The molar solubility of a substance is the number of moles that dissolve per liter of solution. The dissociation equation FeCO3 in water would be as follows:
FeCO3 → Fe2+ + CO3 2-
We calculate as follows:
Ksp = [Fe2+][CO3 2-]
Let X = [Fe2+] - then X = [CO3 2-]
Ksp = X²
(3.13*10^-11) = X²
X = √( 3.13*10^-11)
X = 5.59*10^-6 M
Hope this answers the question. Have a nice day.