Combustion Analysis Calculator: 5 Free Modes, Steps Shown

A combustion analysis calculator turns the two numbers a combustion train gives you — the mass of carbon dioxide and the mass of water collected when a weighed sample burns in excess oxygen — into the empirical formula, and with a molar mass the molecular formula, of the compound. Burn 1.000 g of acetic acid and you trap 1.466 g of CO₂ and 0.600 g of H₂O; those masses contain 0.4001 g of carbon and 0.0671 g of hydrogen, the missing 0.5328 g is oxygen, the mole ratio is 1 : 2 : 1, the empirical formula is CH₂O and the molecular formula is C₂H₄O₂. That chain of arithmetic is what this page teaches and what the free combustion analysis calculator above performs, with every step written out.

The combustion analysis calculator covers the whole family of problems in five modes: empirical and molecular formula from CO₂, H₂O and optionally N₂ and SO₂ masses, with oxygen by difference; the same problem when a halogen or sulfur is measured as a silver halide or barium sulfate precipitate; prediction of the products, the oxygen consumed and the balanced equation for any C, H, N, O, S compound; a CHN elemental-analysis report check against the ±0.4% journal rule, including solvates; and fuel combustion, giving the stoichiometric air–fuel ratio, excess air and flue-gas composition. Every mode shows its steps, draws a chart and lets you copy, download, print or share the result.

Below the tool is a complete guide: what combustion analysis measures and why it still matters, the conversion factors that take you from product masses to element masses, three fully worked examples, a reference table of predicted products for thirty-two common compounds, the ±0.4% rule used by journals, the history from Lavoisier and Liebig to Pregl’s microanalysis, ten practice problems with hidden answers and a set of frequently asked questions. All values use IUPAC 2021 standard atomic weights, the same numbers the combustion analysis calculator uses internally.

What Is Combustion Analysis?

Combustion analysis is the classic method for finding the elemental composition — and from it the empirical formula — of an organic compound. A precisely weighed sample, typically 1–3 mg in a modern instrument or a few hundred milligrams in the old glass trains, is burned completely in a stream of pure oxygen at 900–1000 °C. Every carbon atom leaves as CO₂, every hydrogen atom as H₂O, nitrogen as N₂ after the nitrogen oxides are reduced over hot copper, and sulfur as SO₂. The products are separated and weighed (or, in a CHN analyser, measured by thermal conductivity), and because each product carries a fixed fraction of its element, the mass of every element in the original sample follows directly.

Oxygen is the one element that cannot be measured this way: the sample burns in oxygen, so oxygen in the products comes from both the compound and the gas stream. Instead, oxygen is found by difference — the sample mass minus the masses of all the elements that were measured. That single subtraction is where most textbook errors happen, and it is why a good combustion analysis calculator always shows it explicitly.

The output of the experiment is a set of element masses. The output of the calculation is a formula. The bridge between them is the mole: divide each element mass by its atomic weight, divide all the mole numbers by the smallest, and clear any fractions by multiplying. If the molar mass is known from mass spectrometry, freezing-point depression or vapour density, one more division gives the molecular formula. The combustion analysis calculator on this page runs that bridge in both directions — data to formula, and formula back to the products you should expect.

How to Use This Combustion Analysis Calculator

Pick a mode with the pill tabs at the top of the combustion analysis calculator:

1. Formula from CO₂ & H₂O — type the sample mass, the CO₂ and H₂O masses, and if you have them the N₂ and SO₂ masses. Choose grams or milligrams, say whether the compound may contain oxygen, add the molar mass if known, and press Calculate.
2. With Halogen / Sulfur — the same combustion data plus a second sample that was converted to AgCl, AgBr, AgI or BaSO₄. The tool scales the precipitate result to the combustion sample automatically.
3. Predict Products — enter a formula and a sample mass; get the masses of CO₂, H₂O, N₂ and SO₂, the oxygen consumed, the air required and the balanced equation.
4. CHN Report Check — enter a proposed formula and the found % C, H, N, S from an analytical report; the tool calculates the theoretical values, flags every element outside ±0.4% and suggests a water or solvent count that would fit.
5. Fuel & Flue Gas — enter a fuel formula and either the excess air or the O₂ reading from a flue-gas analyser; get the stoichiometric air–fuel ratio, CO₂ per kilogram and the dry and wet flue-gas percentages.

