How to turn 6 pounds of gasoline into 20 pounds of carbon dioxide

 

Short answer:

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 to make it much heavier than it was in solid form.

Carbon dioxide — or CO2 — is one carbon atom joined to two oxygen atoms. Carbon dioxide. CO2. Oxygen is a little bit heavier than carbon, so when you stick two oxygen atoms onto every available carbon atom, you roughly triple the weight of the gasoline.

Long answer:

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 eight 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 6 pounds of gas into 20 pounds of carbon dioxide is some highly confusing algebra!

Bonus material:

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 gas.

Also, 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 (H20). Every gallon of gas creates roughly 8 pounds of water vapor. And water vapor does, believe it or not, contribute to global warming.

Aren’t you sorry you asked?

Update: A small point of clarification — even though water vapor is a significant contributor to global warming, you don’t have to feel guilty about taking hot showers. Direct human contributions to atmospheric water vapor are small. Rather, humans have a large indirect impact on atmospheric water vapor when we create carbon dioxide, which warms the atmosphere, which warms the oceans, which creates more water vapor, which warms the atmosphere…