Stoichiometry is the study of the quantitative (numerical) relationships that can be derived from chemical formulas and from chemical equations.
8.2 Formula Mass
"Formula Mass" is the sum of the atomic masses of all the atoms represented by the formula of the substance.
Example: What is the formula mass of water? Solution: Water has the formula H2O, which represents 3 atoms of which 2 are H atoms and 1 is an O atom. The formula mass is the sum of the atomic masses of the 3 atoms. Since the atomic mass of 1 H atom is 1.0 u, we must multiply 1.0 u by 2 (since there are 2 H atoms) and add to that 16.0 u (the atomic mass of 0). Therefore, 2.0 u + 16.0 u = 18.0 u. The formula mass of water is 18.0 u.