We can then use a weighted mean of the relative isotopic masses to find the relative atomic mass (Ar) of the element.

For each isotope peak, multiply its m/z value by its percentage abundance. 

Add those products together and divide the total by 100.

For example, chlorine has two isotopes. Cl-35 has 75% abundance and Cl-37 has 25% abundance. 

So Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5 as seen on the periodic table.

For example, chlorine has two isotopes. Cl-35 has 75% abundance and Cl-37 has 25% abundance. 

So Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5 as seen on the periodic table.

If you’re given relative abundances as a ratio rather than percentages, do the same calculation but divide by the sum of the abundances rather than 100.