Element M has two naturally occurring isotopes: M-24 (abundance: 78.99%, mass: 23.985 amu) and M-25 (abundance: 21.01%, mass: 24.986 amu). What is the average atomic mass (in amu) of element M?

• A. 24.98
• B. 2.86
• C. 24.30
• D. 5.36

Answer: (A)

Verified step by step guidance

1. Convert the percentage abundances of the isotopes into decimal form by dividing each by 100. For example, 78.99% becomes 0.7899 and 21.01% becomes 0.2101.
2. Multiply the mass of each isotope by its corresponding decimal abundance to find the weighted contribution of each isotope to the average atomic mass. For isotope M-24, calculate \$0.7899 \times 23.985\( amu, and for isotope M-25, calculate \)0.2101 \times 24.986$ amu.
3. Add the weighted contributions of both isotopes together to get the average atomic mass of element M. This can be expressed as: \(\text{Average atomic mass} = (\text{abundance}_1 \times \text{mass}_1) + (\text{abundance}_2 \times \text{mass}_2)\).
4. Make sure to keep track of units (amu) throughout the calculation to ensure the final answer is in atomic mass units.
5. Review the calculation to confirm that the weighted average reflects the relative abundances and masses of the isotopes, which gives the average atomic mass of element M.