The curious (or suspicious-minded) among you have occasionally wondered about our claim that one gallon of gas turns into about twenty pounds of carbon dioxide exhaust. Your high school chemistry teacher would be very disappointed with you for even thinking the question, but there’s no judging here at TerraBlog. Let’s walk through the math.
When you burn something, it might feel like you’re turning it into lightness, air, nothingness. But what you’re really doing is simultaneously vaporizing it and chemically bonding it with oxygen in the air. It’s the weight of that oxygen that makes up the difference.
Carbon dioxide — or CO2 — is one carbon atom joined to two oxygen atoms. Carbon dioxide. Oxygen is a little bit heavier than carbon, so when you stick two oxygen atoms onto every available carbon atom, you end up with an amount of CO2 that is roughly triple the weight of the gasoline.
One gallon of gas weighs about 6.25 pounds. The weight fluctuates with temperature and octane, but this figure is good enough for government work.
Let’s pretend that gas is entirely made up of octane (more properly referred to as 2,2,4-trimethylpentane). It’s not, but that also doesn’t really matter for our purposes. Octane contains 8 carbon atoms (hence the oct- prefix, like Dr. Octopus) and 18 hydrogen atoms. Carbon has a molecular weight of 12 and hydrogen has a molecular weight of 1, so octane has a total molecular weight of 114 (8 x 12 + 18 x 1).
Oxygen has a molecular weight of 16, so CO2 has a total molecular weight of 44 (12 + 16 + 16). Every molecule of octane makes 8 molecules of CO2, with a total molecular weight of 352 (44 x 8).
6.25 pounds x (352 / 114) = 19.3 pounds
Et voila! All it takes to convert one gallon of gas into 20-ish pounds of carbon dioxide is some highly confusing algebra!
Gas doesn’t burn 100% cleanly. You also get some carbon monoxide and other nasty stuff coming out of your tailpipe. But that doesn’t really affect our math very much. The official World Resources Institute conversion rate that we use in our carbon calculator is 19.564 pounds of CO2 per gallon of gasoline. Although we round this number to 20 pounds when we speak informally of the amount of carbon dioxide from one gallon of gasoline, all of our online calculators use the more precise figures.
Also, bear in mind that the 19.564 pounds of CO2 are just the direct result of burning gasoline. The process of extracting, refining, and transporting the product adds an extra few pounds per gallon to the actual environmental impact of filling your gas tank.
Finally, your exhaust is quite a bit heavier if you count the steam that is generated. Those 16 hydrogen atoms attached to every octane molecule have to go somewhere. They combine with oxygen to create water (H2O). Every gallon of gas creates roughly 8 pounds of water vapor. And water vapor is, believe it or not, a greenhouse gas, although not one we generally concern ourselves with, for a variety of reasons.
Aren’t you sorry you asked?
Update: Fixed a math typo, corrected the number of hydrogen atoms in octane, and changed the headline to reflect more clearly that the gasoline is turned into carbon dioxide, not carbon. Also clarified the statement about water vapor. Thank you to our many sharp-eyed readers.