0.7205
g
0.1209
g
0.1586
g
72.05
%
12.09
%
0
%
15.86
%
0.05999
mol
0.1199
mol
0
mol
0.00991
mol
100
%
0.7205
g
0.1209
g
0.1586
g
72.05
%
12.09
%
0
%
15.86
%
0.05999
mol
0.1199
mol
0
mol
0.00991
mol
100
%
Combustion analysis is a classical analytical technique for determining the elemental composition of organic compounds. When an organic sample is completely burned in excess oxygen, the carbon converts entirely to CO₂ and the hydrogen converts to H₂O. By measuring the masses of these combustion products, chemists can back-calculate the mass of carbon and hydrogen in the original sample. This Combustion Analysis Calculator automates the conversion from product masses to elemental percentages and the empirical formula. The technique, pioneered by Justus von Liebig in the 1830s, remains a cornerstone of organic analytical chemistry. Modern combustion analyzers (CHN/CHNS analyzers) use this same principle with automated gas chromatographic detection, providing rapid and accurate elemental composition data essential for characterizing new compounds, verifying synthetic products, and quality control in pharmaceutical manufacturing.
In combustion analysis, the sample is burned completely in excess oxygen:
$$C_xH_yO_zN_w + O_2 \rightarrow xCO_2 + \frac{y}{2}H_2O + \frac{w}{2}N_2$$
The mass of carbon in the sample equals the mass of carbon in the CO₂ produced:
$$m_C = m_{CO_2} \times \frac{M_C}{M_{CO_2}} = m_{CO_2} \times \frac{12.011}{44.009}$$
Similarly, the mass of hydrogen comes from the H₂O:
$$m_H = m_{H_2O} \times \frac{2 \times M_H}{M_{H_2O}} = m_{H_2O} \times \frac{2.016}{18.015}$$
Nitrogen is determined separately (Dumas method). Oxygen is found by difference:
$$m_O = m_{sample} - m_C - m_H - m_N$$
Elemental percentages are: $$\%X = \frac{m_X}{m_{sample}} \times 100$$
The empirical formula is found by converting masses to moles, then dividing by the smallest mole value to get the simplest whole-number ratio.
The elemental percentages should sum to approximately 100%. A total significantly below 100% suggests the compound contains additional elements not accounted for (such as sulfur, phosphorus, or halogens). The empirical formula shows the simplest whole-number ratio of atoms. If the ratios are not close to integers (e.g., 1.5), multiply all ratios by the smallest integer that converts them to whole numbers (e.g., multiply by 2). Note that the empirical formula may not equal the molecular formula — to find the molecular formula, you need the molar mass from a separate measurement (e.g., mass spectrometry). The molecular formula is always a whole-number multiple of the empirical formula: $$\text{Molecular} = n \times \text{Empirical}$$.
Inputs
Results
Mole ratio C:H:O = 0.0434 : 0.1302 : 0.0217 ≈ 2:6:1, giving empirical formula C2H6O — consistent with ethanol.
Inputs
Results
Mole ratio C:H:N:O ≈ 6:5:1:2, giving empirical formula C6H5NO2 — the molecular formula of nicotinic acid (MW 123.11).
Combustion analysis determines the elemental composition (C, H, N, and by difference O) of organic compounds. It is used to verify the identity and purity of synthesized compounds, characterize new materials, and is a requirement for publishing new compound data in most chemistry journals.
Oxygen is typically determined by difference — subtract the masses of C, H, N, and any other detected elements from the total sample mass. Some modern analyzers can directly measure oxygen by pyrolysis in an inert atmosphere, converting organic oxygen to CO which is then quantified.
The Dumas method determines nitrogen by combusting the sample and converting all nitrogen to N2 gas, which is measured by gas chromatography or gas-volumetric methods. Modern CHN analyzers use this principle with thermal conductivity detection.
Divide the experimentally determined molar mass by the empirical formula weight. The result (rounded to the nearest integer n) is the multiplier: Molecular formula = n × Empirical formula. For example, if the empirical formula CH2O has weight 30 and the molar mass is 180, then n = 6, giving C6H12O6 (glucose).
If the total is below ~98%, the sample likely contains elements not measured (S, P, halogens, metals). If above 102%, there may be a measurement error. Small deviations (98-102%) are normal and reflect experimental uncertainty in the combustion analysis.
Modern microanalytical combustion requires only 1-5 mg of sample. Classical methods used larger amounts (50-100 mg). The sample should be pure and thoroughly dried before analysis, as moisture will artificially inflate the hydrogen and oxygen percentages.
Standard CHN combustion analysis does not detect halogens. Separate methods like the Schoeniger flask combustion or silver halide precipitation are used for halogen determination. Some advanced CHNS-O analyzers have add-on modules for halogen detection.
Common errors include incomplete combustion (giving low C%), moisture in the sample (giving high H%), insufficient sample homogeneity, and contamination. Hygroscopic samples should be dried in a vacuum desiccator immediately before analysis.
Experimental uncertainty in mass measurements produces ratios that are close to, but not exactly, integers. Ratios within ±0.1 of an integer can be rounded. Ratios near 0.5, 0.33, or 0.25 suggest multiplying by 2, 3, or 4 respectively to obtain whole numbers.
Modern automated CHN analyzers achieve absolute accuracy of ±0.3% for each element. This means a reported value of 52.14% C could range from 51.84% to 52.44%. Duplicate analyses are recommended to verify reproducibility.
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