percent composition
/ per-SENT kom-poh-ZIH-shun /
Read a food label and you see how much of a snack is fat, sugar, and protein by weight. Percent composition is the same kind of breakdown for a chemical compound: what fraction of its mass each element contributes.
Percent composition is the mass of each element in a compound divided by the compound's total mass, times one hundred. You get it by taking the mass each element contributes (its count times its atomic weight) and dividing by the formula weight.
It connects a compound's recipe to what you can actually weigh out, and it lets analysts check whether a sample matches its expected formula — a purity or identity clue. Read the other way, percent composition from experiment is the first step toward deducing an unknown's empirical formula.
In water (H₂O, formula weight 18), hydrogen contributes 2 of the 18 mass units, so water is about 11% hydrogen and 89% oxygen by mass.
Percent composition splits a compound's mass element by element.
Percent composition is by mass unless stated otherwise. Don't confuse it with mole fraction, which counts atoms — hydrogen is two of three atoms in water yet only about a tenth of its mass.