Stoichiometry, Moles, and Molecular Weight

In modern chemistry, all elements are compared to the carbon-12 isotope in terms of mass.  The carbon-12 isotope is defined as exactly 12 atomic mass units.  So, one atomic mass unit (amu) is exactly one twelfth the mass of a carbon-12 atom.  The atomic weight of an element is the average atomic mass from the naturally occurring element, expressed in atomic mass units.

Problem.  How is atomic weight calculated?
Answer:  Take the product of the natural abundance and mass of each isotope.  Then sum the values.

For chlorine:
Isotope       Isotopic mass (amu)    Fractional Abundance    Product
Chlorine-35    34.96885                        0.75771                    26.496
Chlorine-37    36.96590                        0.24229                  +  8.956
                                                                                              35.452

The sum of the products is 35.452 amu, the atomic weight of chlorine.