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# Caclcuate the percentage of BR-79 atoms in sample of bromine? watch

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. Well on the periodic table, Bromine has a mass number of 79.9

So...

x(81) + (1-x)(79) = 79.9
3. --
4. (Original post by High Stakes)
Well on the periodic table, Bromine has a mass number of 79.9

So...

x(81) + (1-x)(79) = 79.9

How did u work that out?
5. (Original post by medicaesthetic)
How did u work that out?
It's how averages work - The numbers you see on the periodic table are averages.

When you take: 2 + 2 + 3 + 3 and divide it by 4, you're doing the same as (0.5 x 2) + (0.5 x 3).

So, when it comes to this we know there can only be two isotopes. We call one of their abundances x and we call the other 1-x because that's the remaining abundance.
6. (Original post by High Stakes)
It's how averages work - The numbers you see on the periodic table are averages.

When you take: 2 + 2 + 3 + 3 and divide it by 4, you're doing the same as (0.5 x 2) + (0.5 x 3).

So, when it comes to this we know there can only be two isotopes. We call one of their abundances x and we call the other 1-x because that's the remaining abundance.
I've never seen it like that could you not just put the numbers in ;s
7. (Original post by medicaesthetic)
I've never seen it like that could you not just put the numbers in ;s
Are you AS Chemistry? This should have been covered in like the first few lessons.
8. (Original post by High Stakes)
Are you AS Chemistry? This should have been covered in like the first few lessons.
Ye I've been getting 90/80%'s all year round and this question I don't have a clue lol, I don't think we've been taught it.
9. You did not answer the question though you just worked out the average where the question is asking you to work out the abundance of Br-79.
10. (Original post by High Stakes)
Well on the periodic table, Bromine has a mass number of 79.9

So...

x(81) + (1-x)(79) = 79.9
you did not answer their question.The question is asking you to work out the percentage of Br-79 in the bromine sample,where you just worked out the average.
11. The answer is 55%. If you were given the % of each isotope and asked to calculate the relative atomic mass, you would do the following

(79x55+81x45)/100 = 79.9

Now, in the question you are asked what is the % of Br-79, so replace with "x" (and replace the Br-81 % with "100-x" as both added together must equal 100%).
(79x + 81(100-x))/100 =79.9
79x + 81(100-x)=7990
79x +8100 -81x =7990
-2x=-110 x=55

Therefore Br-79 55%

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Updated: April 1, 2018
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