Every mode of the combustion analysis calculator has example chips that fill the fields with a classic textbook problem so you can see the expected answer, a Reset button that clears only that panel, and an Enter-key shortcut. Results appear in a highlighted answer box, followed by numbered steps, a table and a bar chart. The toolbar copies the result as text, downloads CSV or TXT, prints a clean report or shares to WhatsApp, Facebook, X or the native share sheet. The last twelve calculations are kept in a history list on your own device.

The Five Modes Explained

Mode 1 — Empirical and molecular formula from CO₂ and H₂O

The core combustion analysis calculator problem. The tool converts each product mass to an element mass with the fixed fractions 12.011/44.009 for carbon, 2.016/18.015 for hydrogen, 1 for N₂ and 32.06/64.064 for SO₂, subtracts to find oxygen if you allow it, converts to moles, finds the smallest whole-number ratio (testing multipliers up to 12 to clear fractions such as 1.5 or 1.333) and, if a molar mass is supplied, the molecular formula. If the measured element masses exceed the sample mass, or if you tell the tool the compound has no oxygen but 2% or more of the mass is unaccounted for, it stops and says so.

Mode 2 — Combustion plus a halogen or sulfur precipitate

Chlorine, bromine and iodine are not measured by burning; a separate portion is decomposed (the Carius tube or Schöniger flask) and the halide is weighed as silver halide. Sulfur may be weighed as BaSO₄. Because the two portions rarely have the same mass, the combustion analysis calculator scales the halogen mass from the precipitate sample to the combustion sample before combining the two data sets, then proceeds exactly as in Mode 1. Oxygen is off by default here, since most halogen problems are halocarbons; switch it on for compounds such as chloroacetic acid.

Mode 3 — Predict the products

The inverse problem, and the best way to check a data set before an exam or to plan an experiment. Type C₆H₁₂O₆ and 1.500 g and the tool returns 2.199 g CO₂, 0.900 g H₂O, 1.599 g of O₂ consumed and the balanced equation C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O, with fractional coefficients doubled where needed, as in 2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O.

Mode 4 — CHN report check

Journals ask that found values agree with calculated values to within ±0.4 percentage points. The combustion analysis calculator computes the theoretical percentages for your proposed formula (with any number of water or solvent molecules), compares them with the found values, colours each element pass or fail, and if the plain formula fails it searches 0.5 to 3 solvate molecules for a count that would bring every element inside tolerance. Caffeine found at C 45.30, H 5.70, N 26.40 fails as C₈H₁₀N₄O₂ but fits perfectly as the monohydrate.

Mode 5 — Fuel combustion and flue gas

Engineers use the same stoichiometry to size burners and tune boilers. Enter a fuel such as CH₄, C₈H₁₈ or C₁₂H₂₃ and the excess air; the tool returns the stoichiometric air–fuel ratio by mass (17.23 : 1 for methane), the kilograms of CO₂ per kilogram of fuel, and the flue-gas composition on a dry and a wet basis. Enter a measured O₂ percentage instead and the combustion analysis calculator back-calculates the excess air — 3.83% O₂ in dry methane flue gas means 20% excess air.

The Formulas: From Product Masses to Element Masses

Each combustion product carries a fixed mass fraction of the element it came from. That fraction is simply the mass of the element in one mole of the product divided by the molar mass of the product:

Then the standard empirical-formula routine: n(element) = m(element) ÷ atomic weight; divide every n by the smallest n; if a ratio lands near 1.5, 1.33, 1.25 or 1.2, multiply the whole set by 2, 3, 4 or 5. The molecular multiplier is the molar mass divided by the empirical formula mass, rounded to the nearest whole number. The combustion analysis calculator prints each of these lines with your numbers substituted, so a graded answer can be copied straight from the steps box.

Two checks are worth building into the habit whenever you use a combustion analysis calculator or work by hand. First, the element masses must add up to no more than the sample mass; if they exceed it, a product was over-weighed or the sample was under-weighed. Second, the percent composition implied by the element masses (mass of element ÷ sample mass × 100) should look like a real compound — 40% C, 6.7% H and 53% O is a carbohydrate, 93.7% C and 6.3% H is an aromatic hydrocarbon. Our percent composition calculator works the formula-to-percent direction if you want to cross-check.

A Fully Worked Example: Acetic Acid

A 1.000 g sample of a compound containing only C, H and O is burned completely, producing 1.466 g of CO₂ and 0.600 g of H₂O. The molar mass is 60.05 g/mol. Find the empirical and molecular formulas.

