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Yet another chem question...

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jeremykleeman:
This one sounds complicated but it should be simple.

20mL of a gaseous hydrocarbon is mixed with 150mL of pure oxygen at 20°C and 1 atmosphere pressure. The mixture is ignited and the hydrocarbon undergoes complete combustion. When the resulting gases are returned to 20°C and 1atmosphere pressure it is found that 80mL of carbon dioxide and 20mL of oxygen are present. The molecular formula of the hydrocarbon is  what?
   

cosec(x):
What is there originally
by PV=nRT
6.24x10^-3 mol Oxygen gas
ie. 1.25x10^-2 mol O
and 8.32x10^-4 mol hydrocarbon

At end we have
3.33x10^-3 mol carbon dioxide
8.32x10^-4 mol oxygen gas (O2)
this totals to
8.32x10^-3 mol O

what happened to the remaining Oxygen, it went to H2O: as H2O:O is 1:1 in H2O, there is 4.18x10^-3 mol H2O
therefore 8.36x10^-3 mol H

H:C ratio is
8.36:3.33
2.5:1
5:2
ie. empirical formula is C2H5
Remeber:
and 8.32x10^-4 mol hydrocarbon reacts with 5.41x10^-3 mol O2

from combustion eq of C2H5,

ratio C2H5:O2 is 4:13 (about .3) but n(C2H5)/n(O2) = .15 (roughly)

as .3/.15=2
we multiply the formula by 2

Therefore the hydrocarbon is C4H10

Hope this helps

beezy4eva:
umm not sure if this is the coreect way to do it but...
since all values are measured at the same temp/pressure, the ratio of moles should stay the same so we dont have to calculate the number of moles.
instead, we can just use their volumes.
the volume of oxygen gas used up in the reaction is 150mL-20mL=130ml, volume carbon dioxide is 80mL and volume of hydrocarbon is 20mL so:
20 (C?H?) + 130 O2 --> 80 CO2 + ?H2O
simplyfies to:
2(C?H?) + 13 O2 ---> 8 CO2 + 10 H2O  since there's 5 O2 molecules left which have to form water, we know that 10 H2O molecules will be formed  
so all we have to do to find the formula for the hydrocarbon is pretty much balance the equation (sort of)
so we have 20 H and 8 C, dividing both by two to get the amount in one molecule of the hydrocarbon since two have been used in the equation
so the equation will be C4H10 (butane)

cosec(x):
Hooray, concordance (and a neat way to avoid PVnRT), you can tell I haven't done chem for 4 months

jeremykleeman:
That makes sense. What confused me was the wording of the question. I thought that C?H? + O2 gave CO2 and O2 and that didnt make any sense.
Thanks for your help.

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