Question

How do you deduce the fractional atomic masses of elements from the relative isotopic abundance?

Answer 1

To calculate the fractional mass (M) of element you need to sum up multiplications of isotopic masses (M_i) and their abundances (W_i).

$M=\sum(Mi*Wi)$, if W_i is fraction.

$M=\sum(Mi*Wi/100)$ , if W_i is percentage.