Ahhh another one... check my work?

Question

The free energy change for the dissolution of CoS at 25 C is 121.587 kJ/mol. What is the solubility of the compound?

So i did deltaG = -RT * lnKsp

rearranged to: lnKsp = deltaG/-RT

so: 121587 J/mol / -2476.38

lnKsp = -49.09...

Ksp = 4.75x10^-22

but my answer isn't right and i don't know why that is !!

Answer 1

I believe your answer is off by the concentration of solid CoS.  The dissolution is from CoS(s) = Co+2(aq) + S-2(aq).  Use the Mw ~91 g/mole and the density 5.5 g/cc  to recompute.  You may want to check these numbers.  I got them from a quick web search.