Combustion Analysis (CHNS) Calculator

Free Online Combustion Analysis (CHNS) Calculator

This combustion analysis calculator takes the raw numbers you get out of a combustion experiment — the mass of CO2 and H2O trapped after burning a sample, or the straight percentage printout from a CHNS elemental analyzer — and turns them into a finished empirical formula in seconds. Type in your numbers, and the tool works out %C, %H, %N, %S, and %O by difference, converts every percent to moles, finds the simplest whole-number ratio, and builds the empirical formula for you, with every step written out along the way.

Combustion analysis is one of the oldest and still one of the most reliable ways chemists figure out what an unknown organic compound is made of. This calculator handles both the classic lab version of the technique (weighing CO2 and H2O by hand) and the modern instrument version (reading %C, %H, %N, and %S straight off a CHNS analyzer), so it works whether you're doing a first-year chemistry lab or checking real analytical data.

What Is Combustion Analysis?

Combustion analysis is a lab technique used to find out how much carbon, hydrogen, nitrogen, and sulfur are in an organic compound. A small, carefully weighed sample is burned completely in a stream of pure oxygen, turning every carbon atom into carbon dioxide gas and every hydrogen atom into water vapor. If the sample also contains nitrogen or sulfur, those show up as nitrogen gas (or nitrogen oxides that get reduced back to N2) and sulfur dioxide.

The trick that makes the whole method work is simple: since we know exactly how much carbon is in one mole of CO2, and exactly how much hydrogen is in one mole of H2O, weighing the CO2 and H2O produced tells us exactly how much carbon and hydrogen were in the original sample — even though we can't weigh the carbon or hydrogen directly.

What Does CHNS Actually Stand For?

CHNS stands for the four elements a modern combustion elemental analyzer measures in one run: Carbon, Hydrogen, Nitrogen, and Sulfur. A CHNS analyzer burns a tiny sample (often just a few milligrams) at very high temperature, separates the resulting gases, and measures each one on its own detector — giving a lab report with %C, %H, %N, and %S already calculated for you.

Oxygen almost never gets its own detector in a standard CHNS run, mostly because oxygen doesn't leave behind a gas that's easy to isolate and measure the way CO2, H2O, N2, and SO2 do. Instead, %O is worked out afterward, the same way it's worked out in this calculator: whatever percent is left over once carbon, hydrogen, nitrogen, sulfur, and any known ash are all subtracted from 100%. That's what chemists mean by "oxygen by difference," and it's built into every calculation this tool runs.

How to Calculate %C and %H from CO2 and H2O Masses

This is the classic textbook version of a combustion analysis problem, and it always follows the same three moves.

• Step 1 — Find the mass of carbon from the CO2. Carbon makes up 12.011 out of every 44.009 grams of CO2, so multiplying the CO2 mass by 12.011 ÷ 44.009 gives you the mass of carbon that was in the original sample.
• Step 2 — Find the mass of hydrogen from the H2O. Hydrogen makes up 2.016 out of every 18.015 grams of H2O, so multiplying the H2O mass by 2.016 ÷ 18.015 gives you the mass of hydrogen that was in the sample.
• Step 3 — Turn those masses into percentages. Divide the mass of carbon (and hydrogen) by the original sample mass and multiply by 100 to get %C and %H.
• If nitrogen or sulfur are also present, their percentages are measured separately (either from a Dumas-style nitrogen determination or from a dedicated analyzer channel) rather than calculated from the CO2 and H2O masses — this calculator lets you add those in as extra fields whenever they apply.

Finding Oxygen by Difference (and Why It's Needed)

Oxygen is almost always the one element in an organic compound that combustion analysis can't measure directly, simply because there's no single, easy-to-trap gas that tells you exactly how much oxygen was in the original sample the way CO2 and H2O do for carbon and hydrogen.

Instead, chemists rely on a simple fact: every percentage in a sample has to add up to 100%. So once %C, %H, %N, and %S are known (plus any inert ash or filler that's already been identified), whatever is left over must be oxygen. The formula is short: %O = 100% − %C − %H − %N − %S − %ash. This calculator applies that formula automatically the moment you enter your data.

