The molecular weight of iron (II) iodide is 309.65 atomic mass units. The molar mass of iron (II) iodide, represented by the chemical formula FeI2, is 309.65 grams per mole. The molecular weight of a compound is determined by adding together the atomic weight of each atom in the compound.
The atomic weight of iron is 55.84 amu, and the atomic weight of iodine is 126.90 amu. The molecular mass calculated by adding together 55.84 amu of iron and 253.8 amu of iodine to account for the two atoms of iodine in the compound. About 18.03 percent of the mass of iron (II) iodide comes from the iron atom.