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

Calculating the relative atomic mass

Example: 

Chlorine has two isotopes: chlorine-35 and chlorine-37 

A typical sample of chlorine will be 75% chlorine-35 atoms and 25% chlorine-37 atoms.

Total mass of 100 atoms= (75 x 35) + 5 x 37)= 3550

Mean mass of 1 atom= (3550 ÷ 100)= 35.5

Ar of chlorine is 35.5