An element has two isotopes with masses 10 amu (70%) and 11 amu (30%). What is the average atomic mass?

An element has two isotopes with masses 10 amu (70%) and 11 amu (30%). What is the average atomic mass?

• A. 9.7 amu
• B. 10.3 amu
• C. 11.0 amu
• D. 10.0 amu

Answer: (B)

The average atomic mass is the weighted average of the isotopic masses using their natural abundances as weights. Convert the percentages to decimals: 70% is 0.70 and 30% is 0.30. Multiply and sum: (0.70 × 10 amu) + (0.30 × 11 amu) = 7.0 amu + 3.3 amu = 10.3 amu. Since the lighter isotope is more abundant, the average lies between 10 and 11 amu and closer to 10, giving 10.3 amu.

The average atomic mass is the weighted average of the isotopic masses using their natural abundances as weights. Convert the percentages to decimals: 70% is 0.70 and 30% is 0.30. Multiply and sum: (0.70 × 10 amu) + (0.30 × 11 amu) = 7.0 amu + 3.3 amu = 10.3 amu. Since the lighter isotope is more abundant, the average lies between 10 and 11 amu and closer to 10, giving 10.3 amu.