Percent Composition of a Compound

Gives the percentage by mass of each element in a chemical compound, element by element.

\% \text{element} = \frac{n \times A_r}{M} \times 100\%

% element = (n × atomic weight / molar mass) × 100%

What it means

Percent composition states how much of a compound's mass belongs to each element, and it is fixed for a given compound — pure water is always 11.2% hydrogen by mass, no matter how much you have. This matters because it connects measurable data (a sample weighed before and after combustion analysis) to the compound's formula: if your measured percentages match the calculated ones, your proposed formula is consistent with the evidence. When to use it: when converting combustion-analysis data into an empirical formula, or when checking that a proposed molecular formula agrees with experimental mass data.

Variables

Symbol	Name	Unit	Meaning
$%$	Mass percent of element	%	The percentage of the compound's total mass contributed by one element.
$n$	Atom count	unitless	The number of atoms of that element in one formula unit of the compound.
$Ar$	Atomic weight	g/mol	The standard atomic weight of the element.
$M$	Molar mass	g/mol	The molar mass of the whole compound.

Assumptions

The compound's formula is known and correct, since the calculation depends on the atom count of each element.
The sample is pure; impurities change the measured molar mass and invalidate the percentages.

Worked example

Problem

Find the percent composition of hydrogen and oxygen in water (H2O).

Given:

Compound: H2O
Atomic weights: H = 1.008, O = 15.999
Molar mass of H2O = 2(1.008) + 15.999 = 18.0 g/mol

Find the molar mass: M(H2O) = 2 × 1.008 + 15.999 = 18.015 g/mol, which rounds to 18.0 g/mol.
Mass of H per mole = 2 × 1.008 = 2.016 g; mass of O per mole = 15.999 g.
For hydrogen: (2.016 / 18.015) × 100% = 11.190%, which rounds to 11.2%.
For oxygen: (15.999 / 18.015) × 100% = 88.810%, which rounds to 88.8%.
Check: 11.2 + 88.8 = 100.0%.

%H = 11.2%, %O = 88.8% (sum = 100%)

About 11.2% of the mass of any water sample comes from hydrogen and 88.8% from oxygen, a fixed ratio for pure water that can be checked against experimental analysis.