Each atom on the periodic table has a Molar Mass in grams/mole.

To find the Molar Mass of a molecule, add up the individual Molar Masses of the atoms that make up that molecule.

To find the Mass % of an atom in a molecule, divide the Molar Mass of the atom by the total Molar Mass of the molecule. This will tell you how much of the total Molar Mass of that molecule comes from that atom!