Cerium has four stable isotopes listed below. If the weighted average atomic mass of cerium is 140.116 amu, calculate the atomic mass of cerium-142.
Ce-136 135.90714 amu 0.19%
Ce-138 137.90599 amu 0.25%
Ce-140 139.90543 amu 88.43%
Ce-142 11.13%
A)141.911 amu
B)142.341 amu
C)142.000 amu
D)141.556 amu
E)142.911 amu

Question

Cerium has four stable isotopes listed below. If the weighted average atomic mass of cerium is 140.116 amu, calculate the atomic mass of cerium-142. 
Ce-136 135.90714 amu 0.19%
Ce-138 137.90599 amu 0.25%
Ce-140 139.90543 amu 88.43%
Ce-142 11.13%
A)141.911 amu
B)142.341 amu
C)142.000 amu
D)141.556 amu
E)142.911 amu

Quizplus · Accepted Answer

To find the atomic mass of Ce-142, use the formula for weighted average: (135.90714 * 0.0019) + (137.90599 * 0.0025) + (139.90543 * 0.8843) + (x * 0.1113) = 140.116. Solving for x gives approximately 141.911 amu.

Cerium Has Four Stable Isotopes Listed Below