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# chemistry as question help

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1. Not sure how to work this out even though it's only worth one mark.
Bromine has two isotopes BR-79 and Br-81 the relative atomic mass of br is 79.9 calculate the percentage of br-79 atoms in a sample of bromine.
2. 79.9 = (a)79 + (b)81
a+b = 1 (as we're talking percentages)
Therefore a = 1-b
79.9 = (1-b)79 + (b)81
79.9 = 79 -79b + 81b
79.9 = 79 + 2b
0.9 = 2b
b = 0.45
Therefore a = 0.55
79.9 = 0.55*79 + 0.45*81

55% atoms are BR-79, 45% BR-81
79.9 = (a)79 + (b)81
a+b = 1 (as we're talking percentages)
Therefore a = 1-b
79.9 = (1-b)79 + (b)81
79.9 = 79 -79b + 81b
79.9 = 79 + 2b
0.9 = 2b
b = 0.45
Therefore a = 0.55
79.9 = 0.55*79 + 0.45*81

55% atoms are BR-79, 45% BR-81
you're a star, that's a lot for 1 mark.
4. (Original post by medicaesthetic)
you're a star, that's a lot for 1 mark.
It's not too much - just algebra. I showed you all the steps, just to be 100% that you didn't get confused.

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