or

What is a mole anyway ?

Copyright, 1996 F.W. Boyle, Jr., Ph.D.

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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 10^{23}
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 10^{23} atoms
in 1 mole of the element. For the elements that exist as
molecules, there are 6.022 x 10^{23} molecules. The
elements that exist as molecules are: H_{2},
O_{2}, N_{2}, F_{2}, Cl_{2},
Br_{2}, I_{2}, S_{8}, and P_{4}.
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
10^{23} 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 10^{23} 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 10^{23}.
Thus we define: 1 mole = 6.022 x 10^{23} atoms, molecules, or
formula units

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

What this equivalence means is that if one was to weigh out 2 g
of H_{2}, the sample weighed out would contain 6.022 x
10^{23} molecules of H_{2}.

And so a mole of any substance can be measured by weighing out
the mass of the substance which contains 6.022 x 10^{23} 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:

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:

As the reader can see, the number of parts is always constant and ALWAYS equals 6.022 x 10Substance g/mole Parts in a moleH_{2}SO_{4}98 6.022 x 10^{23}molecules CaCO_{3}100 6.022 x 10^{23}formula units NaCl 58.5 6.022 x 10^{23}formula units NaOH 40 6.022 x 10^{23}formula units HCl 36.5 6.022 x 10^{23}molecules CaO 56 6.022 x 10^{23}formula units MgSO_{4}120 6.022 x 10^{23}formula units CaCl_{2}111 6.022 x 10^{23}formula units Cu 63.5 6.022 x 10^{23}atoms