Question

There are two naturally occurring isotopes of bromine. 79Br has a mass of 78.9183 amu. 81Br has a mass of 80.9163 amu. Determine the abundance of each isotope.

Answer 1

well realize that the molecular mass of bromine listed in the periodic table is the average of the mass of the many natural occurring isotopes in their normally found isotopic ratio

molecular mass of bromine listed on the periodic table = 79.904Da

amu = Da = g/mole

so set up a system of equations

78.9183y + 80.9163x = 79.904

where y is the number of 79Br and x is the number of 81Br found in the ratio

we need now to set up another equation so that we can solve for x and y. I make the assumption that since 79Br and 81Br are the only isotopes found they must be in a proportion equal to 1

x + y =1

solving using system of equations I get,
y = 0.506657
x = 0.493343

Might as well convert these to percentages they are nicer to look at,

79Br is 50.6%
81Br is 49.3%

http://www.webelements.com/bromine/isotopes.html

looks like the answer is correct, note that the link above is experimental data (they used a mass spectrophotometer to determine the isotope ratio)

Hope this was helpful :D

Answer 2

Really the percentages should be 79Br 50.665% and 81Br 49.334%

oh and since it is experimental data don't expect it to fit your answer exactly, because no experiment is perfect and statistics is a harsh mistress