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# Formula Of The Hydrocarbon - Exam Tomorrow, Help if you can. watch

1. When 15cm3 of a gaseous hydrocarbon is completely burned in oxygen, 45cm3 of carbon dioxide and 45cm3 of water vapour are obtained, all volumes being measured at same temp. and pressure.
The formula of the hydrocarbon is?

Answer is C3H6
But how do you get C3H6?

If you have any idea please write it down.. Thanks
2. (Original post by MedAmy)
When 15cm3 of a gaseous hydrocarbon is completely burned in oxygen, 45cm3 of carbon dioxide and 45cm3 of water vapour are obtained, all volumes being measured at same temp. and pressure.
The formula of the hydrocarbon is?

Answer is C3H6
But how do you get C3H6?

If you have any idea please write it down.. Thanks
4.5 O2 + CxHy --> 3CO2 + 3H2O

3 carbons on the right hand side, so there must be 3 carbons in the hydrocarbon.
6 hydrogens on right hand side, so there must be 6 hydrogens in the hydrocarbon.
3. (Original post by MedAmy)
When 15cm3 of a gaseous hydrocarbon is completely burned in oxygen, 45cm3 of carbon dioxide and 45cm3 of water vapour are obtained, all volumes being measured at same temp. and pressure.
The formula of the hydrocarbon is?

Answer is C3H6
But how do you get C3H6?

If you have any idea please write it down.. Thanks
You know that, since the volumes of each of the gases are taken at the same temperature and pressure, each volume is directly proportional to the number of moles of substance.

(So the ratio of moles of hydrocarbon : oxygen gas : water vapour is 15 : 45 : 45, or 1 : 3 : 3.)

Now try constructing the balanced equation for the reaction, and see if you can find the formula from that.

EDIT: Too slow.
4. (Original post by james.h)
You know that, since the volumes of each of the gases are taken at the same temperature and pressure, each volume is directly proportional to the number of moles of substance.

(So the ratio of moles of hydrocarbon : oxygen gas : water vapour is 15 : 45 : 45, or 1 : 3 : 3.)

Now try constructing the balanced equation for the reaction, and see if you can find the formula from that.

EDIT: Too slow.
Better explained though.
5. (Original post by james.h)
You know that, since the volumes of each of the gases are taken at the same temperature and pressure, each volume is directly proportional to the number of moles of substance.

(So the ratio of moles of hydrocarbon : oxygen gas : water vapour is 15 : 45 : 45, or 1 : 3 : 3.)

Now try constructing the balanced equation for the reaction, and see if you can find the formula from that.

EDIT: Too slow.
hmm.. still a bit confused how you got the ratio. as we don't know how much o2 was burnt?

CxHy + O2 => CO2 + H20
15cm3- CxHy
45cm3 - CO2
" H2O.

So what do you do then.

Nice and simply.. Lol
6. (Original post by starofale)
4.5 O2 + CxHy --> 3CO2 + 3H2O

3 carbons on the right hand side, so there must be 3 carbons in the hydrocarbon.
6 hydrogens on right hand side, so there must be 6 hydrogens in the hydrocarbon.
an where did you get that equation from?
7. (Original post by MedAmy)
an where did you get that equation from?
That's like the definition of combustion. You will always have just two products if the combustion is complete - water and carbon dioxide.
8. (Original post by Antzlck)
That's like the definition of combustion. You will always have just two products if the combustion is complete - water and carbon dioxide.
oh i have the equation I wrote it above. I meant where did he get the equation with the 4.5 etc. in front of the hydrocarbon etc.
9. (Original post by MedAmy)
oh i have the equation I wrote it above. I meant where did he get the equation with the 4.5 etc. in front of the hydrocarbon etc.
The ratio of
CO2:H2O:CxHy
= 45:45:15 (from the volumes of gas)
= 3:3:1

combustion is:
O2 + CxHy --> CO2 + H2O

then put in the ration 3:3:1 to try to balance it:
O2 + CxHy --> 3CO2 + 3H2O

but the number of oxygen atoms is not the same on each side, so to balance it completely:
4.5 O2 + CxHy --> 3CO2 + 3H2O
10. (Original post by james.h)
You know that, since the volumes of each of the gases are taken at the same temperature and pressure, each volume is directly proportional to the number of moles of substance.

(So the ratio of moles of hydrocarbon : oxygen gas : water vapour is 15 : 45 : 45, or 1 : 3 : 3.)

Now try constructing the balanced equation for the reaction, and see if you can find the formula from that.

EDIT: Too slow.
(Original post by MedAmy)
hmm.. still a bit confused how you got the ratio. as we don't know how much o2 was burnt?
Ah just read james's post more carefully, I guess saying oxygen gas was a mistake - should have been carbon dioxide
11. (Original post by starofale)
Ah just read james's post more carefully, I guess saying oxygen gas was a mistake - should have been carbon dioxide
ah thats great. So if I get a question like this, then you know both sides are eqaul. and always do ratio. and then formula?!
12. (Original post by MedAmy)
ah thats great. So if I get a question like this, then you know both sides are eqaul. and always do ratio. and then formula?!
Yes, chemical equations always have to be balanced - atoms aren't created or destroyed.
13. (Original post by starofale)
Ah just read james's post more carefully, I guess saying oxygen gas was a mistake - should have been carbon dioxide

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