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# How to work out relative formula masses and empirical formulae? watch

1. 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
2. (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)
3. (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.
4. (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
5. (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.
6. (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.

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