A molecule is a combination of two or more atoms held together by covalent bonds. It is the smallest unit of a compound that displays the properties of that compound. Molecules may contain two atoms of the same element, as in N2 and O2, or may be composed of two or more different atoms, as in CO2 and SOCl2.
In chapter 3, we discussed the concept of the atomic weight. Like atoms, molecules can also be characterized by their weight. The molecular weight is simply the sum of the weights of the atoms that make up the molecule.
| 1S = | 1 × 32 amu | = | 32 amu |
| 1O = | 1 × 16 amu | = | 16 amu |
| 2Cl = | 2 × 35.5 amu | = | 71 amu |
| molecular weight | = | 119 amu | |
Ionic compounds do not form true molecules. In the solid state they can be considered to be a nearly infinite, three-dimensional array of the charged particles of which the compound is composed. Since no actual molecule exists, molecular weight becomes meaningless, and the term formula weight is used in its place, although the calculation is the same: We simply add up the atomic masses of the elements in the compound’s empirical formula (see below). The formula weight of NaCl, for example, is the atomic weight of Na plus the atomic weight of Cl: (23 + 35.5) amu = 58.5 amu.
Remember that a mole of something is about 6.022 × 1023 of that thing. In addition, the atomic mass of an atom, reported in units of amu, is numerically the same as its mass in grams per mole. For example, one mole of an atom with atomic mass x amu has a mass of x grams. The same relationship holds for molecules: One mole of a compound has a mass in grams equal to the molecular weight of that compound in amu, and contains 6.022 × 1023 molecules of the compound. For example, the molecular weight of carbonic acid, H2CO3, is (2 × 1 + 12 + 3 × 16) = 62 amu. 62 g of H2CO3 represents one mole of carbonic acid and contains 6.022 × 1023 H2CO3 molecules. In other words, the molar mass of H2CO3 is 62 g/mol. This can also be arrived at by simply adding the molar atomic mass of the atoms in the compound: 1 mole of H2CO3 contains 2 moles of H atoms, 1 mole of C atoms, and 3 moles of O atoms.
Given the weight of a sample, one can determine the number of moles present with the following formula:
Now, solve for the number of moles.