Notice how much of the sample was oxygen — more than half. Had the problem said “a hydrocarbon” and you forgot to check the mass balance, you would have tried to build a formula from 0.4672 g of a 1.000 g sample. The combustion analysis calculator guards against this: with oxygen switched off and a third of the mass missing, it refuses to continue and tells you how many grams are unaccounted for.

Two More Worked Examples: Nicotine and Chlorobenzene

Example 2 — A nitrogen compound (nicotine)

1.000 g of nicotine gives 2.7127 g CO₂, 0.7773 g H₂O and 0.1727 g N₂. The molar mass is 162.24 g/mol.

m(C) = 2.7127 × 0.27292 = 0.7404 g; m(H) = 0.7773 × 0.11191 = 0.0870 g; m(N) = 0.1727 g. Sum = 1.0001 g — equal to the sample mass, so there is no oxygen. Moles: C 0.06164, H 0.08629, N 0.01233. Divide by 0.01233: C 5.00, H 7.00, N 1.00 → C₅H₇N (81.12 g/mol). Multiplier 162.24 / 81.12 = 2 → C₁₀H₁₄N₂. In the combustion analysis calculator the oxygen line reads “essentially zero” and the formula is printed without O.

Example 3 — A chlorine compound (chlorobenzene)

0.500 g of a compound gives 1.173 g CO₂ and 0.2001 g H₂O. A separate 0.500 g portion, decomposed and treated with silver nitrate, gives 0.6367 g AgCl. M = 112.56 g/mol.

m(C) = 1.173 × 0.27292 = 0.3201 g; m(H) = 0.2001 × 0.11191 = 0.0224 g; m(Cl) = 0.6367 × 35.45 / 143.32 = 0.1575 g (same sample mass, so no scaling). Sum = 0.5000 g, no oxygen. Moles: C 0.02665, H 0.02221, Cl 0.004443. Divide by 0.004443: C 6.00, H 5.00, Cl 1.00 → C₆H₅Cl, empirical mass 112.56 — identical to the molar mass, so the molecular formula is the same. Mode 2 of the combustion analysis calculator reproduces this in one click from the “Chlorobenzene” chip.

Oxygen by Difference: Why It Matters

Because the sample is burned in a stream of oxygen, the oxygen in the CO₂ and H₂O comes partly from the compound and partly from the gas. There is no way to tell the two apart by weighing the products, so oxygen is never measured directly in a classical combustion analysis; it is what is left after every other element has been accounted for. This has three practical consequences.

• You must know the sample mass. The masses of CO₂ and H₂O alone give you the C : H ratio but not the oxygen. A problem that omits the sample mass is asking for a hydrocarbon formula, or is incomplete.
• All errors pile into oxygen. If the CO₂ absorber gained 1% too much mass, carbon is 1% high and oxygen carries the whole error with the opposite sign. Oxygen by difference is the least reliable number in the analysis, which is why modern laboratories sometimes run a separate pyrolysis (oxygen-mode) determination.
• “Contains only C and H” is a claim to test. Add the element masses; if they fall short of the sample by more than the balance uncertainty, the compound contains something else — usually oxygen. The combustion analysis calculator makes this test automatically and reports the unaccounted mass.

A useful sanity check: a pure hydrocarbon has C + H = 100% of the sample, alcohols and ethers are roughly 20–35% oxygen, carbohydrates and carboxylic acids 50–55%. If your oxygen by difference comes out at 70% for a compound described as an ester, look for an arithmetic slip before you look for a new compound.

Nitrogen, Sulfur and Halogens

Nitrogen. In the Dumas method — and in every modern CHN analyser — the nitrogen oxides formed on combustion are passed over hot copper, which reduces them to N₂. The gas is either collected over concentrated KOH and its volume measured, or detected by thermal conductivity. Because N₂ is the element itself, m(N) = m(N₂): no conversion factor. If the problem gives a volume of nitrogen at STP instead, convert first: 1 mol N₂ = 28.014 g occupies 22.414 L at 0 °C and 1 atm. The older Kjeldahl method converts nitrogen to ammonia and titrates it; it gives m(N) directly and slots into the same calculation.

Sulfur. Sulfur burns to SO₂ (with a little SO₃ that instruments reduce or catalytically fix). Either the SO₂ is weighed — m(S) = m(SO₂) × 32.06/64.064 — or, in the gravimetric tradition, the sulfur is oxidised to sulfate and weighed as BaSO₄, in which case m(S) = m(BaSO₄) × 32.06/233.386 = 0.1374 × m(BaSO₄). Mode 1 of the combustion analysis calculator takes the SO₂ mass; Mode 2 takes the barium sulfate.

