The formula weight of Mg(NO3)2 is 210.34 grams. Formula weight is the sum of all of the atomic weights of the atoms that appear in a chemical formula.
To find the formula weight of Mg(NO3)2, one adds the atomic weight of one Magnesium ion, 24.31 grams, to the mass of two NO3 ions, 124.04 grams. The mass of an NO3 ion is calculated by multiplying the formula weight of oxygen, 16 grams, by three, then adding the result to the formula weight of nitrogen, 14.01. An element's atomic weight is the weight of one mole of its atoms; a mole of atoms is 6.022 x 10^23 atoms, according to the Encyclopeadia Britannica. Therefore, adding the atomic weights of the atoms in a given chemical formula yields the weight of one mole of that chemical.