Question

The abundance of 3 isotopes of magnesium of mass numbers 24,25 and 26 are 78.6%, 10.1%, and 11.3% respectively. What is the relative atomic mass of magnesium?

Answer 1

This is really asking for the average mass of a magnesium atom, so multiply the masses by their respective abundance (converting percentage to decimal equivalent), add those products together, and viola! Note: the masses on amu’s are absolute (infinite significant figures. The abundance’s have 3 sig figs, and so should your answer…)

24 x 0.790 = 19.0 amu

25 x 0.100 = 2.50 amu

26 x 0.110 = 2.86 amu

(Because of the 19.0, the sig figs go only to the 1/10 decimal place)

19.0 + 2.5 + 2.9 = 24.4 amu or g/mole
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