International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Given the isotopic composition of bromine-79 at 50.7% and bromine-81 at 49.3%, what is the relative atomic mass of bromine?

1. 79.01

2. 79.99

3. 80.00

4. 78.95

The correct answer is: 79.99

To find the relative atomic mass of bromine given the isotopic composition, one must consider the mass and abundance of each isotope. Bromine has two major isotopes: bromine-79, which has a mass of approximately 79 amu (atomic mass units), and bromine-81, with a mass of approximately 81 amu. In this case, bromine-79 accounts for 50.7% of the naturally occurring bromine, and bromine-81 makes up the remaining 49.3%. The calculation for the relative atomic mass involves taking a weighted average of the isotopes based on their relative abundances. The calculation can be performed as follows: 1. Calculate the contribution of bromine-79: \[ 79 \times 0.507 = 40.053 \] 2. Calculate the contribution of bromine-81: \[ 81 \times 0.493 = 39.873 \] 3. Combine these contributions to find the relative atomic mass: \[ 40.053 + 39.873 = 79.926 \] Rounding this value gives approximately 79.93, which aligns closely