Halogens. Chlorine, bromine and iodine survive combustion as HX and, in a plain train, would contaminate the water absorber. They are therefore determined on a separate portion: in the Carius method the sample is heated with fuming nitric acid and silver nitrate in a sealed tube; in the Schöniger flask it is burned over an alkaline absorber and the halide precipitated afterwards. The silver halide is dried and weighed: m(Cl) = m(AgCl) × 35.45/143.32 = 0.2473 × m(AgCl); m(Br) = m(AgBr) × 79.904/187.774 = 0.4255 × m(AgBr); m(I) = m(AgI) × 126.904/234.77 = 0.5405 × m(AgI). Since the halogen portion and the combustion portion usually differ in mass, express the halogen as a fraction of its own portion and multiply by the combustion sample mass before adding it to the mass balance — the combustion analysis calculator prints this scaling factor so you can see it.

Fluorine, phosphorus and metals need other methods (ion-selective electrode, colorimetry, ICP or ashing) and are entered as element masses if you have them; the empirical-formula step is the same. For percentages rather than masses, our empirical formula calculator accepts % C, % H and so on directly.

From Empirical to Molecular Formula

Combustion data — whether from a laboratory train or a combustion analysis calculator — fix only the ratio of atoms. CH₂O is the empirical formula of formaldehyde (30.03 g/mol), acetic acid (60.05), lactic acid (90.08), glucose (180.16) and a dozen other compounds; the products of burning 1.000 g of any of them are the same 1.466 g CO₂ and 0.600 g H₂O. To choose between them you need the molar mass from an independent measurement — mass spectrometry today, and historically vapour density, freezing-point depression, boiling-point elevation or osmotic pressure, each of which has its own calculator on this site.

The molecular multiplier is n = M(molecular) ÷ M(empirical). It should come out within about 0.1 of a whole number; 2.00 for acetic acid, 6.00 for glucose, 2.00 for nicotine (162.24 / 81.12). If n lands near 2.5 or 1.5 the empirical formula itself is probably wrong — go back to the ratio step and try a larger multiplier. The combustion analysis calculator reports n to two decimals before rounding so you can see how clean the fit is, and warns when the fit is loose.

Two special cases deserve a mention before you trust any combustion analysis calculator output. Sometimes the molar mass is only approximate (from a vapour-density experiment, say, with 5% error); then round n to the nearest integer and recompute M from the molecular formula to see whether the approximate value is consistent. And sometimes the empirical mass already equals the molar mass, as with chlorobenzene above; n = 1 and the two formulas coincide. For the full treatment of this step, see our molecular formula calculator.

Reference Table: Predicted Combustion Products for 32 Compounds

The table gives, for each compound, the grams of CO₂ and H₂O produced per gram of sample burned, together with the percent composition a CHN analyser should report. Use it to check a homework data set, to recognise a compound from its products, or to set a trap for yourself: pick a row, multiply by any sample mass, and feed the products back into Mode 1 of the combustion analysis calculator. Every value was computed with IUPAC 2021 atomic weights.

Patterns worth noticing: hydrocarbons produce more than 3 g of CO₂ per gram (methane 2.74 g is the exception because a quarter of its mass is hydrogen); every compound with the empirical formula CH₂O — acetic acid, glucose — produces exactly 1.4657 g CO₂ and 0.600 g H₂O per gram; and the water yield tracks the hydrogen content, from 0.47 g for vanillin to 2.25 g for methane. Halogen compounds are omitted from the table because their halogen is measured separately; run them in Mode 2 of the combustion analysis calculator instead.

Reading a CHN Report: The ±0.4% Rule

When a research group submits a new compound to a journal, the editors want evidence that the material is what the authors say it is and that it is pure. The traditional evidence is a CHN (or CHNS) analysis: the found percentages of carbon, hydrogen and nitrogen must agree with the values calculated from the proposed formula to within ±0.4 percentage points for each element. The rule is a convention rather than a law of nature, and it is debated, but it remains in the author guidelines of most chemistry journals.

