For a particular isomer of c8h18, the following reaction produces 5093.7 kj of heat per mole of c8h18(g) consumed under standered conditions. what is the standard enthalpy of formation of this isomer of c8h18(g)?

02(g) = 0 kj/mol 
CO2 (g) = -393.5 kj/mol 
H20(g) = -241.8 kj/mol 

H total = -5094 kJ
5094kJ = [8(-393.5) + 9(-241.8)] - [X + 12.5(0)] 
-5094 kJ = [-3148 + (-2176.2)] - [x + 0] 
-5094 kJ = -5324.2 - x 

add -5324.2 to -5094 
to get +230.2 = -x 

move the negative to the other side 
and you get -230 kj/mol

Ions gain or lose electrons