We have already established that quantities are important in science, especially in chemistry. It is important to make accurate measurements of a variety of quantities when performing experiments. However, it is also important to be able to relate one measured quantity to another, unmeasured quantity. In this chapter, we will consider how we manipulate quantities to relate them to each other. 8.01: Masses of Atoms and Molecules The atomic mass unit (u) is a unit that describes the masses of individual atoms and molecules. The atomic mass is the weighted average of the masses of all isotopes of an element. The molecular mass is the sum of the masses of the atoms in a molecule. 8.02: The Mole The mole is a key unit in chemistry. The molar mass of a substance, in grams, is numerically equal to one atom’s or molecule’s mass in atomic mass units. 8.03: Molar Mass 8.04: Percent Composition 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. 8.05: Empirical Formulas 8.06: Molecular Formulas