how to calculate percentage by mass of element?
Divide the mass of the element by the total mass of the compound and multiply by 100. This will give you the mass percent of the element. Example 1: mass percent = (2.01588/18.01528) x 100 = 0.11189 x 100 = 11.18%. Thus, the mass percent of Hydrogen atoms in a water molecule is 11.18%.