1.12 calculate the relative atomic mass of an element from the relative abundances of its isotopes
Relative atomic mass is the average weight of an atom of an element. There is variation in the weight due to the fact that there are different isotopes with in an element. (Different isotopes have different weights due to less or more neutrons.)
It is calculated like this:
(% of isotope x its mass) + (% of isotope x its mass) / 100
e.g for chlorine
75% of all chlorine has a mass of 35. 25% of all chlorine has a mass of 37.