Write down the chemical equation for the combustion of octane.
C8H18 + O2 -> CO2 + H2O
Determine the count of each element in the reactant and product side of the equation.
Reactants -- 8 carbon, 18 hydrogen, 2 oxygen
Products -- 1 carbon, 2 hydrogen, 3 oxygen
Balance the molecules other than oxygen and hydrogen on each side first. There are 8 carbons in the reactants and only 1 in the products; thus, CO2 on the product side should be multiplied by 8 to balance out the carbons on both sides.
C8H18 + O2 -> 8CO2 + H2O
Reassess the amount of each element in the reactants and products.
Reactants -- 8 carbon, 18 hydrogen, 2 oxygen
Products -- 8 carbon, 2 hydrogen, 16 oxygen
Balance the hydrogen molecules on each side. There are 18 hydrogen molecules in the reactants compared with 2 in the products. The H2O should be multiplied by 9 to result in 18 molecules of hydrogen on each side.
C8H18 + O2 -> 8CO2 + 9H2O
Balance the oxygen molecules. There are 25 oxygen molecules on the right and 2 on the left. Therefore, O2 needs to be multiplied by 12.5 molecules to result in 25 molecules of oxygen on the product and reactant side. The final balanced equation for the combustion of one mole of octane would then be:
C8H18 + 12.5O2 -> 8CO2 + 9H2O
Realize that when balancing equations, the coefficients should normally be expressed as whole numbers. However, specific types of balancing problems may require fractions and decimals. The complete balanced equation for the combustion of octane is found by multiplying the resulting equation by 2, which would give a whole number coefficient for each molecule; however, this equation would be for 2 moles of octane instead of 1 mole. Therefore, the equation in Step 6 must be used. The fully balanced equation would be:
2C8H18 + 25O2 -> 16CO2 + 18H2O