#percent composition

The molecular weight of a substance is the sum of the atomic weight of the atoms in a molecule. The atomic weight is the average atomic mass for a naturally occurring element. This means that molecular weight is the average mass of a molecule of a substance. Molecular weight is expressed in atomic mass units. For example, we might want to find the molecular weight of a molecule of water. We have 2 atoms of H, with each hydrogen atom weighing 1. We multiply 2 H atoms by 1.01  a piece to get 2.02 g/mol. We add the 16.00 from one O atom to the 2.02 g/mol from the oxygen to get a total of 18.02 g/mol for one molecule of water.

The formula weight/mass of a substance is the sum of the atomic weights of all atoms in a formula unit of the compound. Formula weight doesn't depend on whether or not the substance is a molecule.For example, sodium chloride, which is NaCl, has a formula weight of 58.44 g/mol. This results from having 22.99 g/mol from Na and 35.45 g/mol from Cl. You would use the formula weight for substances which are not molecules, such as ionic compounds.

Say we have an element X in a compound. This element X is just part of the whole compound. We define the mass percentage of X as the parts of X per hundred parts of the total, by mass. That is:

Wonderful Life with the Elements: The Periodic Table Personified by Bunpei Yorifuji (No Starch Press, 2012) ↬ Laughing Squid

Cute idea -- each element drawn as a unique character -- but I'm not crazy about turning cross sections into charts and graphs. The two charts at bottom right show composition data for Earth's crust and seawater, but the illustrations are weakened by two misconceptions: (1) that each element is found in pure form, and (2) that each layer in the "stack" has uniform composition. A lot of kids would come away from these charts thinking that somewhere in the middle of the Earth's crust is a layer of pure silicon.