Naturally occurring magnesium exists as three isotopes. 78.70% is Mg-24 with a mass of 23.98504 amu, 10.13% is Mg-25 with a mass of 24.98584 amu, and 11.17% is Mg-26 with a mass of 25.98259 amu. What is the atomic mass of magnesium?

The correct answer is 24.31 amu.

Explanation:

The formula for finding the average atomic mass = The sum of the percentage abundance of isotopes × the actual mass of isotopes

The average atomic mass = % abundance of Mg-24 × actual mass + % abundance of Mg-25 × its actual mass + % abundance of Mg-26 × its actual mass

Avg. Atomic mass = 0.7870 × 23.98504 + 0.1013 × 24.98584 + 0.1117 × 25.98259

Avg. atomic mass = 18.88 + 2.53 + 2.90 = 24.31 amu

The atomic mass of magnesium is 24.31 amu.