Question

Type the correct answer in the box. Round your answer to the nearest thousandth.
What is the average atomic mass of the element?

An element, X, has two isotopes, X-14 and X-15. Use the data in the table to find the average atomic mass of element X.

Isotope Atomic Mass Abundance (%)
X-14 14.003 99.636
X-15 15.000 0.364

The average atomic mass of element X is
amu.

Answer 1

j The average atomic mass of element X is 14.007 u.

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

0.996 36 × 14.003 u =13.952 03 u

0.003 64 × 15.000 u = 0.054 60 u

__________TOTAL = 14.007 u

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