Question

An element has two naturally occurring isotopes with atomic masses of 112.90 amu and 114.90 amu. The relative abundances of these isotopes are 4.29% and 95.71%, respectively.

What is the average atomic mass of the element?

A.22.78 amu
B.113.9 amu
C.114.8 amu
D.227.8 amu

Answer 1

C. 114.8 u

Explanation:

The atomic mass of X 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 percent of the total).

Set up a table for easy calculation

0.0429 × 112.90 u = 4.843 u

0.9571 × 114.90 u = 109.97 u

TOTAL = 114.8 u