Percent composition tells you what fraction of a compound's mass is each element. It is read straight from the formula: take how much mass an element contributes — its atom count times its atomic mass — divide by the compound's molar mass, and multiply by 100. Because it depends only on the formula, it is a fixed fingerprint of the substance: every sample of water is 88.8% oxygen by mass, anywhere, always. It underpins purity checks, fertiliser labels (the N-P-K numbers), ore grades and the path from data to an empirical formula.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the definition of mass percent composition.
The percent-composition equations
The mass an element contributes is its atom count multiplied by its atomic mass — two oxygens in CO₂ contribute 2 × 16.00 = 32.00. Divide by the compound's molar mass and multiply by 100 for the percentage. Add up every element's percent and you should get 100% (within rounding), because the parts must account for the whole molar mass. Multiply a percent by a sample mass and you recover how many grams of that element a real sample holds.
Worked example — water and carbon dioxide
Scenario: Find the percent composition of oxygen in water (H₂O, molar mass 18.015) and carbon in carbon dioxide (CO₂, molar mass 44.01).
Water is 88.8% oxygen and 11.2% hydrogen, adding to 100%. Carbon dioxide is 27.3% carbon and 72.7% oxygen — note the same element, oxygen, is a very different percentage in the two compounds because the molar masses and atom counts differ. To find the oxygen in 250 g of water, just take 0.888 × 250 = 222 g.
Frequently Asked Questions
% = (atoms × atomic mass ÷ molar mass) × 100. O in H₂O = (16/18.015)×100 = 88.8%.
The mass fraction of each element in a compound, out of 100. 100 g of water = 88.8 g O + 11.2 g H.
element mass = (% ÷ 100) × sample mass. 70% iron in 50 g ore = 35 g iron.
Yes — all elements together make up the whole molar mass. If not, recheck atoms, atomic masses or molar mass.
Composition = mass fractions; empirical formula = simplest atom ratio. Convert % → moles → ratio to get the formula.