Which of the following numerical setups can be used to calculate the atomic mass of the element bromine, given that bromine has two isotopes: Br-79 (abundance 50.69%, mass 78.918 amu) and Br-81 (abundance 49.31%, mass 80.916 amu)?

• A. (0.5069 × 80.916) + (0.4931 × 78.918)
• B. (0.5069 × 78.918) + (0.4931 × 80.916)
• C. (0.5069 + 0.4931) × (78.918 + 80.916)
• D. (78.918 + 80.916) / 2

Answer: (B)

Verified step by step guidance

1. Understand that the atomic mass of an element with multiple isotopes is calculated as the weighted average of the masses of its isotopes, where the weights are their relative abundances expressed as decimals.
2. Convert the given percentage abundances of the isotopes into decimal form by dividing by 100. For Br-79, 50.69% becomes 0.5069, and for Br-81, 49.31% becomes 0.4931.
3. Set up the weighted average formula for atomic mass: \(\text{Atomic Mass} = (\text{fractional abundance of isotope 1} \times \text{mass of isotope 1}) + (\text{fractional abundance of isotope 2} \times \text{mass of isotope 2})\).
4. Assign the correct masses to their corresponding isotopes in the formula: Br-79 has a mass of 78.918 amu and Br-81 has a mass of 80.916 amu, so multiply 0.5069 by 78.918 and 0.4931 by 80.916.
5. Add the two products together to get the weighted average atomic mass of bromine.