Cerium has four stable isotopes listed below. If the weighted average atomic mass of cerium is 140.116 u, calculate the atomic mass of cerium-142.
Ce-136 135.90714 u 0.19%
Ce-138 137.90599 u 0.25%
Ce-140 139.90543 u 88.43%
Ce-142 11.13%
A) 141.911 u
B) 142.617 u
C) 140.334 u
D) 142.501 u
E) 139.886 u

Question

Cerium has four stable isotopes listed below. If the weighted average atomic mass of cerium is 140.116 u, calculate the atomic mass of cerium-142. 
Ce-136 135.90714 u 0.19%
Ce-138 137.90599 u 0.25%
Ce-140 139.90543 u 88.43%
Ce-142 11.13%
A) 141.911 u
B) 142.617 u
C) 140.334 u
D) 142.501 u
E) 139.886 u

Quizplus · Accepted Answer

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

Cerium Has Four Stable Isotopes Listed Below