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3: Composition of Substances and Solutions

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    Quantitative aspects of the composition of substances and mixtures are the subject of this chapter.

    • 3.1: Formula Mass and the Mole Concept
      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 the molecular mass. 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 \(6.022 \times 10^{23}\), a quantity called Avogadro’s number.
    • 3.2: Applying Formula Mass, Avogadro's Number, the Mole, and Molar Mass
      The molecular mass and the formula mass of a compound are obtained by adding together the atomic masses of the atoms present in the molecular formula or empirical formula, respectively; the units of both are atomic mass units (amu). The mole is defined as the amount of substance that contains the number of carbon atoms in exactly 12 g of carbon-12, Avogadro’s number (\(6.022 \times 10^{23}\)) of atoms of carbon-12.
    • 3.3: Determining 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.
    • 3.4: Molarity
      Solutions are homogeneous mixtures. Many solutions contain one component, called the solvent, in which other components, called solutes, are dissolved. An aqueous solution is one for which the solvent is water. The concentration of a solution is a measure of the relative amount of solute in a given amount of solution. Concentrations may be measured using various units, with one very useful unit being molarity, defined as the number of moles of solute per liter of solution.
    • 3.5: Other Units for Solution Concentrations
      In addition to molarity, a number of other solution concentration units are used in various applications. Percentage concentrations based on the solution components’ masses, volumes, or both are useful for expressing relatively high concentrations, whereas lower concentrations are conveniently expressed using ppm or ppb units. These units are popular in environmental, medical, and other fields where mole-based units such as molarity are not as commonly used.
    • 3.E: Composition of Substances and Solutions (Exercises)
      These are homework exercises to accompany the Textmap created for "Chemistry" by OpenStax.

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