In the video, you learned that the weighted average atomic mass of an element is calculated using the percent abundance of each naturally occurring isotope of that element.
Let's Practice
Practice calculating weighted average atomic mass by completing this activity. Calculate the atomic mass of the element on each tab, then check your answer.
The element, lithium, has two naturally occurring isotopes. One isotope has a mass of 6.017 u and an abundance of 7.30%. The other isotope has a mass of 7.018 u and an abundance of 92.70%.
What is the weighted average atomic mass of lithium?
6.95 u
If you need help arriving at this answer, click the Solution button.
| Step 1: Substitute the mass of each isotope and its percent abundance into the equation. | \(\text{Average Atomic Mass}=\)\(\left( \frac{7.30\%}{100} \times 6.017\ \text{u} \right) + \)\(\left( \frac{92.70\%}{100} \times 7.018\ \text{u} \right)\) |
| Step 2: Convert each percent abundance to a decimal by dividing each one by 100. | \(\text{Average Atomic Mass}\ =\ \)\(\left( 0.073 \times 6.017\ \text{u} \right) +\)\(\left( 0.927 \times 7.018\ \text{u} \right)\) |
| Step 3: Multiply the mass of each isotope by percent abundance. | \(\text{Average Atomic Mass}= 0.439\ \text{u} + 6.506\ \text{u}\) |
| Step 4: Sum the results. | \(\text{Average Atomic Mass }= 6.95 \text{u}\) |
The element, copper, has two naturally occurring isotopes. Copper-63 is the most common, with an abundance of 69.17%, and it has a mass of 62.930 u. Copper-65 has an abundance of 30.83% and a mass of 64.928 u.
What is the weighted average atomic mass of copper?
63.55 u
If you need help arriving at this answer, click the Solution button.
| Step 1: Substitute the mass of each isotope and its percent abundance into the equation. | \(\text{Average Atomic Mass} = \left( \frac{ 69.17 \% }{100} \times 62.930\ \text{u} \right)+\)\(\left( \frac{ 30.83 \%}{100} \times 64.928\ \text{u} \right)\) |
| Step 2: Convert each percent abundance to a decimal by dividing each one by 100. | \(\text{Average Atomic Mass}=\)\(\left( 0.6917 \times 62.930\ \text{u} \right)+\)\(\left( 0.3083 \times 64.928\ \text{u} \right)\) |
| Step 3: Multiply the mass of each isotope by percent abundance. | \(\text{Average Atomic Mass} = 43.529\ \text{u} + 20.017\ \text{u}\) |
| Step 4: Sum the results. | \(\text{Average Atomic Mass} = 63.55\ \text{u}\) |
The element, magnesium, has three naturally occurring isotopes. They are each listed below with their abundance and atomic mass.
| Isotope | Percent Abundance | Atomic Mass (u) |
|---|---|---|
| Mg-24 | 78.70% | 23.985 |
| Mg-25 | 10.13% | 24.986 |
| Mg-26 | 11.17% | 25.983 |
Calculate the weighted average atomic mass for magnesium.
24.31 u
If you need help arriving at this answer, click the Solution button.
| Step 1: Substitute the mass of each isotope and its percent abundance into the equation. | \(\text{Average Atomic Mass} = \left( \frac{ 78.70 \% }{100} \times 23.985\ \text{u} \right) + \)\(\left( \frac{ 10.13\%}{100} \times 24.986\ \text{u} \right)\) +\(\left( \frac{11.17 \%}{100} + 25.983 \text{u} \right)\) |
| Step 2: Convert each percent abundance to a decimal by dividing each one by 100. | \(\text{Average Atomic Mass} =\)\(\left( 0.7870 \times 23.985\ \text{u} \right) + \)\(\left( 0.1013 \times 24.986\ \text{u} \right) + (0.1117 \times 25.983\ \text{u}) \) |
| Step 3: Multiply the mass of each isotope by percent abundance. | \(\text{Average Atomic Mass} = 18.8762\ \text{u} + 2.5311\ \text{u} + 2.9023\ \text{u}\) |
| Step 4: Sum the results. | \(\text{Average Atomic Mass} = 24.31\ \text{u}\) |