Take caffeine, C₈H₁₀N₄O₂, M = 194.19 g/mol, and follow the same arithmetic the combustion analysis calculator uses. Calculated: C 8 × 12.011 / 194.194 = 49.48%; H 10 × 1.008 / 194.194 = 5.19%; N 4 × 14.007 / 194.194 = 28.85%. A report of C 49.31, H 5.22, N 28.70 differs by −0.17, +0.03 and −0.15 — all inside the window, so the analysis supports the formula. A report of C 45.30, H 5.70, N 26.40 misses by more than four points on carbon. That does not mean the compound is wrong; it means the sample was not anhydrous caffeine. Add one water: C₈H₁₀N₄O₂·H₂O (212.21 g/mol) calculates to C 45.28, H 5.70, N 26.40, a perfect fit for the monohydrate.

Mode 4 of the combustion analysis calculator automates this reasoning: it computes the theoretical percentages, colours each element pass or fail against your tolerance (0.4 by default; tighten to 0.3 for a journal that asks for it), and when the plain formula fails it tries 0.5, 1, 1.5, 2, 2.5 and 3 molecules of the solvent you name — water by default, or CH₂Cl₂, EtOAc, hexane — and tells you which count would bring every element inside tolerance. Fractional solvates are real: half a molecule of dichloromethane per formula unit is common in crystals grown from that solvent.

Three things a pass does not prove. It does not prove purity below about 1–2%, because the method is not sensitive enough. It does not distinguish isomers, which have identical composition. And it does not rule out a mixture that happens to average to the right numbers. It is a necessary check, not a sufficient one — which is why journals also want NMR and mass spectra.

Predicting Products, Oxygen Needed and the Balanced Equation

The forward calculation is ordinary stoichiometry. For a compound C_aH_bN_cO_dS_e burned completely:

Glucose, 1.500 g, exactly as the combustion analysis calculator reports it: n = 1.500 / 180.156 = 0.008326 mol; CO₂ = 6 × 0.008326 × 44.009 = 2.199 g; H₂O = 6 × 0.008326 × 18.015 = 0.900 g; O₂ consumed = 6 × 0.008326 × 31.998 = 1.599 g. Mass balance: 1.500 + 1.599 = 3.099 g in, 2.199 + 0.900 = 3.099 g out. TNT, C₇H₅N₃O₆, 0.300 g: 0.4069 g CO₂, 0.0595 g H₂O, 0.0555 g N₂, and only 0.222 g of external O₂ because the molecule carries so much of its own oxygen — the reason it is an explosive.

Mode 3 of the combustion analysis calculator prints the balanced equation with whole-number coefficients (doubling when b/4 or d/2 is fractional), the product masses, the oxygen consumed and the mass of air that carries that oxygen (air is 23.14% O₂ by mass). It accepts C, H, N, O and S; halogen compounds are refused here because their products (HCl, Cl₂) depend on conditions.

Fuel Combustion: Air–Fuel Ratio and Flue Gas

The same equation that gives the analytical chemist a formula gives the combustion engineer an air requirement. For methane, CH₄ + 2 O₂ → CO₂ + 2 H₂O: one mole of fuel (16.043 g) needs 2 mol of O₂ (63.996 g). Air is 20.95% O₂ by volume and 23.14% by mass, so the stoichiometric air is 63.996 / 0.2315 = 276.5 g per 16.043 g of methane — an air–fuel ratio of 17.23 : 1 by mass. Octane gives 15.13 : 1, ethanol 9.00 : 1 (it carries its own oxygen), pure carbon 11.51 : 1. Every kilogram of methane releases 2.743 kg of CO₂; every kilogram of octane 3.082 kg; every kilogram of coal carbon 3.664 kg.

Real burners — and the fuel mode of the combustion analysis calculator — run with excess air to make sure every fuel molecule meets enough oxygen. Excess air dilutes the flue gas and shows up as free O₂ on the analyser. For methane with 20% excess air the dry flue gas is 9.56% CO₂, 3.83% O₂ and the rest nitrogen; at stoichiometric it is 11.70% CO₂ and 0% O₂. Boiler technicians use this the other way round: a reading of 3.8–4.0% O₂ on a natural-gas boiler means about 20% excess air, a typical tuning target; 8% O₂ means far too much air and heat going up the stack.

Mode 5 of the combustion analysis calculator does both directions. Give it the fuel and the excess air and it returns the air–fuel ratio, CO₂ per kilogram, the dry-basis and wet-basis flue-gas percentages and, if you enter a fuel mass, the kilograms of air and products. Give it the measured O₂ instead and it solves for the excess air. It handles gaseous, liquid and solid fuels by formula — C₁₂H₂₃ for diesel, C₈H₁₈ for petrol, C₂H₆O for ethanol, plain C for coke — and the stoichiometry calculator covers any reaction beyond combustion.

