1 gallon gasoline

On average, one gallon of gasoline produces a little over 18 pounds of carbon dioxide (CO₂).

Gasoline

Like jet fuel, gasoline is a mixture of hydrocarbons, the exact composition of which depends upon the crude oil from which it was refined. In addition, most gasoline contains additives like antioxidants, static inhibitors, corrosion inhibitors, and more.

What we purchase at the pump is a mixture of hundreds of compounds. However, given the boiling points used to distill gasoline (120 - 400 degrees Fahrenheit), we know its hydrocarbons have between 6 and 12 carbon atoms. Most people consider octane - C₈H₁₈ - an acceptable average value¹. Octane has a density of 0.703 g/mL or 2,661.14 g/gal.

Combustion

In a gasoline engine, a mixture of gasoline and air is combusted to drive pistons. The pistons turn the drive shaft, and the drive shaft turns the wheels. In the combustion reaction, gasoline (aka, octane, C₈H₁₈) combines with atmospheric oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂0):

C₈H₁₈ + O₂ -> CO₂ + H₂0 (1)

This is the same combustion reaction that occurs in our bodies to generate energy from food. Of course, other elements in the air such as nitrogen and carbon dioxide are burned in the engine as well as oxygen. Those other elements will create other molecules like nitrates and ozone, but we'll ignore them in our calculations.

Of course, we need to balance the equation so the same number of elements enter and exit our reaction:

2C₈H₁₈ + 25O₂ -> 16CO₂ + 18H₂0 (2)

We can see for every two octane molecules that enter our combustion reaction, 16 carbon dioxide molecules are produced (or, reducing the fraction, one molecule of octane produces eight molecules of carbon dioxide). However, octane molecules and carbon dioxide molecules are not the same mass, and mass must be conserved.

To determine how many grams of carbon dioxide are produced, we need to calculate the molecular mass of carbon dioxide, the molecular mass of octane, and the ratio between the two:

(1 m * 12.011 amu) + (2 m * 15.999 amu) = 44.009 amu (3)

(8 m * 12.011 amu) + (18 m * 1.008 amu) = 114.232 amu (4)

44.009 amu / 114.232 amu = 0.385 (5)

We know that for every octane molecule that enters our reaction produces eight carbon dioxide molecules (Equation 2), and we know that one carbon dioxide molecule is 38.5% the mass of an octane molecule (Equation 5). As a result, we can calculate how many grams of carbon dioxide are produced by one gram of octane (aka, gasoline):

8 * 0.385 = 3.080 g CO₂ / g C₈H₁₈ (6)

Given the density of octane (0.703 g/mL or 2,661.144 g/gal), we can calculate the pounds of carbon dioxide generated by one gallon of octane (aka, gasoline):

2,661.144 g C₈H₁₈/gal * 3.080 g CO₂/g C₈H₁₈ * 1 lb/453.592 g = 18.07 lbs CO₂/gal (7)

Conclusion

There you have it! To the best of our ability, on average, one gallon of gasoline produces 18.07 pounds of carbon dioxide. If your car gets 20 miles-per-gallon and you drive 10,000 miles in a year, you'll produce 4.5 short tons of carbon dioxide.

Heads up! In 2012, the US Energy Information Administration estimated one gallon of gas produces 19.6 pounds of carbon dioxide. The discrepancy probably stems from the fact that they tested actual gasoline with all its additives and other compounds, not just octane. In additition, they probably considered the effect of other gases in the air like nitrogen, carbon dioxide, etc.

Don't forget! There are other molecules in the reaction. We aren't saying that you get 18.1 pounds of carbon from 5.7 pounds of carbon (about the weight of a gallon of gas). That would defy physics! We're saying you get 18.1 pounds of carbon dioxide and 8.3 pounds of water from 5.7 pounds of gasoline (1 gallon) and 20.5 pounds of oxygen.

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