For a sample known to contain only C, H and O, recovered carbon dioxide gives its carbon and recovered water gives its hydrogen. Oxygen is the mass left over in the original sample.

Formula: C g = CO₂ g × 12.011/44.009; H g = H₂O g × 2.016/18.015; O g = sample g − C g − H g; each mass % = element g ÷ sample g × 100

Worked example

A 0.60052 g sample yielding 0.88018 g CO₂ and 0.3603 g H₂O contains 0.24022 g C, 0.04032 g H and 0.31998 g O by difference. The mass percentages are approximately 40.002%, 6.714% and 53.284%.

The sample composition assumption matters

Use this calculation only when the original sample contains carbon, hydrogen and oxygen alone. If it contains nitrogen, sulfur, ash or other material, the remainder is not necessarily oxygen. Added combustion oxygen is not original sample oxygen.

Recovered product masses must be net and complete

The calculation assumes complete combustion and quantitative collection of CO₂ and H₂O, with blanks and contamination already corrected. It rejects data that imply more carbon and hydrogen mass than the original sample. Small measurement errors can disproportionately affect oxygen found by subtraction.

Continue from composition to a formula

The results are elemental masses and percentages, not a molecular formula. Use the empirical-formula tool with the percentages to obtain a simplest ratio. A molecular formula additionally needs independent molar-mass information. The calculation uses rounded atomic masses C = 12.011, H = 1.008 and O = 15.999 g/mol.

Explore the related calculation

For a different starting point, try the empirical formula calculator or return to the main calculator. Browse the complete tool directory for this site.

Reference and corrections

The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.

openstax.org reference

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