The chemical formula for octane is #"C"_8"H"_18"#. A combustion reaction is one in which one reactant reacts with oxygen gas, #"O"_2"#. Since octane is a hydrocarbon, the products will be carbon dioxide and hydrogen. The balanced equation for the combustion of octane gas is:
#"2C"_8"H"_18("g")+"25O"_2("g")##rarr##"16CO"_2("g")+"18H"_2"O(g)"#
Explanation:
First place an 8 in front of #"CO"_2# because of the 8 carbon atoms in a molecule of octane, and a 9 in front of #"H"_2"O"# because of 18 H atoms divided by 2 atoms of O (in #"O"_2#).
#"C"_8"H"_18("g")+"O"_2("g")##rarr##"8CO"_2("g")+"9H"_2"O(g)"#
Balance the oxygen atoms on both sides of the equation. There are 25 O atoms on the right side of the equation. So we place a coefficient of #25/2# in front of the #"O"_2# molecule on the left.
#"C"_8"H"_18("g")##+##25/2"O"_2("g")"##rarr##"8CO"_2("g")+"9H"_2"O(g)"#
There cannot be a fraction as a coefficient, so we need to multiply the equation times 2.
#"2C"_8"H"_18("g")+"25O"_2("g")##rarr##"16CO"_2("g")+"18H"_2"O(g)"#
