Chapter 4 - Atomic Structure - 4.3 Distinguishing Among Atoms - Sample Problem 4.5 - Page 119: 25

79.91 amu

Multiply the percent of the relative abundance of each isotope by its atomic mass, and add them get the atomic mass. Atomic mass of isotope 1 = 78.92 amu (50.69%) Atomic mass of isotope 2 = 80.92 amu (49.31%) 78.92 amu (0.5069) + 80.92 amu (0.4931) = 79.91 amu