7.1 The Mole Concept & Avogadro’s Number
A convenient amount unit for expressing very large numbers of atoms or molecules is the mole. Experimental measurements have determined the number of entities composing 1 mole of substance to be 6.022 × 1023, a quantity called Avogadro’s number.
7.2 Molecular Mass, Avogadro’s Number and The Mole
The formula mass of a substance is the sum of the average atomic masses of each atom represented in the chemical formula and is expressed in atomic mass units. The formula mass of a covalent compound is also called molecular mass.
The mass in grams of 1 mole of a substance is its molar mass. Due to the use of the same reference substance in defining the atomic mass unit and the mole, the formula mass (amu) and molar mass (g/mol) for any substance are numerically equivalent (for example, one H2O molecule weighs approximately 18 amu and 1 mole of H2O molecules weighs approximately 18 g).
7.3 Percent Composition
The percent composition is the percentage by mass of each element in the compound. The percent composition of each element in a compound can be used to determine the empirical and molecular formulas of the compound.
7.4 Determining the Empirical and Molecular Formulas
The chemical identity of a substance is defined by the types and relative numbers of atoms composing its fundamental entities (molecules in the case of covalent compounds, ions in the case of ionic compounds). A compound’s percent composition provides the mass percentage of each element in the compound, and it is often experimentally determined and used to derive the compound’s empirical formula. The empirical formula mass of a covalent compound may be compared to the compound’s molecular or molar mass to derive a molecular formula.
Attribution & References
Except where otherwise noted, this page is adapted by Adrienne Richards from “Chapter 3 Summary – Composition of Substances and Solutions” In Chemistry 2e (OpenStax) by Paul Flowers, Klaus Theopold, Richard Langley, & William R. Robinson, licensed under CC BY 4.0. Access for free at Chemistry 2e (OpenStax).