2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

Combustion of octane — balanced chemical equation, step by step.

Octane is a reference component of gasoline, so this is essentially the reaction inside a car engine.

How to balance C8H18 + O2 = CO2 + H2O

A combustion reaction burns a fuel in oxygen (O₂), releasing energy and producing carbon dioxide and water. Balancing means choosing coefficients so that every element has the same number of atoms on both sides of the arrow — the Law of Conservation of Mass. Here is how it's done, step by step.

Step 1 — Write the unbalanced equation

Start with the correct formulas for every reactant and product:

C8H18 + O2 = CO2 + H2O

Step 2 — Count the atoms of each element

With no coefficients yet (everything counted once), the atoms do not match. The ✗ marks show which elements are unbalanced:

Step 3 — Add the smallest whole-number coefficients

Adjust the coefficients in front of each formula until every element balances. The smallest whole-number coefficients are 2 C₈H₁₈, 25 O₂, 16 CO₂, 18 H₂O, giving:

2C8H18 + 25O2 = 16CO2 + 18H2O

For combustion reactions, balance carbon (C) first, then hydrogen (H), and leave oxygen (O) for last — oxygen appears alone as O₂, so it is easiest to adjust at the end.

Step 4 — Verify the balance

Recount every element. Each one now matches on both sides:

All elements are balanced and the coefficients are the smallest whole numbers, so 2C8H18 + 25O2 = 16CO2 + 18H2O is the correct balanced equation.

Molar masses and mole ratio

The balanced coefficients are also the mole ratio of the reaction. Using standard atomic masses, the molar mass of each substance is:

For this reaction the mole ratio is 2 : 25 : 16 : 18. Combine these molar masses with the ratio in the free stoichiometry calculator to convert between moles, grams, and the number of particles for any reactant or product.

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