8.1 Stoichiometry
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.
If a substance is molecular, the formula is called a molecular formula. The formula mass of a molecular substance can also be called the molecular mass. Since water is a molecular substance, it is correct to say that both the molecular and formula mass is 18.0 u. However, if you aren't sure that a substance is molecular, use the term formula mass to be safe.
8.3 Gram Atomic Mass and Gram Formula Mass
The Gram atomic mass of an element is the quantity of the element that has a mass in grams equal to its atomic mass.
Example: The atomic mass of carbon is 12 u, therefore 12 grams of carbon is 1 gram mass of carbon.
Gram atomic masses can also be abbreviated by the term gram-atom.
The gram formula mass is the quantity of a substance that has a mass in grams equal to its formula mass.
Example: Since water's formula mass is 18 u, the gram formula mass of water is 18 grams.
For molecular substances, the term gram molecular mass can be used instead of gram formula mass.