Calculate the average atomic mass of iron given natural abundances: 15% iron-55 and 85% iron-56.

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Multiple Choice

Calculate the average atomic mass of iron given natural abundances: 15% iron-55 and 85% iron-56.

Explanation:
Weighted averaging of isotopic masses using natural abundances. The average atomic mass is found by weighting each isotope’s mass by how abundant it is: convert the percentages to decimals (0.15 and 0.85) and multiply by the isotope masses (55 and 56 amu), then add them up. 0.15×55 + 0.85×56 = 8.25 + 47.6 = 55.85 amu. This value lies between 55 and 56 and is pulled toward 56 because that isotope is more abundant. So the average atomic mass is 55.85 amu.

Weighted averaging of isotopic masses using natural abundances. The average atomic mass is found by weighting each isotope’s mass by how abundant it is: convert the percentages to decimals (0.15 and 0.85) and multiply by the isotope masses (55 and 56 amu), then add them up. 0.15×55 + 0.85×56 = 8.25 + 47.6 = 55.85 amu. This value lies between 55 and 56 and is pulled toward 56 because that isotope is more abundant. So the average atomic mass is 55.85 amu.

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