Question

Bromium has two naturally occurring isotopes: 79br, with an atomic weight of 78.918 amu, and 81br, with an atomic weight of 80.916 amu. if the average atomic weight for br is 79.903 amu, calculate the fraction-of-occurrences of these two isotopes. [hint: work on example problem 2.1]

Answer 1

The two different isotopes have weights :

w1 = 78.918 amu

w2 = 80.916 amu

average weight w3 = 79.903 amu

The mixing of two components can be modeled as

let the fraction of w1 be 'x'

hence
`w1. x + w2.(1-x)  = w3`

now this is a linear equation in 'x'. Substituting the values we get

x = 0.507

hence the percentage of Br79 = 50.7% and the percentage of BR81 = 49.3%