Mg has three stable isotopes with the following masses and abundances:?

Question

Mg-24 23.985 amu 78.99 percent

Mg-25 24.986 amu 10.00 percent

Mg-26 25.983 amu 11.01 percent

Using this information, calculate the atomic weight of Mg out to four significant figures.

Answer 1

Use a weighted average

0.7899 * 23.985 + 0.1000 * 24.986 + 0.1101 * 25.983 = 24.31 amu

Answer 2

Change your percentages to the decimal forms then multiply each by the isotopes.

(0.7899) x (23.985) = isotope 1
(0.1000) x (24.986) = isotope 2
(0.1101) x (25.983) = isotope 3

add the three answers to get the atomic mass for the element.