How this instrument works
Complete combustion is the reaction of a carbon-and-hydrogen-based fuel (optionally containing oxygen, like an alcohol) with oxygen gas, producing carbon dioxide and water as the only products, provided enough O2 is available. For a general fuel with formula CxHyOz, conservation of atoms — the same carbon, hydrogen and oxygen atoms that go in must come out, just rearranged into new molecules — fixes every coefficient in the balanced equation as a function of x, y and z alone, with no other information needed.
Carbon balances directly: every carbon atom in the fuel ends up in a CO2 molecule, so x moles of fuel produce x moles of CO2. Hydrogen balances similarly: every two hydrogen atoms end up in one H2O molecule, so y hydrogen atoms produce y/2 moles of H2O. Oxygen is the one that takes a small extra step, because the fuel itself may already contain some oxygen (the z in CxHyOz): the O2 needed is whatever oxygen the CO2 and H2O require in total, minus whatever oxygen the fuel already supplied, divided by two since O2 is diatomic — which works out to x + y/4 − z/2 moles of O2 per mole of fuel.
This instrument handles any fuel describable as CxHyOz — plain hydrocarbons like methane (CH4) or octane (C8H18) with z = 0, and oxygen-containing fuels like ethanol (C2H6O) or glucose (C6H12O6) with z greater than zero. The atom-counting logic is identical either way; only the numbers change, and a fuel that already contains oxygen simply needs proportionally less O2 supplied from the air to complete its combustion.
- Enter Carbon atoms (x, in CxHyOz) — the number of carbon atoms in one molecule of your fuel.
- Enter Hydrogen atoms (y, in CxHyOz) — the number of hydrogen atoms.
- Enter Oxygen atoms (z, in CxHyOz) — the number of oxygen atoms already in the fuel; enter 0 for a plain hydrocarbon like methane or octane.
- Read O2 needed (mol per mol fuel), CO2 produced (mol per mol fuel), and H2O produced (mol per mol fuel) — the balanced equation's coefficients, all expressed per one mole of fuel burned.
- Enter at least one atom across the three fields — a fuel formula with zero atoms everywhere has nothing to balance.
Worked example — methane, CH4
Methane, natural gas's main component, has the formula CH4: one carbon atom, four hydrogen atoms, and no oxygen. Enter 1 into Carbon atoms (x), 4 into Hydrogen atoms (y), and 0 into Oxygen atoms (z); O2 needed (mol per mol fuel) reads 2.0, CO2 produced (mol per mol fuel) reads 1.0, and H2O produced (mol per mol fuel) reads 2.0.
That reproduces the textbook-standard balanced equation for methane combustion: CH4 + 2 O2 → CO2 + 2 H2O. Every mole of methane burned consumes exactly two moles of oxygen gas and produces one mole of carbon dioxide and two moles of water — coefficients you can verify by counting atoms on both sides of the equation.
Questions
Why does the O2 needed formula subtract z/2 for oxygenated fuels?
Because a fuel that already contains oxygen atoms — like ethanol or glucose — supplies part of what the combustion products need internally, so less O2 has to come from the air. The full oxygen requirement for x moles of CO2 and y/2 moles of H2O is x + y/4 moles of O atoms worth, and the fuel's own z oxygen atoms (z/2 moles of O2 equivalent) get credited against that total, leaving x + y/4 − z/2 moles of O2 still needed from outside the fuel.
Does this work for fuels containing nitrogen, sulfur, or other elements?
No — this instrument is built specifically for the CxHyOz case: carbon, hydrogen, and optionally oxygen. Fuels containing nitrogen, sulfur, or other elements produce additional products beyond CO2 and H2O (such as nitrogen oxides or sulfur dioxide) and require balancing those extra atoms separately, which this simpler carbon-hydrogen-oxygen formula doesn't account for.
What does 'per mole of fuel' mean in the results?
It means every coefficient is scaled so that exactly one mole of fuel is being burned — the smallest possible whole-number-free basis. For ethane, C2H6, that basis gives a fractional O2 coefficient (3.5 mol O2 per mol fuel) rather than the whole-number 7 you'd see in the doubled textbook equation 2 C2H6 + 7 O2 → 4 CO2 + 6 H2O; both describe the identical reaction, just scaled differently.
Is this 'complete' combustion, or could it produce carbon monoxide instead?
This instrument calculates complete combustion specifically — the case where there's enough oxygen present for every carbon atom to become CO2 and every hydrogen atom to become H2O. Incomplete combustion, which happens in oxygen-limited conditions, can produce carbon monoxide (CO) or even soot (elemental carbon) instead, following different balanced equations that this instrument doesn't compute.
Can I use this for a fuel formula with a coefficient other than 1, like C2H6?
Enter the atom counts exactly as they appear in the molecular formula — for ethane, C2H6, that's 2 carbon atoms and 6 hydrogen atoms, entered directly into Carbon atoms (x) and Hydrogen atoms (y). The instrument always computes coefficients per one mole of that fuel molecule, so there's no separate 'coefficient' field to fill in beyond the atom counts themselves.