Inferring empirical and molecular formulas from composition
Element mass ratios become atom ratios through molar masses, with uncertainty and sample identity retained.
Key knowledge
For a pure compound of fixed composition, convert each constituent element's measured mass to amount using its appropriate molar mass. Divide those amounts by a common reference amount and identify a smallest whole-number atom ratio consistent with the measurements. This gives an empirical formula, not atom connectivity. For a molecular compound with independently known molar mass, the ratio to the empirical formula-unit molar mass gives the positive integer multiplier for molecular subscripts, within measurement uncertainty.
Values and equations
n_i = m_i/M_i; k = M_molecule/M_empirical
- n_i
- amount of atoms of constituent element i in the analyzed sample mol
- m_i
- measured mass attributable to constituent element i kg
- M_i
- molar mass of atoms of element i for the stated composition kg mol^-1
- k
- positive integer molecular-subscript multiplier inferred within uncertainty 1
- M_molecule
- independently determined molecular molar mass kg mol^-1
- M_empirical
- molar mass of one empirical formula unit on the same composition basis kg mol^-1
Conditions and scope
- All constituent elements and relevant purity, hydration and isotope composition must be accounted for. A mixture or variable-composition solid need not yield one small-integer formula.
- Do not round arbitrary ratios blindly: assess measurement uncertainty and possible integer multiples. Both molar masses in the multiplier must use the same composition basis and units.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Element mass-to-amount conversion, smallest atom ratios and molecular-to-empirical molar-mass comparison.
2026-10-10 · Independent Mathematics and Information editor (AI editorial/source review; not the Chemistry/Biology/Earth author). The specified introductory inventory has no unresolved demonstrated material gap after the changes and body-based reuse. Existing advanced references retain their historical scoped approvals. English-only reference prose and explicit Lithuanian-translation-pending status are preserved.
Foundational review
2026-10-10 · Independent AI editorial inspection of this new compact reference entry, its equations, applicability, sources and knowledge connections.
Sources from other places
Agreement within this scope
Mass-to-mole ratios and distinguishing empirical from molecular formula.
A molecular formula also needs molecular-mass information.
Siyavula
- Physical Sciences 10: Composition of substances South AfricaEnglish
§19.2 Empirical and molecular formula, mass-to-mole worked examples
- Physical Sciences 10: Composition of substances
Entry details
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- foundation-chem-empirical-formula-inference
- Revision
- foundations-audit.1
- Structured reference record
- Foundational core completion