Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta

chem AS help??? watch

Announcements
    • Thread Starter
    Offline

    6
    ReputationRep:
    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.
    • Political Ambassador
    • Welcome Squad
    Offline

    20
    ReputationRep:
    Political Ambassador
    Welcome Squad
    (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
    • Thread Starter
    Offline

    6
    ReputationRep:
    (Original post by zXcodeXz)
    This has nothing to with this, but I really want to do Chemistry next year

    Start revising now then
    • Political Ambassador
    • Welcome Squad
    Offline

    20
    ReputationRep:
    Political Ambassador
    Welcome Squad
    (Original post by medicaesthetic)
    Start revising now then
    Yeah, what did you get in GCSE chemistry if you don't mind me asking?
    • Thread Starter
    Offline

    6
    ReputationRep:
    (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.
    Offline

    6
    ReputationRep:
    (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.
    • Thread Starter
    Offline

    6
    ReputationRep:
    (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.
    Offline

    3
    ReputationRep:
    Is 55% the answer by any chance?

    Edit: Nm lol
    Offline

    11
    ReputationRep:
    (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%
    • Political Ambassador
    • Welcome Squad
    Offline

    20
    ReputationRep:
    Political Ambassador
    Welcome Squad
    (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
    • Thread Starter
    Offline

    6
    ReputationRep:
    (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.
    Offline

    12
    ReputationRep:
    (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
    • Political Ambassador
    • Welcome Squad
    Offline

    20
    ReputationRep:
    Political Ambassador
    Welcome Squad
    (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?
    • Political Ambassador
    • Welcome Squad
    Offline

    20
    ReputationRep:
    Political Ambassador
    Welcome Squad
    (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
    • Thread Starter
    Offline

    6
    ReputationRep:
    (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
    • Political Ambassador
    • Welcome Squad
    Offline

    20
    ReputationRep:
    Political Ambassador
    Welcome Squad
    (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
 
 
 
Poll
Do I go to The Streets tomorrow night?
Help with your A-levels

All the essentials

The adventure begins mug

Student life: what to expect

What it's really like going to uni

Rosette

Essay expert

Learn to write like a pro with our ultimate essay guide.

Uni match

Uni match

Our tool will help you find the perfect course for you

Study planner

Create a study plan

Get your head around what you need to do and when with the study planner tool.

Study planner

Resources by subject

Everything from mind maps to class notes.

Hands typing

Degrees without fees

Discover more about degree-level apprenticeships.

A student doing homework

Study tips from A* students

Students who got top grades in their A-levels share their secrets

Study help links and info

Can you help? Study help unanswered threadsRules and posting guidelines

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.