Average Atomic Mass Calculations

If 75.0% of the isotopes of an element have a mass of 35.0 u and 25.0% of the isotopes have a mass of 37.0 u, what is the atomic mass of the element? Show your work.

Naturally occurring boron is composed of two isotopes. The percent abundance and the mass of each isotope are listed below.

• 19.9% of the boron atoms have a mass of 10.013 atomic mass units.

• 80.1% of the boron atoms have a mass of 11.009 atomic mass units. Show your work.

Two isotopes of Bromine naturally occur:

Isotopes of Bromine: Percent Abundance: Atomic Mass:

Bromine-79 50.69% 78.92 amu

Bromine-81 49.31% 80.92 amu

Calculate the average atomic mass for the two isotopes of Bromine.