The elemental makeup of a compound defines its chemical identity, and chemical formulas are the most succinct way of representing this elemental makeup. When a compound’s formula is unknown, measuring the mass of each of its constituent elements is often the first step in the process of determining the formula experimentally. The results of these measurements permit the calculation of the compound’s percent composition, defined as the percentage by mass of each element in the compound.

For example, consider a gaseous compound composed solely of carbon and hydrogen. The percent composition of this compound is represented as follows:

%H = mass H mass compound × 100%

%C = mass C mass compound × 100%

If the analysis of a 10.0-g sample of this gas showed it to contain 2.5g H and 7.5g C, the percent composition is calculated to be 25% H and 75% C:

%H = 2.5g H 10.0g compound × 100% = 25%

%C = 7.5g C 10.0g compound × 100% = 75%