## Percent Composition of Compounds

#### Learning Objective

• Translate between a molecular formula of a compound and its percent composition by mass

#### Key Points

• The atomic composition of chemical compounds can be described in a variety of ways, including molecular formulas and percent composition.
• The percent composition of a compound is calculated with the molecular formula: divide the mass of each element found in one mole of the compound by the total molar mass of the compound.
• The percent composition of a compound can be measured experimentally, and these values can be used to determine the empirical formula of a compound.

#### Term

• percent by massThe fraction, by weight, of one element of a compound.

The atomic composition of chemical compounds can be described using a variety of notations including molecular, empirical, and structural formulas. Another convenient way to describe atomic composition is to examine the percent composition of a compound by mass.

## Percent Composition by Mass

Percent composition is calculated from a molecular formula by dividing the mass of a single element in one mole of a compound by the mass of one mole of the entire compound. This value is presented as a percentage.

For example, butane has a molecular formula of C4H10. Butane’s percent composition can be calculated as follows:

• Mass of H per mol butane: $10\:mol\:H \cdot \frac{1.00794\:g}{1\:mol\: H} = 10.079\:g\:H$
• Mass of C per mol butane: $4\:mol\:C\cdot\frac{12.011\:g\:C}{1\:mol\:C}=48.044\:g\:C$
• Mass percent H in butane: $\frac{10.079\:g\:H}{58.123\:g\:butane} \cdot 100$ = 17.3% H
• Mass percent C in butane: $\frac {48.044\:g \:C}{ 58.123 \:g \:butane} \cdot 100$ = 82.7% C

Therefore, the atomic composition of butane can also be described as 17.3% hydrogen and 82.7% carbon, and, as expected, these values sum to 100%.