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# chem AS 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. (Original post by medicaesthetic)
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.
This has nothing to with this, but I really want to do Chemistry next year
3. (Original post by zXcodeXz)
This has nothing to with this, but I really want to do Chemistry next year

Start revising now then
4. (Original post by medicaesthetic)
Start revising now then
Yeah, what did you get in GCSE chemistry if you don't mind me asking?
5. (Original post by zXcodeXz)
Yeah, what did you get in GCSE chemistry if you don't mind me asking?
A* dropped 2 marks in one paper and 3 marks in the other lol, it's nothing compared to a level.
6. (Original post by medicaesthetic)
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.
So you know that the average mass is 79.9, and that the two abundances add up to 100%.

From there, let x = the % abundance of BR-79. (taking 1 to be 100%)

You know then that the % abundance of BR-81 is 1-x.

From that you can get this equation:

79x+(1-x)81 = 79.9

Rearrange and you''ll end up with a decimal e.g 0.76 or something, which would mean 76%.

I recommend trying a couple questions like this to get the hang of it.
7. (Original post by Strom)
So you know that the average mass is 79.9, and that the two abundances add up to 100%.

From there, let x = the % abundance of BR-79. (taking 1 to be 100%)

You know then that the % abundance of BR-81 is 1-x.

From that you can get this equation:

79x+(1-x)81 = 79.9

Rearrange and you''ll end up with a decimal e.g 0.76 or something, which would mean 76%.

I recommend trying a couple questions like this to get the hang of it.

Thank-you for your help but the answers 55.5% just not sure how to get i.
8. Is 55% the answer by any chance?

Edit: Nm lol
9. (Original post by medicaesthetic)
Thank-you for your help but the answers 55.5% just not sure how to get i.
81x + 79(100 - x) / 100 = 79.90

Expand the bracket and multiply both sides by 100

81x + 7900 - 79x = 7990

81x - 79x = 7990 - 7900

2x = 90

x = 45

100 - 45 = 55%
10. (Original post by medicaesthetic)
A* dropped 2 marks in one paper and 3 marks in the other lol, it's nothing compared to a level.
Nice and that's true, but to be honest in all fairness they are making GCSE's really hard now I'll be lucky to even get a B or an A
11. (Original post by zXcodeXz)
Nice and that's true, but to be honest in all fairness they are making GCSE's really hard now I'll be lucky to even get a B or an A
It's the work you put in at a level, you can be a genuis and get a D. Or you can scrape a B at gcse work so hard at as level and do every past paper and get an A. So don't be put off at ALL.
12. (Original post by zXcodeXz)
Nice and that's true, but to be honest in all fairness they are making GCSE's really hard now I'll be lucky to even get a B or an A
A level chem is probably the subject with the biggest step up from gcse to a level
13. (Original post by medicaesthetic)
It's the work you put in at a level, you can be a genuis and get a D. Or you can scrape a B at gcse work so hard at as level and do every past paper and get an A. So don't be put off at ALL.
Ok thanks, what other a levels do you do?
14. (Original post by samb1234)
A level chem is probably the subject with the biggest step up from gcse to a level
Ah ok, I've seen some work of chemistry and it is
15. (Original post by zXcodeXz)
Ok thanks, what other a levels do you do?
french and biology, dropped eng lit as it was pointless and too late to switch to physics
16. (Original post by medicaesthetic)
french and biology, dropped eng lit as it was pointless and too late to switch to physics
Oh ok nice, I'm planning on doing biology chemistry physics and maths

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