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

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.

Reply 1

High Stakes

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

So...

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

(edited 7 years ago)

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?

Reply 4

High Stakes

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.

Reply 5

medicaesthetic

Original post by High Stakes

I've never seen it like that could you not just put the numbers in ;s

Reply 6

High Stakes

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.

Reply 7

medicaesthetic

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.

Reply 8

NoClueGuy

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.

Reply 9

NoClueGuy

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.

Reply 10

Maths&SciTutor

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%

(edited 6 years ago)

Reply 11

ibyss

Original post by Maths&SciTutor

And I thought it was a 1 marker..

Reply 12

otakuplantsky

I still dont get it ;-;

Reply 13

slinkcube

Fear no further, I have the full working! How has this been unsolved for 4 years 😂...

To work out Br-79, we must use the equation for relative atomic mass.

RAM = Sum of (Isotopic mass x Relative abundance) / 100

Sub in the values that you are given that being 79 and 81 (we will use y and z for the unknowns)

(79 x Y) + (81 x Z) / 100 = 79.9

Percentages add up to 100:

Because the percentage relative abundances will all add up to 100, we can do the following. Y + Z = 100.

This means that Y = 100 - Z.

Sub this conclusion back into the equation:
Now we can replace Y in the equation we made initially:

(79 x (100 - Z) + (81 x Z) / 100 = 79.9

Now solve the new equation for Z:
(7900 - 79Z) + 81Z / 100 = 79.9

7900 + 2Z = 7990

2Z = 90

Z = 45

Now that we have worked Z (being Br-81) out we can work out Y easily (Y being Br-79) by substituting Y = 100 - Z which would be Y = 100 - 45 as the percentage has to add up to 100 giving us an answer of 55.

You can also do this the other way round to work out Y directly without taking away from 100 at the end although in most cases it does not matter as they will tend to ask you for both percentages.

This is proofread and is 100% correct. If you have any questions ill be happy to answer them for you. Thank you.

(edited 3 years ago)

Reply 14

GabiAbi84

Original post by slinkcube

Fear no further, I have the full working! How has this been unsolved for 4 years 😂...

.

It was solved twice in this thread...now three times.

Reply 15

slinkcube

Actually there is only one other answer and the format is very messy to say the least... further more it does not make sense to anyone reading this problem for the first time, Thanks.