1.12 calculate the relative atomic mass of an element from the relative abundances of its isotopes

Multiply each isotopic mass by its percentage divided by 100, and add the lot together.

e.g. calculate the relative atomic mass of chlorine, which has the following isotopic abundances:

35Cl = 75 % and 37Cl = 25 %.

Ar = (35 x 75/100) + (37 x 25/100) = 35.5