Element X has two naturally occurring isotopes. Isotope A (mass 140 u) has an abundance of 75 % and isotope B (mass 141 u) has an abundance of 25 %. Calculate the average atomic mass of element X.

Step-by-step solution

1. Determine the mass contribution of each isotope using its relative abundance.
2. Compute contribution of isotope A: $0.75\times 140=105,\text{u}$.
3. Compute contribution of isotope B: $0.25\times 141=35.25,\text{u}$.
4. Add the two contributions: $105+35.25=140.25,\text{u}$.
5. Thus the average atomic mass of element X is 140.25 u.

Concept tested: Atomic mass (isotopes)

Published 10 October 2026 · How this solution was prepared and checked