The molecular weight of a compound is the sum of all the atomic weights in the compound. To illustrate: H1O is composed of two atoms of hydrogen and one atom of oxygen. According to the table of atomic weights (page 129), water has a molecular weight of 18, i.e., (1 X 2 (H1) + 16 X 1(O) = 18). This means that 2 parts of hydrogen combined with 16 parts of oxygen form 18 parts by weight of water.