The average atomic mass of Eu is 151.96 amu.There are only two naturally occurring isotopes of europium,¹⁵¹Eu with a mass of 151.0 amu and ¹⁵³Eu with a mass of 153.0 amu.The natural abundance of the ¹³¹Eu isotope must be approximately A)60%. B)20%. C)50%. D)80%. E)40%.

Question

Quizplus · Accepted Answer

The average atomic mass of Eu is closer to the masses of its isotopes, suggesting their natural abundances are relatively balanced. Using the formula for average atomic mass: (mass of isotope 1 * its abundance) + (mass of isotope 2 * its abundance) = average atomic mass, and knowing the two abundances must add up to 100%, we can set up an equation. If we let the abundance of the 151Eu isotope be x, then the abundance of 153Eu isotope would be (100% - x). Plugging into the formula: 151x + 153(1-x) = 151.96, solving for x gives an abundance close to 50% for the 151Eu isotope.

The Average Atomic Mass of Eu Is 151