Bromine has two naturally occurring isotopes, Br-79 and Br-81. The atomic mass of Br-79 is 78.9183 u with a percent abundance of 50.69% and the atomic mass of Br-81 is 80.9163 u. Calculate the atomic mass of bromine. A) 80.92 u B) 80.03 u C) 80.91 u D) 78.92 u E) 79.90 u
The Correct Answer and Explanation is:
To calculate the atomic mass of bromine, we can use the weighted average formula for atomic masses, which accounts for the percent abundances of the isotopes. The formula is:Atomic Mass of Bromine=(Mass of Br-79×Abundance of Br-79)+(Mass of Br-81×Abundance of Br-81)\text{Atomic Mass of Bromine} = (\text{Mass of Br-79} \times \text{Abundance of Br-79}) + (\text{Mass of Br-81} \times \text{Abundance of Br-81})Atomic Mass of Bromine=(Mass of Br-79×Abundance of Br-79)+(Mass of Br-81×Abundance of Br-81)
Step-by-Step Calculation:
- Mass of Br-79 = 78.9183 u
- Abundance of Br-79 = 50.69% = 0.5069
- Mass of Br-81 = 80.9163 u
- Abundance of Br-81 = 100% – 50.69% = 49.31% = 0.4931
Now, plug the values into the formula:Atomic Mass of Bromine=(78.9183×0.5069)+(80.9163×0.4931)\text{Atomic Mass of Bromine} = (78.9183 \times 0.5069) + (80.9163 \times 0.4931)Atomic Mass of Bromine=(78.9183×0.5069)+(80.9163×0.4931)=(39.9860)+(39.9534)= (39.9860) + (39.9534)=(39.9860)+(39.9534)=79.9394 u= 79.9394 \, \text{u}=79.9394u
Final Answer:
Rounding to two decimal places, the atomic mass of bromine is 79.94 u.
The closest option is E) 79.90 u.
Explanation:
The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, taking into account their respective abundances. This ensures that the value reflects the proportion of each isotope found in nature.
