Question

Bromine has two isotopes that naturally occur in nature. Given the following information, what is the average atomic mass of bromine?

bromine-79 with 50.69% abundance

bromine-81 with 49.31% abundance

a
82.99 amu
b
79.99 amu
c
81.11 amu
d
81.99 amu

Answer 1

To calculate the average atomic mass of bromine based on the given isotopes and their abundances, you can use the following formula:

\[
\text{Average atomic mass} = ( \text{mass of isotope 1} \times \text{fractional abundance of isotope 1} ) + ( \text{mass of isotope 2} \times \text{fractional abundance of isotope 2} )
\]

For bromine, we have:

- **Bromine-79**:
- Mass = 79 amu
- Abundance = 50.69% = 0.5069

- **Bromine-81**:
- Mass = 81 amu
- Abundance = 49.31% = 0.4931

Now, we can substitute these values into the formula:

\[
\text{Average atomic mass} = (79 \, \text{amu} \times 0.5069) + (81 \, \text{amu} \times 0.4931)
\]

Calculating each term:

1. \( 79 \times 0.5069 = 40.0901 \, \text{amu} \)
2. \( 81 \times 0.4931 = 39.9391 \, \text{amu} \)

Now, add the two results together:

\[
\text{Average atomic mass} = 40.0901 + 39.9391 = 80.0292 \, \text{amu}
\]

Therefore, rounding to two decimal places, the average atomic mass of bromine is approximately 80.03 amu.

None of the provided answer options exactly match this calculation, but it is closest to:

**b. 79.99 amu**

Thus, the best choice based on your given options is **b. 79.99 amu**.