Inside the Apparatus: From Liebig’s Train to the CHN Analyser

The classical combustion train that generations of students drew in their notebooks has four parts. An oxygen supply, dried and purified. A combustion tube packed with copper(II) oxide, heated to red heat, in which the weighed sample (in a small boat) is burned; the CuO supplies extra oxygen and guarantees complete oxidation of any CO to CO₂. A water absorber — a U-tube of anhydrous magnesium perchlorate or calcium chloride — weighed before and after. And a CO₂ absorber — soda lime or Ascarite (NaOH on asbestos) — also weighed before and after. The order matters: water first, because the CO₂ absorber also traps water. Two weighings give two masses, and the combustion analysis calculator does the rest.

A modern CHN analyser miniaturises and automates the same chemistry. A 1–3 mg sample in a tin capsule drops into a furnace at about 1000 °C; the tin flashes exothermically and raises the local temperature to 1800 °C in a pulse of pure oxygen. The gases pass over oxidation catalysts, then over hot copper to reduce nitrogen oxides to N₂ and remove excess oxygen. A gas chromatography column or a set of selective traps separates N₂, CO₂, H₂O and SO₂, and a thermal-conductivity detector measures each one. The instrument is calibrated with a standard of known composition (acetanilide, C₈H₉NO: C 71.09%, H 6.71%, N 10.36%) and reports percentages directly — which is why Mode 4 takes percentages while Modes 1 and 2 take masses.

Oxygen, when it is needed directly rather than by the difference a combustion analysis calculator computes, is done by pyrolysis in the absence of oxygen: the sample is cracked over carbon at 1060–1100 °C so that all its oxygen becomes CO, which is then measured. This closes the mass balance independently and is the check that catches an unexpected element.

Who Uses a Combustion Analysis Calculator?

• Students in first-year chemistry, where the combustion problem is the standard test of stoichiometry; in organic chemistry, where it introduces structure determination; and in analytical courses, where the apparatus itself is examined.
• Synthetic chemists checking that a new compound’s CHN report satisfies the ±0.4% rule before submission, or working out how much solvent a crystal retains.
• Pharmaceutical analysts confirming the hydration state of an active ingredient, since a monohydrate and an anhydrate differ in dose per milligram.
• Materials and polymer scientists determining the composition of copolymers, carbon fibres and doped carbons where NMR is unhelpful.
• Soil, fuel and environmental laboratories reporting total carbon, nitrogen and sulfur in soils, coals, biomass and sediments.
• Combustion and HVAC engineers sizing air supply, estimating CO₂ emissions per unit of fuel and interpreting flue-gas analyser readings.
• Teachers and tutors who need a data set that comes out to clean whole-number ratios — Mode 3 of the combustion analysis calculator generates one from any formula.

Step-by-Step Walkthrough in the Tool

Suppose the problem is: 0.2500 g of vanillin gives 0.5785 g CO₂ and 0.1184 g H₂O; M = 152.15 g/mol.

1. Open the combustion analysis calculator and stay on the first tab, Formula from CO₂ & H₂O.
2. Type 0.2500 in Mass of sample burned, 0.5785 in Mass of CO₂, 0.1184 in Mass of H₂O. Leave N₂ and SO₂ blank.
3. Keep the unit at grams and Oxygen in the compound at “Yes, by difference”. Enter 152.15 as the molar mass.
4. Press Calculate. The answer box reads: empirical formula C₈H₈O₃, molecular formula C₈H₈O₃.
5. Read the steps: C 0.1579 g, H 0.0133 g, O by difference 0.0789 g; moles 0.01315, 0.01315, 0.00493; ratio 2.667 : 2.667 : 1 → ×3 → 8 : 8 : 3; n = 152.15 / 152.15 = 1.
6. The table lists each element’s mass, moles, ratio and percent; the bar chart shows the percent composition (C 63.15%, H 5.30%, O 31.55%).
7. Use Copy to paste the working into a lab notebook, TXT or CSV to save it, or Print for a clean one-page report. The calculation is saved in the history strip; Reset clears the panel for the next problem.

The 2.667 ratio is a good example of why the multiplier search matters: a student who rounds 2.67 to 3 gets C₃H₃O, which has the wrong C : O ratio; the combustion analysis calculator recognises 2.667 as 8/3 and multiplies by 3.

