Moles and Molar Masses

Most chemical experiments involve enormous numbers of atoms or molecules. For this reason, the SI system of units defines the mole, abbreviated mol, as the amount of a substance that contains the same number of atoms as 12 g of ¹²C. This number is called Avogadro's number, NA = 6.0221 × 10²³ mol^-1. The mole thus is a counting term analogous to "dozen." Just as a dozen eggs corresponds to 12 eggs, a mole of atoms is 6.0221 × 10²³ atoms. The mole can be applied to counting atoms, molecules, ions, electrons, protons, neutrons, etc.—it always corresponds to NA of the counted species. The mass of one mole of an element with atomic mass Ar(E) u is NAAr u or simply Ar g/mol, e.g., the atomic mass of gold is 197.0 u or 197.0 g/mol.

The chemical symbol for an element—H, C, O, etc.—is used to designate that element. Molecular substances consist of independent molecules containing two or more atoms bound together. A molecular formula specifies the identity and number of the atoms in the molecule. For instance, the formula for carbon dioxide is CO₂, one carbon atom and two oxygen atoms. The molecular mass of CO₂ is Ar(C) + 2Ar(O) = 12.0107 + 2 × 15.9994 = 44.0095 u. The molar mass of CO₂ is the mass in grams numerically equal to the molecular mass in u, 44.0095 g/mol, i.e., 44.0095 g contains NA CO₂ molecules.

Many common substances are ionic, e.g., sodium chloride, NaCl. A crystal of NaCl contains sodium ions, Na⁺, and chloride ions, Cl^-, arranged in a regular spatial array. Although there are no NaCl molecules, the formula indicates the relative number of atoms of each element present in the crystal, and we can speak of the molar mass of NaCl, 22.98977 + 35.4527 = 58.4425 g/mol, as the mass of sodium chloride which contains NA sodium ions and NA chloride ions. We also speak of the molar mass of an ion, e.g., OH-, 15.9994 + 1.00794 = 17.0073 g/mol, as the mass of NA hydroxide ions.

Older books and chemical journals used the terms atomic weight and molecular weight rather than atomic mass and molar mass. Because "weight" is a force rather than a mass, such usage is potentially confusing and is now discouraged, but many chemists trained in an earlier time still use the old terms, so you may encounter them.