Chemistry
Textbooks
Boundless Chemistry
Mass Relationships and Chemical Equations
Compound Composition
Chemistry Textbooks Boundless Chemistry Mass Relationships and Chemical Equations Compound Composition
Chemistry Textbooks Boundless Chemistry Mass Relationships and Chemical Equations
Chemistry Textbooks Boundless Chemistry
Chemistry Textbooks
Chemistry
Concept Version 10
Created by Boundless

Percent Composition of Compounds

The percent composition (by mass) of a compound can be calculated by dividing the mass of each element by the total mass of the compound.

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 mass

    The fraction, by weight, of one element of a compound.


Full Text

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.

Percentage Composition - YouTube

This video shows how to calculate the percent composition of a compound.

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%.

Butane

The structural formula of butane.

In practice, this calculation is often reversed. Mass percents can be determined experimentally via elemental analysis, and these values can be used to calculate the empirical formula of unknown compounds. However, this information is insufficient to determine the molecular formula without additional information on the compound's molecular weight.

[ edit ]
Edit this content
Prev Concept
Converting between Mass and Number of Moles
Combustion Analysis
Next Concept
Subjects
  • Accounting
  • Algebra
  • Art History
  • Biology
  • Business
  • Calculus
  • Chemistry
  • Communications
  • Economics
  • Finance
  • Management
  • Marketing
  • Microbiology
  • Physics
  • Physiology
  • Political Science
  • Psychology
  • Sociology
  • Statistics
  • U.S. History
  • World History
  • Writing

Except where noted, content and user contributions on this site are licensed under CC BY-SA 4.0 with attribution required.