# Homework Help: Percent Abundance

1. Mar 1, 2012

### sjung915

1. The problem statement, all variables and given/known data
Naturally occurring bromine contains two isotopes; Br-79, which has a weight of 78.9183 and Br-81 with a weight of 80.9163. If the atomic weight of Br is 79.904, determine the % abundance of Br-79 and Br-81.

2. Relevant equations

To solve this equation I try to put the equation as

(x)(78.9183) + (x)(80.9163) = 79.904

The value of x = .499946789

3. The attempt at a solution

The answers were given because this is a practical exam I'm studying for. Can someone point me in the right direction. Something seems to be missing from the equation.

Br-81 49.33%
Br-79 50.66%

2. Mar 1, 2012

### sjung915

Nevermind I found the answer.. Thank you

3. Mar 2, 2012

### Staff: Mentor

For the record:

is wrong, as abundances of both isotopes are different, not identical - so you need two variables. The correct equation is

(x1)(78.9183) + (x2)(80.9163) = 79.904

and you need another equation:

x1 + x2=1