From Percent Composition to Empirical Formula

Once you know the percent of every element in the compound, finding the empirical formula follows the same routine used for any percent-composition problem in general chemistry.

• Assume a 100 g sample, so each percent turns directly into a mass in grams.
• Convert each element's mass to moles by dividing by its atomic mass.
• Divide every mole value by the smallest one, giving a mole ratio with the smallest element sitting at exactly 1.
• If any ratio isn't a whole number, multiply every ratio by the same small whole number (2, 3, 4, and so on) until they all land close enough to whole numbers to round.
• Write the final whole numbers as the subscripts in the empirical formula, in the standard order: carbon first, hydrogen second, then the rest alphabetically.

Getting the Molecular Formula, Not Just the Empirical One

The empirical formula only shows the simplest ratio of atoms — it doesn't necessarily match the true number of atoms in one real molecule. Naphthalene, for example, has the molecular formula C10H8, but its empirical formula is the smaller C5H4.

If you already know the compound's actual molar mass (from a mass spectrometer, a freezing-point depression experiment, or just because you're checking a known compound), this calculator uses it to go one step further: it divides the molar mass by the empirical formula mass to find the whole-number multiplier n, then multiplies every subscript by n to give the real molecular formula. Just turn on the molar mass option and type in the value to see this happen automatically.

Advanced Feature: Degree of Unsaturation (DBE) Check

As a bonus, whenever the calculator finds a formula that contains carbon, it also works out the degree of unsaturation — sometimes called the Double Bond Equivalent, or DBE — using the standard formula DBE = C − H⁄2 + N⁄2 + 1.

This number tells you how many rings and/or pi bonds (double or triple bonds) the molecule has to contain in total. It's a fast, genuinely useful sanity check: if your combustion data points to a formula with a DBE that doesn't make chemical sense for the compound you expect (a negative number, for instance, which is impossible), that's usually a sign a measurement or an input somewhere needs a second look.

Two Ways to Use This Calculator

This tool covers both common versions of a combustion analysis problem, so switch between them depending on what data you're starting from.

• Combustion Masses mode — enter the mass of the original sample, plus the mass of CO2 and H2O produced when it was burned. This is the setup used in most general chemistry textbooks and introductory lab courses, and it's the classic way the technique has been taught for decades.
• CHNS Analyzer % Output mode — enter %C, %H, and (if present) %N and %S exactly as they appear on a modern elemental analyzer printout. This is the setup used in real analytical, pharmaceutical, and materials-science labs today, where the instrument reports percentages directly instead of raw gas masses.
• Both modes support an optional ash / non-combustible residue percentage, an optional nitrogen and sulfur reading, and an optional molar mass field for finding the molecular formula — so the calculator adapts to a simple two-element problem or a full four-element CHNS workup without switching tools.

Worked Example: Naphthalene

A 50.0 mg sample of a hydrocarbon is burned completely, producing 171.7 mg of CO2 and 28.1 mg of H2O.

Mass of carbon = 171.7 × (12.011 ÷ 44.009) ≈ 46.86 mg. Mass of hydrogen = 28.1 × (2.016 ÷ 18.015) ≈ 3.14 mg. That gives %C ≈ 93.7% and %H ≈ 6.3%, adding up to almost exactly 100%, which tells us the compound contains only carbon and hydrogen — no oxygen, nitrogen, or sulfur.

Converting to moles per 100 g: 93.7 ÷ 12.011 ≈ 7.80 mol C and 6.3 ÷ 1.008 ≈ 6.25 mol H. Dividing both by the smaller value (6.25) gives a ratio close to C 1.25 : H 1, which scales up ×4 to C5H4 — the empirical formula. Since naphthalene's real molar mass is about 128.17 g/mol and the empirical formula mass of C5H4 is about 64.08 g/mol, 128.17 ÷ 64.08 ≈ 2, giving the correct molecular formula, C10H8.

Worked Example: A Compound Containing Sulfur

A CHNS analyzer reports a sample as 57.14% carbon, 4.80% hydrogen, and 38.06% sulfur, with no detectable nitrogen.

