Naturally occurring europium (Eu) consists of two isotopes was a mass of 151 and 153. Europium-151 has an abundance of 48.03% and Europium-153 has and abundance of 51.97%. What is the atomic mass of europium?

151.94 u

Explanation:

The atomic mass of Eu 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 abundance).

The atomic masses of Eu-151 and Eu-153 are 150.92 u and 152.92 u, respectively.

Set up a table for easy calculation.

0.4803 * 150.92 u = 72.472 u

0.5197 * 152.92 u = 79.473 u

TOTAL = 151.94 u