A certain element consists of two stable isotopes. the first has a mass of 68.9 amu and a percent natural abundance of 60.4 %. the second has a mass of 70.9 amu and a percent natural abundance of 39.6 %. what is the atomic weight of the element?

The average atomic weight of this element can be calculated as follows:

average atomic weight =

(atomic weight of first isotope) (its percentage of abundance) +

(atomic weight of second isotope) (its percentage of abundance)

atomic weight of first isotope = 68.9 amu

its percentage of abundance = 0.604

atomic weight of second isotope = 70.9 amu

its percentage of abundance = 0.396

So, just substitute in the above equation to get the atomic weight of the element as follows:

atomic weight = (68.9) (0.604) + (70.9) (0.396) = 69.692 amu