Question

fictitious element Jz has two common isotopes. Three-fifth of the isotopes have a mass of 329.1 amu, while the rest have a mass of 334.5 amu. Given this information what is the average atomic mass (in amu) of Jz? Report the answer to 4 significant figures

Answer 1

The average atomic mass of Jz is 331.3 u.

The average atomic mass of Jz is the weighted average of the atomic masses of its isotopes.

We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its fractional abundance).  

Thus,

⅗ × 329.1 u = 197.46  u

⅖ × 334.5 u = 133.80 u

      TOTAL = 331.3   u