A Short History: Lavoisier, Liebig and Pregl

Long before any combustion analysis calculator, Antoine Lavoisier burned alcohol and oils in the 1780s and weighed the water and “fixed air” produced, establishing that organic matter is made of carbon, hydrogen and oxygen — but his apparatus was unwieldy and his results rough. Joseph Louis Gay-Lussac and Louis Jacques Thénard improved the method with potassium chlorate as oxidant in 1810, and Jöns Jacob Berzelius made it quantitative enough to support his tables of atomic weights.

The decisive step was Justus von Liebig’s Kaliapparat of 1831: a five-bulb glass vessel filled with potassium hydroxide solution that absorbed CO₂ quantitatively and could simply be weighed. With copper(II) oxide as the oxidant and calcium chloride for water, Liebig’s train reduced an analysis from days to under an hour and made it reliable enough for students. The five-bulb shape is still the emblem of the American Chemical Society. Jean-Baptiste Dumas added the nitrogen determination in 1833, reducing nitrogen oxides over copper and collecting N₂ over potash.

Fritz Pregl, at Graz, scaled the whole procedure down between 1911 and 1917 so that 2–5 mg of sample sufficed, using a microbalance sensitive to a microgram. Micro-analysis made it possible to characterise the tiny quantities of natural products — hormones, vitamins, alkaloids — that biochemists were isolating, and Pregl received the 1923 Nobel Prize in Chemistry for it. The automated CHN analyser of the 1960s replaced the absorption tubes with a thermal-conductivity detector; the arithmetic that a combustion analysis calculator performs has not changed since Liebig.

Limitations and Sources of Error

• Incomplete combustion. Any CO that escapes oxidation carries carbon past the CO₂ absorber; soot left in the boat does the same. Result: carbon low, oxygen by difference high. Modern instruments use oxygen pulses and catalysts to prevent this.
• Hygroscopic samples and absorbers. Water picked up by the sample before weighing inflates the hydrogen and oxygen; water picked up by the CO₂ absorber inflates carbon. This is why the water trap comes first.
• Nitrogen oxides. Without the copper reduction stage, some nitrogen appears as NO₂, is absorbed with the water, and hydrogen reads high.
• Halogens and sulfur in a plain train form HX and SO₂/SO₃ that are trapped with the water. Silver wool or lead chromate packing removes them; better still, determine them separately as Mode 2 assumes.
• Balance precision. A 1 mg sample weighed to ±0.001 mg is 0.1%; a 0.5 g sample on a 0.1 mg balance is 0.02%. The empirical formula is robust to 1% errors for small molecules but not for large ones: C₂₀H₄₁ and C₂₀H₄₂ differ by 0.3% in hydrogen.
• Isomers. Combustion analysis cannot distinguish compounds with the same formula — ethanol and dimethyl ether both give C₂H₆O.
• Rounding. Keep four significant figures through the element masses and moles; rounding to two before dividing by the smallest turns 1.333 into 1.3 and loses the ×3 multiplier. The combustion analysis calculator carries full precision and rounds only for display.

Practice Problems (With Answers)

Work each problem by hand first, then verify with the free combustion analysis calculator above. Answers are hidden until you click.

Tips for Accurate Results

• Write the mass balance first. Sample mass on the left, element masses on the right. Whatever is missing is oxygen (or an error). This one line prevents most wrong answers.
• Use the conversion fractions, not memorised percentages. 12.011/44.009 and 2.016/18.015 with four figures; 0.27 and 0.11 are not accurate enough for a molecular formula.
• Keep the product masses in the same unit as the sample. Milligrams throughout or grams throughout; the combustion analysis calculator has a unit switch so you never convert by hand.
• Do not round the mole ratio too early. 1.33 means ×3, 1.25 means ×4, 1.5 means ×2, 1.67 means ×3. A ratio of 1.1 or 1.9 usually means an arithmetic error, not a multiplier.
• Check n before you trust the molecular formula. M / M(empirical) should be within 0.1 of an integer.
• For halogen problems, scale the second portion. Cl as a fraction of the Carius sample, then times the combustion sample mass.
• Sanity-check the percent composition. Compare with the reference table above; a “sugar” at 75% carbon is not a sugar.
• Save the steps. Copy or print the working from the combustion analysis calculator so you can show the method, not just the answer.

