Answer:
C) 8.9x10-19M
Step-by-step explanation:
Whe disolving a solid in water, this solid splits in anions and cations, in our example for CuS solid:
CuS (s) -------> Cu+2 (ac) + S-2 (ac)
Whe this solution is saturated, and balance is reached, this balance can be represented with Ksp formula (solubility constant), as following:
Ksp= [Cu+2][S-2], and for tgis ecuation, you only consider aqueous ions, but not the solid salt
When balance is reached, you may suppose that "X" moles of CuC gives you "X" moles of Cu+2 and "X" moles of S-2 (according to the stoicheometry of the above reaction):
XCuS (s) ------> XCu+2(ac) + XS-2(ac) , for which:
[Cu+2]=X and [S-2]=X so this gives us: Ksp=8x10-37=X.X=X²
X= √8x10-37 and finally X=[Cu+2]=8.9x10-19M (expressed in molar units)