These three percentages already add up to 100.0%, so there's no oxygen in this compound. Moles per 100 g: 57.14 ÷ 12.011 ≈ 4.76 mol C, 4.80 ÷ 1.008 ≈ 4.76 mol H, and 38.06 ÷ 32.06 ≈ 1.19 mol S. Dividing every value by the smallest (1.19) gives a ratio of C 4 : H 4 : S 1 — the empirical formula C4H4S, which matches thiophene, a common sulfur-containing ring compound.

Common Mistakes in Combustion Analysis Problems

The single most common mistake is forgetting that the CO2 and H2O masses aren't the masses of carbon and hydrogen themselves — they're the masses of the whole molecules that carbon and hydrogen ended up trapped inside. You always need to multiply by the mass fraction (12.011/44.009 for carbon, 2.016/18.015 for hydrogen) before those numbers mean anything as %C or %H.

Another frequent error is assuming oxygen is present just because a compound is organic — plenty of organic compounds, including simple hydrocarbons like naphthalene above, contain no oxygen at all. Always check whether %C + %H (+ %N + %S, if measured) already adds up close to 100% before assuming there's oxygen left over to find by difference. A third mistake is rounding a mole ratio too early — a ratio like 1.25 should be recognized and scaled by 4, not rounded straight down to 1, which would give the wrong formula entirely.

Where Combustion (CHNS) Analysis Is Used in Real Labs

This isn't just a classroom exercise — combustion analysis and CHNS elemental analysis are everyday tools across chemistry and materials science.

• Pharmaceutical quality control — confirming that a newly made drug batch matches its expected elemental composition before it goes any further in development.
• Organic synthesis — checking that a compound made in the lab is actually the compound the chemist intended to make, and that it's reasonably pure.
• Environmental and soil science — measuring the carbon, nitrogen, and sulfur content of soil, sediment, or plant material.
• Petroleum and fuel chemistry — measuring sulfur content in fuels, since sulfur levels are tightly regulated for environmental reasons.
• Academic teaching labs — combustion analysis is one of the standard experiments in general and organic chemistry courses, usually feeding directly into an empirical formula calculation just like the one this tool performs.

How to Use This Calculator

Pick the mode that matches your data: use "Combustion Masses" if you have a sample mass plus CO2 and H2O masses from an experiment, or "CHNS Analyzer % Output" if you already have percentages from an instrument printout. Fill in the fields, and turn on the nitrogen or sulfur toggles only if your compound actually contains them — leave them off for a simple hydrocarbon or a compound made only of C, H, and O.

The result panel updates instantly, showing the empirical formula, its formula mass, and (if you've entered a molar mass) the molecular formula too, along with a percent-composition breakdown, a mole-ratio chart, and a downloadable table. Scroll down to the step-by-step solution to see the complete working laid out the way a textbook or lab report would show it — useful for checking your own by-hand work or for building an answer you can show your steps for.

Frequently Asked Questions

How do you calculate percent composition from combustion analysis? Multiply the mass of CO2 collected by 12.011/44.009 to get the mass of carbon, and the mass of H2O by 2.016/18.015 to get the mass of hydrogen, then divide each by the original sample mass and multiply by 100.

Why is oxygen found by difference in combustion analysis? Because oxygen doesn't produce a separate trapped gas the way carbon and hydrogen do, so it can't be measured directly — instead, it's whatever percent is left over once every other measured element (and any ash) is subtracted from 100%.

What's the difference between combustion analysis and a CHNS analyzer? Classic combustion analysis is done by hand, weighing the CO2 and H2O absorbers before and after burning a sample. A CHNS analyzer automates the whole process in one instrument run and reports %C, %H, %N, and %S directly, without you needing to weigh any trapped gas yourself.

Does every compound in a combustion analysis contain oxygen? No — plenty of compounds, especially simple hydrocarbons, contain no oxygen at all. Check whether the measured percentages already add up close to 100% before assuming oxygen makes up the rest.

Can this calculator handle nitrogen and sulfur too? Yes — turn on the nitrogen and/or sulfur toggles and enter their measured percentages, and the calculator folds them into the empirical formula and the oxygen-by-difference calculation automatically.