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Water from Burning Gasoline


Tuesday, April 30, 2002

name         Brandon D.
status       student
age          20s

Question -   i need to know how much water is produced for each
gallon of gasoline burned. the actual question I am trying to answer is..."for
each gallon of fuel burned, more than ______ of water is formed." this
question pertains to motor vehicles, so that is the reason i specified gasoline.
-------------------------------------------------------
The inquiry has no specific answer because gasoline is a mixture of
hydrocarbons, and in some places up to 10% ethanol. In addition, the
composition of gasoline is different in different climates. For example, in
the northern states in winter there must be a higher ratio of more volatile
components. Otherwise the car will not start at -20F. There are also some
aromatic hydrocarbons present in gasoline. There is also the complicating
factor that combustion is not always complete, so the mix of products will
change.

You may find the following equation useful to find the approximate amount of
water produced, that you could use in some model calculations. For a linear
aliphatic hydrocarbon the general formula is:C(n)H(2n+2). For octane n=8 so
the formula is C8H18. Octane is typical of the hydrocarbons found in
gasoline. So, the general chemical equation for combustion is:

C(n)H(2n+2)    +  [(3n+1)/2] O2  ------->  (n) CO2  + (n+1) H2O

It would be fairly easy to set this up on a spread sheet (e.g. Excel) and
look at various compositions. The mass units need not be metric, so you
could look at the lbs of H2O produced per lb of hydrocarbon, and the density
conversion for water is: 8.33 lbs./ gal.

Vince Calder
=========================================================
Gasoline is a complex mixture of hydrocarbons, and other ingredients, so
there is not an exact answer, but assuming that gasoline is octane (C8H18)
the complete combustion (which does not occur in the real world) chemical
reaction is:
2C8H18  +  25O2  =  16CO2  +  18H2O
Assuming octane has a density of 0.8 kg/liter and liquid water has a density
of 1.0 kg/liter, you can calculate the volume of water produced by a given
volume of hydrocarbon.

Vince Calder
==========================================================
By now, you have probably received this answer from others, but if my
resources and calculations are correct, (at room temperature) LESS than one
gallon of water is formed in a complete, clean reaction of gasoline with
oxygen to produce water and carbon dioxide.  Actually, it's only around five
milliliters less.

Tim Spry



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