Question

Br-79 has an atomic mass unit (amu) of 78.92 at 50.69% abundance, and Br-81 has an amu of 80.92 at 49.31% abundance. What is the correct calculation of the average atomic mass of Br?

a) (78.92 x 0.5069) + (80.92 x 0.4931)
b) (78.92 x 50.69) + (80.92 x 49.31)
c) 78.92 + 80.92
d) (78.92 + 80.92) / 2

Answer 1

The average atomic mass of bromine is calculated by taking the sum of the products of the isotopes' amus and their respective abundances, resulting in 79.90 amu. The correct option is (a) (78.92 × 0.5069) + (80.92 × 0.4931).

Step-by-step explanation:

The correct calculation of the average atomic mass of Br (bromine) using the given abundances and atomic mass units (amus) is done by taking the weighted average of the isotopes. This is calculated as follows:

Br-79:

78.92 amu × 0.5069 (50.69% abundance as a decimal) = 40.00 amu

Br-81:

80.92 amu × 0.4931 (49.31% abundance as a decimal) = 39.90 amu

Adding these two weighted masses:

40.00 amu + 39.90 amu = 79.90 amu

This method yields the average atomic mass of bromine based on the given isotopic abundances and masses. Therefore, the correct choice for the calculation is (a) (78.92 × 0.5069) + (80.92 × 0.4931).