Question: how to calculate percentages by mass?
Answer: 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%.