Using the Molar Mass of Octane (Advanced)
Gasoline is not a simple chemical compound like water or ethanol. It is a mixture of hundreds of different compounds.
The reason for this is that gasoline is made from crude oil (petroleum) which is made up of thousands of different compounds.
After the oil is produced (or taken out of the ground via oil wells) it is shipped to an oil refinery where it is separated into
different products including gasoline, jet fuel, and diesel fuel, among others.
The separation of the oil takes place in a process called distillation. (There are other processes too, but the
simplification to consider just distillation is not too far off.) In distillation, the oil is heated until it starts to boil. Some of the
thousands of chemical compounds that make up the oil start to boil at low temperatures, and some do not boil until the
petroleum has been heated up to higher temperatures. In this way the oils are physically separated.
While we can't say for sure exactly what compounds are present in gasoline, we can define gasoline to some extent.
First, it is made up almost entirely of hydrocarbons, which are molecules made up of carbon and hydrogen. At normal
atmospheric pressure, gasoline’s hydrocarbons boil at temperatures between about 50 and 200 degrees Celsius (120 to 400
degrees Fahrenheit). This means that these compounds are mostly hydrocarbons that have between 6 and 12 carbon atoms in
each molecule, with chemical formulas between C6H14 and C12H26. So a good "average" compound is octane, C8H18.
When a hydrocarbon is burned (that is, reacted with oxygen) it forms carbon dioxide (CO2) and water (H2O). For our
"average" gasoline of C8H18, an ideal chemical reaction (meaning it probably won’t happen exactly in real life) is 2 molecules
of octane reacting with 25 molecules of oxygen (O2) to form 18 molecules of water and 16 molecules of carbon dioxide.
Chemical reaction of octane burning: 2C8H18 + 25O2 —> 18H2O + 16CO2
1. Octane has a density of 703.00 kg/m³, or 703.00 g/L. A gallon is 3.78541 L. What is the mass (in g) of 1.0000 gallon of
octane?
2. What is the molar mass for octane?
3. How many moles of octane are in 1.0000 gallon of octane? (Hint: Look at #1 and #2.)
4. If (exactly) 2 moles of octane react with (exactly) 25 moles of oxygen, as shown in the chemical reaction above, a. how
many moles of oxygen are needed to burn the 1.0000 gallon of octane?
b. What is the mass of this many moles of O2? (Remember that it is O2, not O.)
5. Using the chemical equation before #1, see if you can figure out what mass of water and what mass of CO 2 are produced.
6. What is the empirical formula for octane?