Energy in a Gallon of Octane

Q.
Pure liquid octane (C₈H₁₈) (d= 0.702 g/ml) is used as the fuel in a test of a new automobile drive train. How much energy is released by the complete combustion of all the octane in a 21.) gal fuel tank (Hcomb= -5.45 E 3 kJ/mol)?
A.
You must convert the gallons of octane to grams:
(21.0 gal)(3.785 L/1 gal )(1000 mL/1 L )(0.702 g/ 1 mL) = 55800 grams
1 mole of C8H18 = 114.23 g

Convert grams to moles:
(55800 g)(1 mol / 114.23 g) = 488.5 moles

Heat = ( 488.5 mol)(5450 kJ/mol) = 2.66x10^6 kJ

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