The Mole Unit

In laboratory practice, you work with much larger quantities of the elements than single atoms or single molecules. A convenient standard quantity is the mole, the amount of the substance equal in grams to the sum of the atomic masses. Therefore, one mole of carbon dioxide is 44.01 grams:

The mole is a convenient unit for expressing the relative amounts of substances in chemical reactions. The burning of carbon in oxygen can be written with the 2 oxygen atoms bonded in a single O 2 molecule:

The mole is the most common unit used to express the quantity of a chemical substance. For all solids, liquids, and gases, you can convert mass to moles (or moles to mass). For gases, you should memorize the following conversion of volume to moles (or moles to volume): at 0°C and 1 atmosphere pressure (known as standard temperature and pressure or STP), one mole of any gas occupies approximately 22.4 liters. Therefore, the preceding reaction describing the oxidation of carbon means that 12 grams of carbon burned at STP in 22.4 liters of O 2 yields 22.4 liters of carbon dioxide.

Many substances do not exist as molecules. For example, the atoms in most inorganic solids are in a three‐dimensional structure in which each atom is surrounded by a number of other atoms. In crystals of sodium chloride, no distinct Na and Cl pair can be called a molecule, because each sodium is surrounded by 6 chlorine and each chlorine is surrounded by 6 sodium.

For nonmolecular substances like sodium chloride, the use of the word mole, with its connotation of molecules, is inappropriate. A comparable unit, the gram formula mass, is used; it is defined as the sum in grams of the atomic masses of all the atoms in the chemical formula of the substance. For sodium chloride (NaCl), the gram formula mass is calculated as

The mole unit and the gram formula unit are employed in similar calculations. For this reason, many chemists use the term mole to describe quantities of both molecular and nonmolecular substances. Try answering the following problems:

  • How many moles of bromobenzene are in 1 kilogram of bromobenzene, C 6H 5Br?
  • What is the mass in grams of neon gas that occupies a volume of 5 liters at 0°C and 1 atmosphere of pressure?