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A little Chem. help

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Ecto
 
Reply Thu 11 Dec, 2003 09:28 pm
Heya! My first time here, and sadly it's on a bit of a down note. Sad I've been stuck on question #70 of this Chemistry sheet for more time than I'd like to admit, and figured some of you guys could help me in setting it up.

Here goes:

A car gets 9.2km to a liter of gasoline. Assuming that gasoline is 100% octane, which has a specific gravity of 0.69, how many liters of air (21% oxygen by colume at STP) will be required to burn the gasoline for a 1250-km trip, assuming complete combustion?

Any help would be immeasurably appreciated.
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lab rat
 
  1  
Reply Fri 12 Dec, 2003 07:23 am
Octane is C8H18; the reaction for combustion in this case is
2C8H18 + 25O2 --> 18H2O + 16CO2
So, each mole of octane you have requires 12.5 moles of oxygen to combust.
From the gas mileage we know the car will burn ~136 liters of gas (1250 / 9.2).
Using specific gravity, convert liters to g; then use the molecular weight of octane to convert g to moles of octane. From here you can obtain moles of oxygen, then calculate g oxygen.
Does this help?
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lab rat
 
  1  
Reply Fri 12 Dec, 2003 10:42 am
additional info you need:
specific gravity of O2 at STP = 0.00004466 g/ml*
Use this to convert g oxygen to ml; you can then calculate the required volume of air.

*(from the CRC Handbook of Chemistry and Physics, 72nd edition)
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princessash185
 
  1  
Reply Fri 12 Dec, 2003 10:48 am
beat me to it, lab rat. . . ah well. . . :-) Welcome to A2k, ecto!
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patiodog
 
  1  
Reply Fri 12 Dec, 2003 11:50 am
You're probably allowed to treat gases as ideal gases at this stage, so I would just use whatever the volume of one mole of an ideal gas at STP is (can't remember exactly -- somewhere around 22 L? -- certainly easier than calculating from moles to grams to volume) and not forget that about 21% of the atmosphere is oxygen.
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Ecto
 
  1  
Reply Sun 14 Dec, 2003 11:14 pm
Thanks so much for the help, sorry it's belated. It was very helpfull, I tend to draw a blank when it comes to this stuff. Sad
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KevinCarlson
 
  1  
Reply Sat 20 Dec, 2003 11:56 pm
They didn't talk about the Ideal Gas Law in your Chemistry class?

PV = nRT.
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patiodog
 
  1  
Reply Sun 21 Dec, 2003 08:35 am
STP, so not really necessary.
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