[denizen]

Here is the problem:
When backpacking in the wilderness, hikers often boil water to sterilize it for drinking.
Suppose that you are planning a backpacking trip and will need to boil 36 L of water for your group.

What volume of fuel should you bring? Assume each of the following:
the fuel has an average formula of C7H16 ; 15% of the heat generated from combustion goes
to heat the water (the rest is lost to the surroundings);
the density of the fuel is 0.78 g/mL; the initial temperature
of the water is 25.0C; and the standard enthalpy of
formation of C7H16 is -224.4 kJ/mol..

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Here is my attempt (incorrect):

std enthalpy is -224.4 kJ
density is .78g/mol

36,000g * 4.184 * 75 = 11296.8 kJ required to heat the water

which leaves (224.4*.15)x = 11296.8 or x = 335.62 moles of C7H16

335.62 moles * 100 g/mol = 33562 g / .78 = 43027.56 mL of C7H16 required to heat the water...

Except the answer 43027.56 is not right...what am I missing?  Thanks for your help.

[sjb]

In the first instance, why are you using 224.4 kJ mol^-1 as your value for heat?