Write the chemical formula of the substance
Find the atomic mass of each element from the periodic table
Count the number of atoms of each element in the formula
Multiply each atomic mass by its number of atoms
Add all the results together
The total is the molecular mass in atomic mass units (amu) or g/mol
Example: H2O = 2(1) + 16 = 18 amu
Example: CO2 = 12 + 2(16) = 44 amu