-11. This is very insoluble unless you make the solution more acid. This will promote the dissolution of more S[-2] ion because more and more S[2-] will disappear when it forms related ion HS[-] and the nasty H2S gas (which smells like rotten eggs). Assuming you allow that nasty stuff to form, the answer will be determined by the equilibrium state of S[2-], HS[-] and H2S which varies with pH (and if the gas is allowed to escape the solution and smell up your lab, even more S[2-] will be drawn in to dissolve to replace it). Your Fe[2+] is still in solution and doesn't react, so at saturation you expect Fe[2+] > S[2-] when their product equals the Ksp.

But please, please don't make a smell...

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