Water from gasoline

I give the following challenge as a take-home assignment. A pre-discussion can flesh out the need for a balanced, complete combustion equation and the need for the average formula for gasoline and its density. Teachers can decide on how much direction is needed for their students. I ask students to hand in their answer on one sheet, neatly written and organized — no rough work.

A major consideration in fighting a war is the provision of supplies and materiel to front-line troops. This includes fuel and water, both of which are required in extremely large amounts.

The US Army considered extracting the water in a Humvee’s exhaust to decrease reliance on shipped-in water. [Reference from Chemical and Engineering News, October 31, 2005, Newscripts column on back page — available online in CEN archives at pubs.acs.org]

How is water produced in the exhaust of an internal combustion engine?

The US Army people claim that the combustion of each gallon of gasoline yields about one gallon of liquid water.

Is this true?

Carry out calculations to verify this claim.

What information do you need to solve this problem?  

Provide a neat set of calculations to solve this problem.

If necessary I direct students to these assumptions:

  • Assume 100% extraction of water
  • Assume gasoline is pure octane, C8H18
  • assume complete combustion
  • density of octane is 0.703 g/mL

[Answer: 1 volume of liquid gasoline produces an equal volume of liquid water.]