Question

How do I calculate the average atomic mass of element X?

isotope: abundance: mass:

221x 74.22 220.9
220x 12.78 220.0
218x 13.00 218.1

Answer 1

Here's how I do it.

The atomic mass is the weighted average of the atomic masses of each isotope.

In a weighted average, we multiply each value by a number representing its relative importance.

In this problem, the percent abundance represents the relative importance of each isotope.

Here's how to do the calculation:

`ulbb("Isotope" color(white)(m)bb"% abundance"color(white)(m)A_text(r) color(white)(m)bb"Contribution")`
`color(white)(m)""^221"X"color(white)(mmmmm) 74.22color(white)(mmll)220.9color(white)(mm)163.95`
`color(white)(m)""^220"X"color(white)(mmmmm)12.78color(white)(mmll)220.0color(white)(mml)28.12`
`ul(color(white)(m)""^218"X" color(white)(mmmmm) 13.00color(white)(mmll)218.1color(white)(mml)28.35)`
`color(white)(mmmmmmmmmm)"Atomic mass = 220.4"`

The average relative atomic mass of element X is 220.4.