Precipitation problem?

The mole fraction of calcium in the ocean is 4.11 x 10-4. The concentration of carbonate ion (CO32-) is 1.2 x 10-7 M. The precipitation of CaCO3 is favored under conditions in the ocean. The excess reactant is carbonate for this precipitation. If all of the CaCO3 precipitated what would be the concentration of the excess reactant? M