# Bromine has two isotopes, Br- 79 and Br- 81. The relative atomic mass of bromine is 79.9.

CALCULATE THE PERCENTAGE of Br-79 atoms in a sample of bromine.

Someone said:
The total for 79 and 81 is 100% and the total for fraction is 1, so
let X = fraction 79 and 1-X = fraction of 81.
Then 79(X) + 81(1-X) = 79.9
Solve for X.

I dont get it the total is 160 and there are 2 numbers so 2 fractions? and I dont get the x thing with 1-x AND 79(X)+81(1-X) iM VERY CONFUSED PLEASE HELP.
I need a simple clear explanation please.

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1. assume you have x percent of 79. Then you have 100-x percent of 81, and you have 100 percent of the mixture.

x*79+(100-x)81=79.9*100
solve for x, then 100-x.
Of course there are two fractions, each isotope has a differing content. Now if you cant solve that equation above, you have very serious chemistry issues ahead, this is algebra I.

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bobpursley
2. is it 40.1 percent??

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3. Itâ€™s 55%

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