John, Actually, the statement should have been 6 pounds of gasoline yields approximately 20 pounds of carbon dioxide. Each molecule of octane contains 8 carbons and 18 hydrogens. The atomic weight of carbon is 12, the atomic weight of hydrogen is 1. So each molecule of octane has an atomic weight of 114. In burning octane, all of the carbon goes to carbon dioxide. So for each of the eight carbons, you must take 2 oxygens (O2) from the surrounding air. The atomic weight of oxygen is 16. Therefore, if you start out with one molecule of octane, total atomic weight of 114, you create 8 molecules of carbon dioxide. Each molecule of carbon dioxide has an atomic weight of 44, and you now have 8 of them, which yields a total atomic weight of 352. 352 divided by 114 equals 3.1. 6 pounds times 3.1 equals 18.6 pounds of carbon dioxide from 6 pounds of gasoline. (Gasoline is approximately 84 to 90 % octane and 10 to 16 % heptane. The heptane is 7 carbon atoms rather than eight. This makes less than 2% difference in the final numbers.) Now, as to the gallons versus pounds of gas. I believe the original calculation was for 1 gallon of gas, which weighs approximately 6.3 pound. Either Patrick or the BBC made an error by saying gallons instead of pounds. 6.3 pounds times 3.1 would equal 19.53 pounds, and yield the rounded "twenty pounds of carbon dioxide to 1 gallon of gas". On the internet, you can find different values for the weight of gasoline, but the one that most of the people in the industry use is 6.3 pounds per gallon of gas. Sincerely, Jo Grahn At 10:10 AM 8/7/2004 -0600, you wrote:
I think there was a part of the original statement that was converted incorrectly ... The statement listed 6 GALLONS of gasoline. Then later, this was changed to 6 pounds.
I agree that from this formula the ratio is 6 lbs of octane to 18.4 lbs of CO2. But how much octane is contained in a gallon of gasoline? No matter what you do to a chemical compound, when you add up their molecular weights, they must remain the same. Yes each molecule can have different weights, but the sum of ALL components will be the same going in as going out of the process.
I believe that a gallon of gasoline weighs about 7.5 lbs (Not sure exactly), so the original statement of 6 gallons of gas would be around 45 lbs of gas. When burned, producing about 20 lbs of CO2 sounds plausible to me.
John Zeigler www.johnstelescopes.com www.mirrorkits.com