Chemistry Help

[collisster]

Just doing some chemistry homework.. got stuck with this question, wondering if anyone could help me out please?

If 1dm^3 of petrol weighs 750g, what mass of CO2 is produced for each 1dm^3 burned?

C8H12 + 12.5 O2 ------>>>> 8CO2 + 9H2O

I've given it a go and fot that 750g of petrol is 6.58 moles, and then the 1:8 ratio gives 52.68 moles of CO2, mass = moles x Mr = 2317 odd grams of Co2 which is daft.

Cheers
Collisster

 

[SlyReaper]

Enough mass of CO2 to annoy hippies.

 

[The Halk]

I don't recognise that at all. It's over 16 years since I did any chemistry though....

What level of Chemistry are you doing? I mean university, school etc.

 

[collisster]

A level

 

[Alex74]

It isnt daft. Remember that you're combining the heavy parts of the hydrocarbon with a rather heavy reactant. So instead of having 3 hydrogens on there for every 2 carbons, you've got 2 oxygens to every carbon! A greater mass of CO2 makes sense here just make sure that the total mass is equal (i.e. add the mass of O2 on the left and the mass of the H2O on the right and make sure that both sides balance)

 

[The Halk]

Hmm, I'm pretty sure I didn't do it in Higher Chemistry, and I thought A levels were a notch down from Highers.

Perhaps educational standards are actually increasing, despite what the papers say.

Good luck with the question, sorry I can't help. The only thing that comes to mind about school chemistry is that those metal sharpeners burn great.

 

[platinum87]

the formula is actually

2 C8H12 + 25O2 -> 18H2O + 16CO2

because there is no such thing as 0.5, putting it is a fail, where simply doubling it is a pass

edit: your answer is right

edit: im drunk so that above formula is wrong, but the point stands

 

[Alex74]

..I think you have a problem with hydrogens? There are 24 on the left in the above post (or 12 in the OP) but 36 (18) in the right. You may be getting the wrong answer because the formula is wrong?

 

[petemyster]

A level

seems an alright question. We did that sort of stuff in GCSE's. Too late to be thinking about chemistry though Do it in the morning?

 

[Alex74]

Your formula's wrong. (EDIT: This does not actually affect the solution though) This is a balanced equation:

C8H12 + 11 (O2) -> 6 (H2O) + 8 (CO2)


EDIT: Ok, I think I can explain the solution..

C8H12 has a molar mass of 108. 11 Molecules of O2 have a total molar mass of 352. So most of the mass by far in our balanced equation is with the oxygens. Or in other words, for every one mol of C8H12, there is triple the mass of oxygen to be used in our reaction, a number which is not included in the question and therefore makes it deceptive.

On the other side, 6 molecules of water have a total molar mass of 108, while 8 molecules of CO2 have a total molar mass of 352. We, by coincidence, have the same numbers. What fun. This means that most of the mass in our balanced equation is now with the Carbon Dioxide. So we have added mass to the reactant. Your answer should be correct, you just made an arithmetic error, as corrected below (i actually get 2444g but..)

 

[Disco Boy]

C8H12 +11O2 -> 8CO2 + 6H2O

750/ ((12*8)+12) = 6.94 moles petrol

6.94* 8 = 55.2 moles carbon dioxide produced

55.2 * (12+32) = 2442.88g

= 2443g carbon dioxide (3s.f.)

the formula is actually

2 C8H12 + 25O2 -> 18H2O + 16CO2

because there is no such thing as 0.5, putting it is a fail, where simply doubling it is a pass

lol Platinum86. I think that you'll find that you can have 0.5 moles.

 

[touch]

..I think you have a problem with hydrogens? There are 24 on the left in the above post (or 12 in the OP) but 36 (18) in the right. You may be getting the wrong answer because the formula is wrong?

Thats what i was thinking. But its a long time since i've done chemistry.
I would guess at this formula:
C8H12 + 11O2 ------>>>> 8CO2 + 6H2O

 

[platinum87]

I've given it a go and fot that 750g of petrol is 6.58 moles, and then the 1:8 ratio gives 52.68 moles of CO2, mass = moles x Mr = 2317 odd grams of Co2 which is daft.

Cheers
Collisster

because the O2 is in the atmosphere, and oxygen weighs more than carbon

 

[touch]

lol Platinum86. I think that you'll find that you can get 0.5 moles.

Doesnt that say 0.5 O2 (or just O) tho rather than 0.5 moles of it? which i dont think exists naturally so you should use?

 

[Disco Boy]

That does not say 0.5 moles tho, that says 0.5 O2 (or just O) which i dont think exists naturally so you should use?

In an equation the numbers represent a number of moles, one does not deal with individual molecules.

 

[platinum87]

lol Platinum86. I think that you'll find that you can have 0.5 moles.

writing an equation with 0.5 oxygen molecules is wrong

 

[Disco Boy]

writing an equation with 0.5 oxygen molecules is wrong

No it isn't. Trust me; I'm at a top university reading for a degree in the subject, I know what I am talking about.

 

[Dunks]

Hmm, I'm pretty sure I didn't do it in Higher Chemistry, and I thought A levels were a notch down from Highers.

Perhaps educational standards are actually increasing, despite what the papers say.

Good luck with the question, sorry I can't help. The only thing that comes to mind about school chemistry is that those metal sharpeners burn great.

A Levels are a notch above Highers actually. Somewhere between Highers and Advance Highers.

 

[touch]

No it isn't. Trust me; I'm at a top university reading for a degree in the subject, I know what I am talking about.

Are there different formats for writing the formulae maybe? Wikipedia seems to agree with the numbers = number of atoms theory.

 

[The Halk]

A Levels are a notch above Highers actually. Somewhere between Highers and Advance Highers.

Ah. Explained then! It was Highers and SYSs. Although I didn't do any SYSs. I think Advanced Highers replaced SYS.