First we write the reactants and the products in this manner shown below:
C5H12 + O2 = H20 + CO2
This equation is not yet balanced so we add coefficients to make the equation balance:
C5H12 + 8O2 = 6H20 + 5CO2
The principle of concern here is the Law of Conservation of Mass.
This law states that for a system that is isolated, the mass is not created nor destroyed. Hence for this system, the amount of CO2 and water created is exactly from the reactants pentane and oxygen.
