Combustion Analysis Calculator - Empirical and Molecular Formula

The combustion analysis calculator is a chemistry tool that turns CO2 and H2O masses from a burned compound into its empirical formula, and uses an optional molar mass to extend that empirical formula into the molecular formula of the original sample. Use this combustion analysis calculator for general chemistry homework, organic chemistry unknowns, and CHN combustion analysis problems where you need to convert grams of combustion products into element-by-element formulas.

• • General chemistry homework: Solve lab-report problems where you are given CO2 and H2O masses from a combustion train and asked for the empirical formula.
• • Organic chemistry unknowns: Identify an unknown organic compound from combustion data and a separately measured molar mass.
• • Compounds containing oxygen: Compute oxygen by difference when the sample also contains oxygen, the standard CHN workflow.
• • Cross-checking elemental analysis results: Verify percent-composition or CHNS analyzer outputs by re-running the mass math.

The underlying assumption is the same one a CHN analyzer makes: when a compound is burned in a stream of excess oxygen, every carbon atom ends up in CO2 and every hydrogen atom ends up in H2O, so the masses of those two products tell you exactly how much carbon and hydrogen the sample contained.

Oxygen has no combustion product of its own, so it is reported by difference: the sample mass minus the carbon mass minus the hydrogen mass. That approach works when the compound contains only C, H, O, and any other elements that are measured separately.

Once you have the empirical formula in hand, a stoichiometry reaction calculator turns the same mole ratios into theoretical product yields for the balanced reaction.

The combustion analysis calculator applies the standard CHN combustion analysis steps in order. Each step uses IUPAC atomic weights or the molar masses of CO2 and H2O, so the math is traceable to a single source of truth.

• sampleMass: Mass of the unknown compound you burned, in grams.
• massCO2: Mass of carbon dioxide captured from the combustion train, in grams.
• massH2O: Mass of water captured from the combustion train, in grams.
• molarMass: Optional molar mass of the unknown, in grams per mole. Leave at 0 to skip the molecular formula output.
• atomic weights: IUPAC 2021 atomic weights: C 12.011, H 1.008, O 15.999.

After the moles of C, H, and O are known, the smallest non-zero mole count divides every other element to produce the raw empirical subscripts. Those raw ratios are rounded to the nearest whole number to satisfy the convention that empirical subscripts are integers.

If you supply a molar mass, the calculator divides it by the empirical formula mass, rounds to the nearest whole number n, and multiplies every empirical subscript by n. For glucose that factor is 6, so CH2O becomes C6H12O6.

According to Omni Calculator Combustion Analysis, burning 1.00 g of glucose produces 1.466 g of CO2 and 0.587 g of H2O, which gives empirical formula CH2O and molecular formula C6H12O6.

The combustion step itself needs a balanced reaction such as CxHyOz + O2 -> x CO2 + y/2 H2O, which a chemical equation balancer calculator handles before the mass math.

Four ideas show up in every combustion analysis problem. Once you understand them, the calculator's outputs map onto the textbook steps you would show on paper.

These four ideas are the same steps you would write out by hand on a worksheet, which is why the calculator's intermediate outputs are useful for showing your work.

If you remember only one of them, remember that oxygen by difference only works when the sample truly contains only C, H, and O; otherwise you would need a CHNS analyzer that reports nitrogen and sulfur separately.

Combustion analysis is the lab-scale version of oxygen-by-difference, and a chemical oxygen demand calculator covers the same idea at wastewater scale where oxygen consumed by organic matter is reported in mg/L.

Plug in the masses you measured during the combustion and let the calculator do the moles, division, and rounding. The whole flow takes a minute once you have the data.

1. 1 Weigh the sample: Record the mass of the unknown compound to four significant figures on an analytical balance.
2. 2 Capture the CO2 mass: Use the absorption tube readings before and after combustion to record the mass of CO2, in grams.
3. 3 Capture the H2O mass: Use the water absorption tube readings before and after combustion to record the mass of H2O, in grams.
4. 4 Enter the three masses: Type the sample mass, CO2 mass, and H2O mass into the first row of inputs.
5. 5 Add the molar mass if you have it: Enter the molar mass in grams per mole. Leave it at 0 to skip the molecular formula step.
6. 6 Read the formula outputs: Use the empirical formula for the simplest ratio and the molecular formula for the actual atomic composition.

For a homework problem where burning 1.00 g of an unknown produces 1.466 g of CO2 and 0.587 g of H2O, enter 1.00, 1.466, and 0.587. With no molar mass supplied, the calculator reports CH2O as the empirical formula and prompts you to add the molar mass if you know it.

When the lab steps branch into electrochemistry, the same balance of inputs you used here pairs with a Nernst equation calculator for cell potentials on the next worksheet.

The combustion analysis calculator handles the tedious part of the lab report, so you can focus on the chemistry decision that comes after you have the formula in hand.

• • Faster empirical-formula homework: Skip the long division of moles by the smallest mole count and get the empirical subscripts directly.
• • Built-in oxygen by difference: See the oxygen mass without doing the sample-mass subtraction yourself, removing the most common arithmetic slips.
• • Optional molecular formula step: Hand the calculator the molar mass and let it scale the empirical subscripts to the matching integer multiple.
• • Traceable constants: Atomic weights and molar masses are from IUPAC 2021, so the numbers match the constants your textbook uses.
• • Useful intermediate values: Carbon, hydrogen, and oxygen masses are shown alongside the formulas, convenient for percent-composition follow-ups.
• • Mobile-friendly for lab and study: Real-time recalculation lets you try different sample masses on a phone in the lab.

Use the empirical formula output to answer the first half of the worksheet, then move to the molecular formula only when the problem gives a molar mass.

If the oxygen-by-difference value is close to zero or negative, that is a signal that the sample may contain an element you did not measure.

For the atomic-structure side of the same chapter, a Bohr model calculator handles electron shells and energy levels so you can confirm the carbon and oxygen atoms in your formula.

The arithmetic is exact, but the assumptions baked into combustion analysis can change what the calculator tells you about the sample.

• • The combustion analysis calculator assumes the compound contains only C, H, and O; if nitrogen or sulfur is present the molecular formula will be wrong because those masses are silently rolled into the oxygen-by-difference value.
• • Rounding empirical subscripts to the nearest integer can misclassify compounds whose true ratio is close to a half-integer (1.49 vs 1.51); cross-check with the molecular-formula step whenever subscripts look unusual.

Treat the calculator's outputs as a starting point and confirm them against the percent-composition table you would normally add to a lab report.

If the oxygen mass comes out negative, the sample mass is too small or the compound contains something other than C, H, O; double-check the input values before trusting the formula.

According to IUPAC Standard Atomic Weights, carbon is 12.011 g/mol, hydrogen is 1.008 g/mol, and oxygen is 15.999 g/mol, which are the atomic masses used to convert measured CO2 and H2O masses into empirical-formula subscripts.

If your unknown is a peptide instead of a small organic, the protein molecular weight calculator handles the amino-acid-by-amino-acid mass math that complements combustion analysis.