I get a lot of questions like this: "Can burning 1 gallon of gasoline really release almost 20 pounds of carbon dioxide (CO2)? That does not make sense. Does the carbon dioxide really weigh 20 pounds? Can you weigh it? How can this be true?"

First of all, yes, this is true.

My genius PhD chemist friend Maria promised to share with me the calculations that demonstrate how this works. I am going to share these with you and then explain them in simple English so your head does not explode (unless you are an AP Chemistry student in high school and then you might want to calculate the numbers yourself JUST FOR FUN). From Maria (note that a "Mole", abbreviated as mol, is a unit of molecular measurement):

Here is a calculation that I did for gasoline...


Energy Information Administration (EIA) stated that 1 gal (3.78 L) of gasoline (density = 0.70 g/mL and approximate composition C8H18) gives 8 kg (18 lbs) of CO2.

Combustion of gasoline
C8H18 + (25/2) O2 -> 8CO2 + 9H2O

3.78 L(1000 mL/L)(0.70g/ mL) = 2646 g gasoline
2646 g gasoline(8 mol CO2/ 1 mol gasoline)(1 mol/114 g gasoline) = 185.68 mol CO2
185.68 mol CO2 (44 g/mol CO2) =8,170 g or 8 kg
8,170 g * 0.0022 lb/g = 17.7 lb or 18 lb
First, Maria shares the mass balance of the equation. That means, the same number of each type of atom will be in the equation before they react and after. They will just exist in different combinations. In other words, gasoline has a lot of carbon and hydrogen in it. When you burn gasoline, you mix it with oxygen. The end result of burning is the energy you get to power your engine plus water and carbon dioxide. Water is made up of hydrogen and oxygen. Carbon dioxide is made up of carbon and oxygen.

Second of all, Maria figures out how much of this gas is released when you burn one gallon of gasoline. She does the calculation in metric, so she gets the answer in kilograms and converts it to pounds. Carbon dioxide, even though it is gaseous, has a weight to it. For example, if you compress carbon dioxide and turn it into a liquid, it is pretty heavy. You can also measure the weight of it in gaseous form. If you burn the fuel and oxygen and trap the gases released from burning in a big balloon and then weigh the carbon dioxide in the balloon, the carbon dioxide in the balloon will weigh 18 pounds. CO2 is heavier than air. If you add all of the other greenhouse gases that are in the balloon from burning the fuel and figure that the burning efficiency is about 99%, as the EIA does to estimate the way engines really work, you will get 19.56 pounds of carbon dioxide equivalent. Maria says that chemistry labs in college sometimes do this combustion experiment with a small quantity of propane.

Don't believe me? Read up at Scientific American.

So there you have it. This experiment brought to you by Maria, the chemistry genius of our Carbon Reduction Action Group. The experiment really shows just how significant the emissions are from the combustion of just 1 gallon of gasoline- and the obvious benefits of using less.

Posted by smoo at Monday, January 28, 2008

Labels: burn, carbon dioxide, climate change, CO2, combustion, emissions, gallon, gasoline, global warming, how, pounds

Anonymous said...
This is a wonderful illustration of free energy. Apparently nothing actually is converted to energy in this calculation. I understand that the carbon that is not burned is combined with o2 to form co2. I always thought that some of the matter had to be destoyed to create energy. If not, some of the principles of science i was taught must be wrong.

February 22, 2008 6:23 PM
smoo said...
Um. Wow. No. Atoms do not get destroyed during combustion. They get rearranged. The amazing thing about the previous comment is that someone actually thought they had been taught their belief as "principles of science." Here's this for a principle of science: the Lomonosov-Lavoisier law of classical physics, which states that matter can not be destroyed or created, though it can be rearranged


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