An unkown element has been found to occur in two different stable isotopes. One isotope has an abundance of 57.21 percent and has a mass of 120.9 amu. The other isotope has an abundance of 42.79 percent and has a mass of 122.9 amu. What is the average atomic mass of the unknown element?

Answer: 121.7558 amu

Explanation:

Average atomic mass of the unknown element =

(Mass of isotope 1 x Relative Abundance of Isotope 1) + (Mass of Isotope 2 x Relative Abundance of Isotope 2)

(120.9 x 0.5721) + (122.9 x 0.4279) = 69.16689 + 52.58891

Average mass of the unknown element = 121.7558 amu