What is combustion analysis?

It's a method to determine the elemental composition of organic compounds by burning them in excess oxygen and measuring the masses of CO₂ and H₂O produced. This keeps planning practical and lowers the chance of preventable errors.

How do you find oxygen in the sample?

Oxygen is found by difference: mass of O = sample mass - mass of C - mass of H - mass of any other elements (N, S, etc.).

What's the difference between empirical and molecular formulas?

The empirical formula is the simplest whole-number ratio of atoms (e.g., CH₂O). The molecular formula is the actual number of atoms (e.g., C₆H₁₂O₆ = glucose). You need the molar mass to determine the molecular formula.

Can combustion analysis detect nitrogen?

Yes. In a CHN analyzer, nitrogen is measured as N₂ gas using a thermal conductivity detector. This calculator supports optional nitrogen input.

Why must combustion be complete?

Incomplete combustion produces CO (carbon monoxide) and soot (elemental carbon) instead of just CO₂, leading to incorrect carbon measurements. This keeps planning practical and lowers the chance of preventable errors.

What are sources of error in combustion analysis?

Incomplete combustion, moisture in reagents, impurities in the sample, and inaccurate weighing are common sources of error. This keeps planning practical and lowers the chance of preventable errors.

Combustion Analysis Calculator

Determine empirical and molecular formulas from combustion analysis data. Enter masses of CO₂ and H₂O produced to find percent composition and molecular formula.

Common Compounds

Sample Mass

Mass of CO₂ Produced

Mass of H₂O Produced

Compound contains nitrogen

Known Molar Mass (optional)For determining molecular formula

g/mol

Empirical Formula

C3H8O

Empirical mass: 60.10 g/mol

Molecular Formula

C3H8O

Same as empirical (or no molar mass given)

% Carbon

60.04%

0.1531 g from 0.561 g CO₂

% Hydrogen

13.43%

0.0342 g from 0.306 g H₂O

% Oxygen

26.53%

Found by difference from sample mass

Percent Composition

C 60.0%

H 13.4%

O 26.5%

Detailed Analysis

Element	Mass (g)	Moles	Mole Ratio	Empirical #	Mass %
C	0.1531	0.01275	3.015	3	60.04%
H	0.0342	0.03397	8.034	8	13.43%
O	0.0677	0.00423	1.000	1	26.53%

Planning notes, formulas, and examples

About the Combustion Analysis Calculator

Combustion analysis is the classic method for determining the empirical formula of organic compounds. A known mass of a compound containing carbon, hydrogen, and possibly oxygen is burned completely in excess oxygen. The carbon is converted entirely to CO₂ and the hydrogen to H₂O. By measuring the masses of CO₂ and H₂O produced, chemists can calculate the masses of C and H in the original sample, and determine oxygen by difference.

This technique, developed by Justus von Liebig in the 1830s, revolutionized organic chemistry by allowing accurate determination of molecular formulas. Modern combustion analyzers (CHN analyzers) automate the process but the underlying chemistry is identical. The method remains a standard analytical technique in organic chemistry and is a staple of introductory chemistry courses.

This calculator takes the sample mass and the masses of CO₂ and H₂O produced, then calculates the moles and mass percentages of C, H, and O. It determines the empirical formula by finding the simplest whole-number mole ratio, and if you provide the molar mass, it will determine the molecular formula as well.

When This Page Helps

Combustion analysis calculations involve multiple steps of unit conversion and mole ratio determination. This calculator automates the entire process from raw combustion data to molecular formula, reducing errors and saving time.

How to Use the Inputs

Enter the mass of the organic compound sample in grams.
Enter the mass of CO₂ produced from the combustion.
Enter the mass of H₂O produced from the combustion.
Optionally check if the compound contains nitrogen and enter N₂ mass.
Optionally enter the experimentally determined molar mass.
Click a preset to explore common examples.
Review the empirical formula, molecular formula, and percent composition.

Formula used

Mass of C = mass CO₂ × (12.011 / 44.010)
Mass of H = mass H₂O × (2 × 1.008 / 18.015)
Mass of O = sample mass - mass C - mass H

Moles: n_C = mass C / 12.011, n_H = mass H / 1.008, n_O = mass O / 15.999

Divide each by the smallest to get the mole ratio → empirical formula

Molecular formula = empirical formula × (molar mass / empirical mass)

Example Calculation

Result: C₂H₆O (empirical: CH₃O)

From 0.561 g CO₂: mass C = 0.561 × 12.011/44.010 = 0.1531 g. From 0.306 g H₂O: mass H = 0.306 × 2.016/18.015 = 0.0343 g. Mass O = 0.255 - 0.1531 - 0.0343 = 0.0676 g. Moles: C = 0.01275, H = 0.03400, O = 0.004225. Ratio: 3.02 : 8.05 : 1 → CH₃O. With molar mass 46, molecular formula = C₂H₆O (ethanol).

Tips & Best Practices

Always check that mass C + mass H + mass O ≤ sample mass. If the sum exceeds the sample mass, check your input data.
If the mole ratios are close to x.5 (like 1.5), multiply all ratios by 2 to get whole numbers.
For compounds without oxygen, the mass of O will be zero — the calculator handles this correctly.
Modern CHN analyzers typically achieve ±0.3% accuracy for C, H, and N.
If your empirical formula seems unreasonable, double-check that masses are in the same units (grams).
The method works for any CₓHₓOₓ compound, including carbohydrates, alcohols, acids, and hydrocarbons.

History of Combustion Analysis

Justus von Liebig developed the combustion analysis apparatus in the 1830s, using a series of absorption tubes filled with drying agents and alkali to capture H₂O and CO₂ respectively. This invention was crucial for establishing the field of organic chemistry, allowing chemists to determine molecular formulas for the first time.

Modern CHN Analyzers

Today, automated CHN analyzers (like the PerkinElmer 2400 or Elementar vario MICRO) flash-combust samples at 950-1050°C in pure oxygen, then separate the combustion gases chromatographically. Results are available in minutes with parts-per-million accuracy, processing dozens of samples per hour.

Common Combustion Analysis Problems

Students often encounter problems where they must work backward from combustion data to the molecular formula. The key steps are: (1) convert CO₂ → mass C, and H₂O → mass H, (2) find mass O by difference, (3) convert all masses to moles, (4) divide by the smallest to get the empirical formula, and (5) use the molar mass to find the molecular formula.

Sources & Methodology

Last updated: June 1, 2026

Frequently Asked Questions

It's a method to determine the elemental composition of organic compounds by burning them in excess oxygen and measuring the masses of CO₂ and H₂O produced. This keeps planning practical and lowers the chance of preventable errors.