About

Combustion analysis remains the primary quantitative method for determining the elemental composition of organic compounds. A sample of known mass is burned in excess O₂. The carbon converts entirely to CO₂, hydrogen to H₂O, nitrogen to N₂, and sulfur to SO₂. From the measured product masses, you back-calculate the moles of each element in the original sample. Errors in product mass measurement propagate directly into the empirical formula. A 0.5% error in CO₂ mass can shift a C₆ compound to C₇. This tool applies the standard gravimetric stoichiometry used in CHN/CHNS analyzers, handles oxygen-by-difference, and resolves non-integer mole ratios up to a multiplier of 10.

Oxygen content is calculated by difference: m_O = m_sample − (m_C + m_H + m_N + m_S). This assumes no other elements are present. If halogens or phosphorus exist, you must account for them separately or the oxygen value will be incorrect. The tool approximates results assuming complete combustion and quantitative product recovery. Real-world instrument drift, incomplete combustion, and moisture contamination are not modeled.

Formulas

The mass of each element in the original sample is extracted from the measured combustion product masses using stoichiometric ratios.

m_C = m_CO₂ × M_CM_CO₂ = m_CO₂ × 12.01144.010

m_H = m_H₂O × 2 × M_HM_H₂O = m_H₂O × 2.01618.015

m_N = m_N₂ × 2 × M_NM_N₂ = m_N₂

m_S = m_SO₂ × M_SM_SO₂ = m_SO₂ × 32.06064.066

m_O = m_sample − (m_C + m_H + m_N + m_S)

Moles of each element are computed by dividing element mass by its molar mass.

n_X = m_XM_X

The empirical formula is determined by dividing each mole value by the smallest mole count, then rounding to the nearest integer or multiplying to resolve fractional ratios.

ratio_X = n_Xmin(n_all)

If a molecular weight M_actual is provided, the molecular formula multiplier is calculated.

k = M_actualM_empirical

Where m = mass in grams, M = molar mass in g/mol, n = moles, k = integer multiplier for molecular formula.

Reference Data

Element	Symbol	Molar Mass (g/mol)	Combustion Product	Product Molar Mass (g/mol)	Atoms per Product Molecule
Carbon	C	12.011	CO₂	44.010	1
Hydrogen	H	1.008	H₂O	18.015	2
Nitrogen	N	14.007	N₂	28.014	2
Sulfur	S	32.060	SO₂	64.066	1
Oxygen	O	15.999	By difference	-	-
Chlorine	Cl	35.450	HCl / AgCl	36.461 / 143.32	1
Bromine	Br	79.904	HBr / AgBr	80.912 / 187.77	1
Fluorine	F	18.998	CaF₂	78.075	2
Phosphorus	P	30.974	P₂O₅	141.943	2
Common Compound: Glucose	C₆H₁₂O₆	180.156	6CO₂ + 6H₂O	-	-
Common Compound: Ethanol	C₂H₆O	46.069	2CO₂ + 3H₂O	-	-
Common Compound: Benzoic Acid	C₇H₆O₂	122.123	7CO₂ + 3H₂O	-	-
Common Compound: Urea	CH₄N₂O	60.056	CO₂ + 2H₂O + N₂	-	-
Common Compound: Aspirin	C₉H₈O₄	180.158	9CO₂ + 4H₂O	-	-
Common Compound: Caffeine	C₈H₁₀N₄O₂	194.191	8CO₂ + 5H₂O + 2N₂	-	-
Common Compound: Naphthalene	C₁₀H₈	128.174	10CO₂ + 4H₂O	-	-
Common Compound: Acetic Acid	C₂H₄O₂	60.052	2CO₂ + 2H₂O	-	-
Common Compound: Thiophene	C₄H₄S	84.140	4CO₂ + 2H₂O + SO₂	-	-
Common Compound: Aniline	C₆H₇N	93.129	6CO₂ + 3.5H₂O + 0.5N₂	-	-

Frequently Asked Questions

A negative oxygen mass means the sum of carbon, hydrogen, nitrogen, and sulfur masses exceeds the original sample mass. This usually indicates measurement error in the product masses, incomplete drying of the CO₂ or H₂O absorbers, or the presence of elements (halogens, phosphorus) not accounted for. Verify your product masses are in grams consistent with the sample mass. If halogen is present, its mass must be subtracted before computing oxygen by difference.

The algorithm divides all mole values by the smallest one, then checks if the resulting ratios are within 0.1 of a whole number. If not, it multiplies all ratios by successive integers from 2 through 10 until all values round cleanly to integers. For example, a ratio of 1.5 is multiplied by 2 to yield 3. A ratio of 2.333 is multiplied by 3 to yield 7. If no clean integer set is found within ×10, the closest rounding is used and a warning is displayed.

The empirical formula gives the simplest whole-number ratio of atoms. For example, glucose and acetic acid both have an empirical formula of CH₂O. The molecular formula reflects the actual number of atoms: C₆H₁₂O₆ for glucose (molecular weight 180.16 g/mol) vs C₂H₄O₂ for acetic acid (60.05 g/mol). You need an independent molecular weight measurement (mass spectrometry, freezing point depression) to distinguish them.

The gravimetric method measures product masses, not volumes, so temperature and pressure do not directly affect the calculation. However, if you are measuring gas volumes instead of masses (e.g., nitrogen gas volume), you must convert using the ideal gas law: n = PV / RT at your laboratory conditions. Standard conditions are 0 °C and 1 atm (STP: 22.414 L/mol) or 25 °C and 1 bar (SATP: 24.790 L/mol). This calculator accepts mass inputs only.

This tool is designed for organic CHNS analysis with oxygen by difference. It does not account for metals, silicon, boron, or other elements. For organometallic compounds, the metal residue (ash) must be weighed separately, and its mass subtracted from the sample before computing oxygen. The tool does not model ash analysis. For purely inorganic samples, use gravimetric or instrumental methods specific to the elements involved.

Classical combustion analysis (Pregl method) achieves accuracy of ±0.3% absolute for C and H when performed correctly. Modern automated CHN analyzers (e.g., Elementar vario MICRO) report accuracy of ±0.1% absolute. The main sources of error are incomplete combustion, hygroscopic samples absorbing moisture, and catalyst degradation. This calculator assumes ideal complete combustion. Real instrument calibration typically uses acetanilide (C₈H₉NO, 71.09% C, 6.71% H, 10.36% N) or benzoic acid as standards.