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Old 04-08-08, 09:05 PM   #1
devildogmech
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Stupid Science Question

Hey all,

I just saw this Cliff Bar Ad on Youtube...

http://www.youtube.com/watch?v=Zz3zMzVdlX8

In the ad it says that a gallon of Gasoline has 20 pounds of CO2 in it.... HOW THE **** CAN THAT BE? A gallon of gas does not mass 20 pounds! Matter is Matter.... It cant change mass, just form. If a gallon of Gasoline weighs 10 pounds (the weight of a gallon of water... I figure its close enough), How the hell do you get twenty pounds of CO2 out of that?????? Even adding the Oxygen from the atmosphere it wont go up that much! I mean the gasoline is a complex hydrocarbon, made up of H, C & O.....??????

Or am I just loopy?

TIA

Billy
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Old 04-08-08, 09:12 PM   #2
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http://www.fueleconomy.gov/feg/co2.shtml

The chemical reaction draws Oxygen from the air, that's where the mass is 'made up'.
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Old 04-08-08, 09:19 PM   #3
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Quote:
Originally Posted by devildogmech View Post
Hey all,

I just saw this Cliff Bar Ad on Youtube...

http://www.youtube.com/watch?v=Zz3zMzVdlX8

In the ad it says that a gallon of Gasoline has 20 pounds of CO2 in it.... HOW THE **** CAN THAT BE? A gallon of gas does not mass 20 pounds! Matter is Matter.... It cant change mass, just form. If a gallon of Gasoline weighs 10 pounds (the weight of a gallon of water... I figure its close enough), How the hell do you get twenty pounds of CO2 out of that?????? Even adding the Oxygen from the atmosphere it wont go up that much! I mean the gasoline is a complex hydrocarbon, made up of H, C & O.....??????

Or am I just loopy?

TIA

Billy
Technically, you're right, but gasoline needs to react with oxygen to do it's thing, which produces CO2 as a byproduct. I won't vouch for a Clif Bar ad's math as far as how much is actually produced per gallon.
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Old 04-08-08, 09:20 PM   #4
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hydrocarbon + air = carbon dioxide + water

HC + O2 = CO2 + H2O

taking atomic numbers, i.e., proton count only

atomic number of C is 6

atomic number of O is 8

atomic number of H is 1

so at the simplest molecule

CH4 > 10

2*O2 > 32

CO2 > 22

2*H2O > 20

The overall mass is conserved (you're right no matter is destroyed or created) but CO2 is more than double the mass of the original CH4 molecule.

Anyhoo that's the best quick explanation I can give. It's very simplistic and is in no way complete but should give you an idea of why the mass increases so dramatically.

Now, if someone has the actual figures for gasoline mix combustion products per litre/gallon that'd probably help more.
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Old 04-08-08, 09:24 PM   #5
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See! I told you it was a stupid question

And to think I aced Chem114 at Purdue Cal !
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Old 04-08-08, 10:15 PM   #6
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Gasoline weighs approximately 6 lbs/gal. Water weighs approximately 8 lbs/gal. The common isotope of carbon has an atomic weight of 12, the common isotope of oxygen has an atomic weight of 16. A CO2 molecule thus has a weight of 44.

So, we can't say for sure exactly what compounds are present in gasoline, but we can define gasoline to some extent. First, it is made up almost entirely of hydrocarbons, which are molecules made up of carbon and hydrogen. (Some of the compounds present in gasoline also contain small amounts of other elements, including sulfur, nitrogen, oxygen, and some trace metals.) Next, those hydrocarbons which make up most of gasoline boil at temperatures between about 50 and 200 degrees Celsius (120 to 400 degrees Fahrenheit). This means that these compounds are mostly those hydrocarbons that have between 6 and 12 carbon atoms in each molecule. A good average is probably octane, which has eight carbon atoms and 18 hydrogen atoms and is written C8H18

When a hydrocarbon is burned (that is, reacted with oxygen), it forms carbon dioxide (CO2) and water (H2O). For our "average" gasoline of C8H18, the reaction is 2 molecules of octane reacting with 25 molecules of oxygen (O2) to form 18 molecules of water and 16 molecules of carbon dioxide.
http://www.madsci.org/posts/archives...4809.Ch.r.html

Since the common isotope of hydrogen has an atomic weight of 1, two molecules of gasoline weighing 228 will produce 18 molecules of water (total weight 324) and and 16 molecules of carbon dioxide (total weight 704). So...6 pounds of gasoline, when burned, will produce 6*(704/228) = 18.5 pounds of CO2 and 6*(324/228) = 8.5 pounds of H2O.
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Old 04-08-08, 10:16 PM   #7
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Did I do that right? Someone please check my work...
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