Common Mistakes

How This Calculator Is Different

This combustion analysis calculator is one of a family of free tools on molecularweightcalculator.org. Start with the molecular weight calculator for any formula’s molar mass, the atomic mass calculator for individual elements and the formula weight calculator for salts. Combustion data feed straight into the empirical formula calculator, the molecular formula calculator and the percent composition calculator. The molar mass you need for the molecular formula can come from the vapor density calculator, the molar mass from gas density calculator, the freezing point depression calculator, the boiling point elevation calculator or the osmotic pressure calculator. For the mass spectrometry route use the exact mass calculator, the monoisotopic mass calculator, the isotopic mass calculator and the guide to average vs monoisotopic mass. Related stoichiometry lives in the stoichiometry calculator, the grams to moles calculator, the molarity calculator, the molar mass of hydrates calculator, the equivalent weight calculator and the van der Waals equation calculator. Gas mixtures and fuels connect to the average molecular weight of a mixture, and the polymer set — number average, weight average, polydispersity index and molecular weight from viscosity — covers materials whose composition is often confirmed by CHN analysis.

Where the Numbers Come From

The atomic weights in this combustion analysis calculator are the IUPAC 2021 standard atomic weights as published by the Commission on Isotopic Abundances and Atomic Weights, using the conventional value for elements given as an interval: H 1.008, C 12.011, N 14.007, O 15.999, S 32.06, Cl 35.45, Br 79.904, I 126.904, Ag 107.868, Ba 137.327. The derived molar masses are CO₂ 44.009, H₂O 18.015, N₂ 28.014, SO₂ 64.064, O₂ 31.998, AgCl 143.32, AgBr 187.774, AgI 234.77 and BaSO₄ 233.386 g/mol. Air is taken as 20.95% O₂ by volume with a mean molar mass of 28.96 g/mol, giving 23.14% O₂ by mass and 3.773 mol N₂ (plus argon) per mol O₂. Every example on this page was recomputed with these values; where a textbook quotes 0.273 or 0.2727 for the carbon fraction of CO₂ the difference is rounding or an older atomic weight, not data.

If you spot a discrepancy worth checking, our corrections policy explains how reports are handled, and the contact page is the fastest route to us. More about the site and its author is on the about page.

Trusted external references for further combustion analysis calculator reading:

• LibreTexts — Determining Empirical and Molecular Formulas (libretexts.org): textbook treatment of combustion analysis with worked examples
• IUPAC CIAAW — Standard Atomic Weights (ciaaw.org): the atomic weights used in every calculation on this page
• Wikipedia — Combustion analysis (wikipedia.org): history, apparatus and modern instrumentation
• The Nobel Prize in Chemistry 1923 — Fritz Pregl (nobelprize.org): the invention of organic micro-analysis

Exam Checklist

Frequently Asked Questions

10 Facts Worth Knowing

1. Carbon is exactly 27.29% of the mass of CO₂ and hydrogen 11.19% of the mass of water — the two numbers behind every combustion analysis calculator.
2. Liebig’s five-bulb Kaliapparat of 1831, which absorbed the CO₂, is still the logo of the American Chemical Society.
3. Fritz Pregl won the 1923 Nobel Prize in Chemistry for shrinking the analysis to a few milligrams of sample.
4. A modern CHN analyser burns the sample in a tin capsule whose flash oxidation briefly reaches about 1800 °C.
5. Oxygen is the only common element in organic compounds that a combustion train cannot measure directly.
6. Formaldehyde, acetic acid, lactic acid and glucose all give identical combustion data per gram: they share the empirical formula CH₂O.
7. Acetanilide (C 71.09%, H 6.71%, N 10.36%) is the standard most CHN laboratories use to calibrate their instruments.
8. Burning one kilogram of methane produces 2.74 kg of CO₂; one kilogram of coal carbon produces 3.66 kg.
9. The ±0.4% agreement rule for CHN data has been in journal author guidelines for more than a century and is still debated.
10. Ethanol needs only 9.0 kg of air per kilogram, against 15.1 for petrol, because oxygen already makes up a third of its mass.

Final Thoughts

Combustion analysis — and the combustion analysis calculator that automates it — is two centuries old and still the first quantitative test a new organic compound faces. The experiment is elegant — burn it, weigh the smoke — and the calculation is a straight line from product masses to element masses to moles to a formula, with one subtraction for oxygen along the way. The places students stumble are always the same: the conversion fractions, the mass balance, the multiplier that clears 1.33, and the extra molar mass needed for a molecular formula. The worked examples, the reference table and the practice problems above are meant to make each of those steps automatic.

Whenever you need to check a data set, predict what a compound should give, read a CHN report or size the air for a burner, the free combustion analysis calculator at the top of this page will do the arithmetic and show its working. Bookmark it, share it with a study group, and if a problem behaves strangely, use the mass balance line to find out why.