Hey there! Sign in to join this conversationNew here? Join for free

How to work out relative formula masses and empirical formulae? Watch

    • Thread Starter
    Offline

    2
    ReputationRep:
    Hi,

    I was going through some of the questions in my revision guides and there are a couple of questions that I really don't understand. I have looked in the back for the answers and there is no explanation and google doesn't seem to be of any help either.

    Relative formula question:

    work out the relative formula mass of pb(C2H5)4

    The correct answer is 323.2 and the answer I got was 275.2. I got confused about the ()4 (brackets with formula inside and then 4 on the outside), and I wasn't quite sure how to go about doing it.

    Empirical formulae question:

    give the empirical formulae of the following:

    a)C6H6
    b)H202
    c)C12H22O11

    I'm also unsure how to do these.

    If anyone has any advice on how to do these or know of any good websites that helpfully explain how to correctly work these out I would really appreciate it if you could help.

    Thankyou
    • Community Assistant
    • Study Helper
    Offline

    14
    ReputationRep:
    (Original post by Caraphernelia)
    Hi,

    I was going through some of the questions in my revision guides and there are a couple of questions that I really don't understand. I have looked in the back for the answers and there is no explanation and google doesn't seem to be of any help either.

    Relative formula question:

    work out the relative formula mass of pb(C2H5)4

    The correct answer is 323.2 and the answer I got was 275.2. I got confused about the ()4 (brackets with formula inside and then 4 on the outside), and I wasn't quite sure how to go about doing it.
    Everything inside the bracket multiplies by the number outside the bracket.

    207.2 + (29*4)
    • TSR Support Team
    Offline

    21
    ReputationRep:
    (Original post by Caraphernelia)
    Hi,

    I was going through some of the questions in my revision guides and there are a couple of questions that I really don't understand. I have looked in the back for the answers and there is no explanation and google doesn't seem to be of any help either.

    Relative formula question:

    work out the relative formula mass of pb(C2H5)4

    The correct answer is 323.2 and the answer I got was 275.2. I got confused about the ()4 (brackets with formula inside and then 4 on the outside), and I wasn't quite sure how to go about doing it.
    Could I ask how you got that answer?

    The (formula)4 means that there's 4 lots of that compound in the molecule, so with that, if you were to write out the formula of that compound it'd be: PbC8H20

    So, you calculate the RAM by adding up the atomic mass of all the components - so, the Ethane has a RAM of 29. 4 of those therefore has a RAM of 116. Add the 207.2 atomic mass of the Lead and you should get 323.2


    Empirical formulae question:

    give the empirical formulae of the following:

    a)C6H6
    b)H202
    c)C12H22O11

    I'm also unsure how to do these.

    If anyone has any advice on how to do these or know of any good websites that helpfully explain how to correctly work these out I would really appreciate it if you could help.

    Thankyou
    Right, what is the empirical formula in terms of a definition? It's the simplest integer ratio between the molecules - so as an example: Hexane, it has a molecular formula C6H14 - the common factor is 2, divide both by two - empirical formula therefore is C3H7

    So, take those, work out what the lowest common factor is and divide to find the empirical formula.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by Stiff Little Fingers)
    Could I ask how you got that answer?

    The (formula)4 means that there's 4 lots of that compound in the molecule, so with that, if you were to write out the formula of that compound it'd be: PbC8H20

    So, you calculate the RAM by adding up the atomic mass of all the components - so, the Ethane has a RAM of 29. 4 of those therefore has a RAM of 116. Add the 207.2 atomic mass of the Lead and you should get 323.2



    Right, what is the empirical formula in terms of a definition? It's the simplest integer ratio between the molecules - so as an example: Hexane, it has a molecular formula C6H14 - the common factor is 2, divide both by two - empirical formula therefore is C3H7

    So, take those, work out what the lowest common factor is and divide to find the empirical formula.
    Thank you for your help. I have managed to work out the empirical formula questions.

    For the relative formula question, I did:
    C2= (12x2) 24. 24x4=48
    H5= (1x5) 5. 5x4=20
    Pb=207.2

    207.2+48+20=275.2
    • TSR Support Team
    Offline

    21
    ReputationRep:
    (Original post by Caraphernelia)
    Thank you for your help. I have managed to work out the empirical formula questions.

    For the relative formula question, I did:
    C2= (12x2) 24. 24x4=48
    H5= (1x5) 5. 5x4=20
    Pb=207.2

    207.2+48+20=275.2
    There's your mistake - you multiplied 12 by 4, not 24 - 24 x 4 is 96.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by Stiff Little Fingers)
    There's your mistake - you multiplied 12 by 4, not 24 - 24 x 4 is 96.
    I see what I did. Thank you for your help.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Did TEF Bronze Award affect your UCAS choices?
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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

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