Molecular Weight and Molar Mass

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 N 2 and O 2 , or may be composed of two or more different
atoms, as in CO 2 and SOCl 2.

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.

Example: What   is  the molecular   weight  of  SOCl 2?

Solution: To    find    the molecular   weight  of  SOCl 2 ,    add together    the atomic  weights of  each    of

the atoms.

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 × 10^23 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