Percent Composition of an Element
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A chemical formula is a recipe by mass: each element's share equals its contribution to the compound's molar mass. In water, hydrogen contributes 2 × 1.008 = 2.016 g/mol out of 18.02 g/mol — just 11.2% by mass, with oxygen carrying the other 88.8%. The same arithmetic run in reverse is how chemists deduce empirical formulas from combustion analysis, and how fertilizer bags justify their nitrogen percentages.
Percent Composition of an Element
Where
- = Mass percent of element X
- = Atoms of X per formula unit
- = Molar mass of element X
- = Molar mass of compound
Missing one of these? Work it out first, then come back
- Mass percent of element X — Mass Percent of a Solution, Percent Yield
- Atoms of X per formula unit — Specific Gravity, Radioactive Activity (A = λN)
- Molar mass of element X — Faraday's Law of Electrolysis (m = QM/nF), Moles from Mass (n = m/M)
- Molar mass of compound — Moles from Mass (n = m/M), Gas Density from Molar Mass