››More information on molar mass and molecular weight. In chemistry, the formula weight is a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a chemical formula by the number of atoms of that element present in the formula, then adding all of these products together. In this video we'll write the correct formula for Ammonium iodide (NH4I).To write the formula for Ammonium iodide we'll use the Periodic Table, a Common Ion ...(a) AgI, silver iodide, a solid with antiseptic properties (b) CaCO 3, calcium carbonate, the active ingredient in many over-the-counter chewable antacids (c) Mg(OH) 2, magnesium hydroxide, the active ingredient in Milk of Magnesia (d) Mg(NH 4)PO 4, magnesium ammonium phosphate, an essentially insoluble substance used in tests for magnesium