The Elusive Mole
or
What is a mole anyway ?
Copyright, 1996 F.W. Boyle, Jr., Ph.D.

The mole, a term used quite often in chemistry, is not always readily understood by the new student. The reasons vary but it may be simply do to the use of certain words when describing a mole.

First, a mole is NOT a mass or a weight of substance. Sometimes it is stated that X g of whatsis equals 1 mole of whatsis. This terminology is not quite correct. So in place of using the phrase "is equal to", the central theme of this paper is to get the reader to begin using the phrase "is equivalent to".

Everyone recognizes that a dozen eggs means that there are 12 eggs. So how much is a mole ? Understanding the count in a mole is easy. A mole of any pure substance contains 6.022 x 1023 parts.

Now what are those parts ? In a dozen eggs, the parts were the individual eggs. Since pure substances can be different things, the parts are called by different names.

For an element, the parts are called atoms. For all the elements that are not molecules, there are 6.022 x 1023 atoms in 1 mole of the element. For the elements that exist as molecules, there are 6.022 x 1023 molecules. The elements that exist as molecules are: H2, O2, N2, F2, Cl2, Br2, I2, S8, and P4. S and P can be used as atoms in many chemical equations unless the molecular form is given in the reaction.

Compounds are combinations of 2 or more atoms of DIFFERENT elements. There are two kinds of compounds: molecular and ionic.

The parts of a molecular compounds are called molecules and contain only nonmetal elements. There are 6.022 x 1023 molecules in 1 mole of ANY molecular compound.

The parts of an ionic compound are called formula units. Ionic compounds contain at least one metal atom and at least one nonmetal atom. There are 6.022 x 1023 formula units in 1 mole of ANY ionic compound.

So up to this point the reader should begin grasping the idea that a mole is not a mass, a mole is a count and the count is ALWAYS 6.022 x 1023. Thus we define: 1 mole = 6.022 x 1023 atoms, molecules, or formula units

Now there often arises some confusion when the following "equality" is mentioned. Many times it has been said that

1 mole H2 = 2 g H2

This is not exactly true. What is true is that

1 mole H2 2 g H2

The is the mathematical symbol for "is equivalent to". Thus, one can more correctly say that one mole of H2 is equivalent to 2 g of H2.

What this equivalence means is that if one was to weigh out 2 g of H2, the sample weighed out would contain 6.022 x 1023 molecules of H2.

And so a mole of any substance can be measured by weighing out the mass of the substance which contains 6.022 x 1023 atoms, molecules, or formula units of the substance. The mass is then stated to be equivalent to 1 mole of substance.

For CaO: CaO has a mass of 40 g Ca + 16 g O, giving a formula unit mass of 56 g. This mass is often written as follows:

56 g CaO/mole

The measure is 56 g of CaO contain 6.022 x 1023 formula units of CaO and since 6.022 x 1023 formula units of CaO IS equal to one mole we say that the molar mass of CaO is 56 g/mole.

For every substance, there is a mass which is equivalent to one mole of that substance. The mass needed for any substance can be calculated from the formula of the substance. Here are some examples:

          Substance        g/mole         Parts in a mole     

           H2SO4          98         6.022 x 1023 molecules

           CaCO3         100         6.022 x 1023 formula units

           NaCl          58.5        6.022 x 1023 formula units

           NaOH           40         6.022 x 1023 formula units

            HCl          36.5        6.022 x 1023 molecules

            CaO           56         6.022 x 1023 formula units
                           
           MgSO4         120         6.022 x 1023 formula units

           CaCl2         111         6.022 x 1023 formula units

             Cu          63.5        6.022 x 1023 atoms
As the reader can see, the number of parts is always constant and ALWAYS equals 6.022 x 1023 (called Avogadro's number). The weights vary because the different atoms that make up the substance have different masses. Although each compound is made up of a number of different atoms, these atoms are attached to each other in such a way that the compound behaves like a big new parts rather than as each of its individual parts.