stoichiometric coefficient
 

When I try to use the formula for reaction in the provided space( in reaction system sub folder in chemical reactions folder), I face with an error in the command output window which says the stoichiometric constants are not correct and (molecular weight times number of moles) of the product and reactant in left and right side of the reaction are different!! The reaction that I use is as follows:
_/C12H26 + 18.5O2 -> 12CO2 + 13H2O/_
But the software dose not accept this coefficient !
It just accept it If I increase the constant of O2 to 19.12999 ?

Could you please let me know how I can solve this problem with stoichiometric constants.

Where dose this problem comes from?
with thanks