Molecular Mass and Formula Mass

The molecular mass - is the mass of one molecule of a molecular compound. The unit of molecular mass is the amu.

The formula mass - is the sum of atomic masses of all atoms in its formula. The unit of formula mass is the amu.

Steps to to calculate molecular mass of water H₂O:

Molecular mass = the sum of the atomic masses of each atom in the molecule
Molecular mass H₂O = 2(1.00794) + 15.9994 = 18.0153 amu

Steps to to calculate formula mass of calcium chloride:

Formula for calcium chloride is CaCl₂.
Formula mass = Sum of the atomic masses of atoms in the formula (1 Ca atom + 2 Cl atoms)

calcium = \frac{40.08 amu}{1 Ca atom} x 1 Ca atom = 40.08 amu

chloride = \frac{35.45 amu}{1 Ca atom} x 2 Cl atoms = 70.90 amu

Formula for calcium chloride CaCl₂ is 110.98 amu.