Bromine has two naturally occurring isotopes, Br-79 and Br-81. The mass of Br-79 is 78.92 u with a percent abundance of 50.69%. If the atomic mass of bromine is 79.904 u, calculate the mass and percent abundance of Br-81.
A) 80.916 u and 50.69%
B) 78.921 u and 49.31%
C) 79.904 u and 49.31%
D) 80.916 u and 49.31%
E) 80.028 u and 50.31%

Question

Quizplus · Accepted Answer

We know that the atomic mass of bromine, which takes into account the percent abundance of each isotope, is 79.904 u. We can set up the following equation to solve for the percent abundance of Br-81:

(0.5069)(78.92 u) + (x)(molar mass of Br-81) = 79.904 u

Solving for x, we get:

x = (79.904 u - 39.963 u) / 80.916 u

x = 0.4931 or 49.31%

Therefore, the mass and percent abundance of Br-81 are 80.916 u and 49.31%, respectively. The answer is choice D.

Bromine Has Two Naturally Occurring Isotopes, Br-79 and Br-81