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#1




How much CO2 does a gallon of gas generate?
If you go to the Terrapass website, they have a calculator that allows you to determine how much CO2 your vehicle spews into the air in one year. Entering information for both my car and my wife's, I get figures that seem ridiculously high. Working the math backwards from the results they provide, it appears they are using a constant of one gallon of gasoline creating around 19.5 pounds of CO2 emissions. Is that figure anywhere close to accurate? Or is this just marketing hype to get you to buy in to their system? (Not that I think it's a bad idea  I just think their math or science is off.)

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#2




It seems like a lot, given that a gallon of gas weighs around six pounds, but that figure is approximately correct. The extra weight comes from the oxygen that combines with the gasoline when it burns.

#3




As Absolute says, the added weight comes from the oxygen. You can get a rough estimate like this. Say the gasoline is octane having 8 carbon atoms and 18 hydrogen atoms. The molecular weight is then 18 + 8*12 = 114. So the weight of a gallon of octane is proportional to 114.
When burned completely one molecule of octane will produce 8 molecules of CO_{2} which is one molecule for each carbon atom. The molecular weight of CO_{2} is 12 + 2*16 = 48 and there are 8 molecules so the weight of the gas that a gallon of octane produces is proportional to 8*48 = 384. Octane weighs about 6 lb./gal so the total weight of CO_{2} is 6*384/114 or about 20.5 lb. Close enough for a rough estimate since a gallon of gasoline isn't pure octane but contains other things, like water for example, that don't produce CO_{2}. 
#4




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Nitpick: 12 + (2 x 16) = 44, not 48, so the total comes out at 18.5 lb, not 20.5 lb. And, of course, the burning uses up eight molecules of oxygen (O_{2}), each with a molecular weight of 32, which means that as well as creating 18.5 lb of carbon dioxide, you're destroying {6 x (8 x 32) / 114} = 13.5 lb of oxygen. Well, OK, not destroying, but tying up. Scary, huh? 
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