From Formula to Mass Percent Composition

Mass percent of an element = (mass contribution of that element / molar mass of compound) × 100%. In water, oxygen contributes about 88.8% of the mass despite being only one of three atoms.

Empirical formula problems reverse this process: convert percent to grams (assume 100 g), grams to moles, then divide by the smallest mole count to find ratios. Rounding ratios to integers is where many students lose precision.

Comparing mass percents across related compounds (CO vs CO₂, SO₂ vs SO₃) builds intuition for oxidation-state trends and combustion analysis results.