Full question: In several recent articles, reference was made to the amount of carbon dioxide (CO₂) released from burning some quantity of fuel. It seemed that the tons of CO₂ exceeded the weight of the fuel. What is the ratio of CO₂ released to fuel burned by weight?

Susan Trumbore, professor in and chair of the earth system science department at the University of California, Irvine, calculates an answer to this question.

This seems like a question about conversion of mass to moles. (A mole is a unit of measurement to quantify the amount of a substance made up of atoms, where one mole is equal to 6.02 x 10²³ units of that substance; 6.02 x 10²³ is a chemical constant known as Avogadro's number.)

When considering fuel, let's take the example of gasoline. One of the primary components of gasoline is octane, which is made up of molecules that have eight carbon atoms and 18 hydrogen atoms. The weight of one mole of octane molecules will be equal to the summation of the weights of eight carbon atoms (at 12 grams/mole each, from carbon's mass number) plus 18 hydrogen atoms (at 1 gram/mole each). So, doing the math (8 x 12 + 1 x 18), we see that octane weighs 114 grams/mole.

If you combusted all of the octane to carbon dioxide, each of the eight carbon atoms would become part of a CO₂ molecule. Thus, you would have eight CO₂ molecules per octane molecule burned--or eight moles of CO₂ per mole of octane burned. The weight of CO₂ is 44 grams per mole (1 x 12 grams/mole for the carbon and 2 x 16 grams/mole for the oxygen atoms). Burning one mole of octane (114 grams), therefore, would produce eight moles of CO₂, with a weight of 352 grams (8 x 44).

Thus, the weight ratio of CO₂ produced per octane molecule burned is 352/114, or roughly 3 to 1.

Actual weight ratios will of course vary, since gasoline is not purely octane. In general, however, when you burn carbon fuels they are in "reduced" form, that is, the carbons in the molecules are attached mostly to hydrogen atoms. When they are burned, the carbon becomes "oxidized" (literally, combined with oxygen) to make CO₂. Since oxygen is far heavier than hydrogen, the product is heavier than what is burned.