Answer
Gasoline provides more energy per mol and per gram.
Work Step by Step
Enthalpies of formation (kJ/mol):
$C_6H_6O(l): -277.0, C_8H_18(l): -250.1, CO_2(g): -393.5, H_2O(l): -285.8$
Ethanol:
$C_6H_6O+7\ O_2\rightarrow 6\ CO_2+3\ H_2O$
$\Delta H_c= 6\times-393.5+4\times-285.8+277.0=-3227.2\ kJ/mol$
$-3227.2\ kJ/mol\div 94.11\ g/mol=-34.29\ kJ/g$
Octane:
$C_8H_{18}+25/2\ O_2\rightarrow 8\ CO_2+9\ H_2O$
$\Delta H_c= 8\times-393.5+9\times-285.8+250.1=-5470.1\ kJ/mol$
$-5470.1\ kJ/mol\div 114.23\ g/mol=-47.89\ kJ/g$
Gasoline provides more energy per mol and pet gram.
