The typical structure of a hydrocarbon is HCnHnH where for every carbon atom, there's 2 hydrogen atoms plus a single hydrogen atom at each end of the hydrocarbon chain. So the expected output will be 1 CO2 molecule and 1 H2O molecule per carbon atom plus 1 extra H2O molecule. These molecules have to have a total of 16 oxygen atoms. So
2n + (n+1) = 16
where n = number of carbon atoms
Solving for n gives
3n + 1 = 16
3n = 15
n = 5
So the equation should be:
C5H7 + 8O2 ==> 5CO2 + 6H2O
