Pentane gas + oxygen gas -> water + carbon dioxide, right? so substitute the chemical symbols in: C5H12 + O2 -> H2O + CO2 i'll leave for you to balance. The principle is called the law of conservation of mass: "The Law of Conservation of Mass states that matter can be changed from one form into another, mixtures can be separated or made, and pure substances can be decomposed, but the total amount of mass remains constant. We can state this important law in another way. The total mass of the universe is constant within measurable limits; whenever matter undergoes a change, the total mass of the products of the change is, within measurable limits, the same as the total mass of the reactants. "