http://www.ck12.org Chapter 10. The Mole
In the case of pure metals, the representative particle is simply the atom. One mole of zinc refers to a mole of Zn
atoms.
Example 10.2
Calculate the number of moles of iron atoms that are present in 2 moles of Fe 2 O 3.
2 mol Fe 2 O 3 ×(1 mol Fe2 mol Fe 2 O 3 ) =4 mol Fe
Example 10.3
Calculate the number of atoms that must be present in 3.05 moles of Pb.
3 .05 mol Pb×(^6.^022 ×^10
(^23) atoms Pb
1 mol Pb ) =^1.^84 ×^10
(^24) atoms Pb
Molar Mass
Because we are not able to count individual atoms, it is important to have a way to convert between amounts, which
are expressed in moles, and a unit of quantity that we can more easily measure, such as mass. We begin by looking
at the periodic table, which tells us the relative masses of various elements.
Molar Masses of the Elements
As you learned previously, the average atomic masses found on the periodic table are in atomic mass units (amu).
For example, one atom of the most abundant isotope of hydrogen has a mass of approximately 1 amu, and one atom
of helium has a mass of about 4 amu. Atomic masses are relative masses; they are based on the definition that one
amu is equal to 1/12thof the mass of a single atom of carbon-12. Therefore, one atom of carbon-12 has a mass of
12 amu, which is three times heavier than an atom of helium. This ratio would hold for any number of carbon and
helium atoms. One hundred carbon-12 atoms would have three times the mass of one hundred helium atoms. By
extension, 12.00 g of carbon-12 would contain the same number of atoms as 4.00 g of helium.
The relative scale of atomic masses in amu is also a relative scale of masses in grams. We said before that the mole
is officially equal to the number of carbon atoms in exactly 12 g of carbon-12. In other words, one carbon-12 atom
has a mass of exactly 12 amu, and one mole of carbon atoms has a mass of exactly 12 grams. This relationship is
true for all substances. If one atom of helium has a mass of 4.00 amu, one mole of helium atoms has a mass of 4.00
g. One molecule of water has a mass of 18.0 amu, so one mole of water molecules has a mass of 18.0 grams.Molar
massis defined as the mass of one mole of representative particles of a substance. It is expressed in units of grams
per mole (g/mol).
The values on the periodic table can be read either as average atomic masses or as molar masses. For example,
the mass associated with chlorine is 35.45, even though no atoms of chlorine actually have a mass of 35.45 amu.
However, in naturally occurring samples of chlorine atoms, about 75% will be chlorine-35 atoms (which have a mass
of 35.0 amu), and about 25% will be chlorine-37 atoms (which have a mass of 37.0 amu). Since any usable quantity
of chlorine contains a very, very large number of atoms, one mole of chlorine atoms will contain a mixture of these
two isotopes in this ratio, resulting in a sample with a mass of 35.45 grams. Therefore chlorine has a molar mass of
35.45 g/mol.
Molar Masses of Compounds
The molecular formula of carbon dioxide is CO 2. One molecule of carbon dioxide consists of 1 atom of carbon and
2 atoms of oxygen. We can calculate the molar mass of carbon dioxide by adding together the molar masses of each
atom present in the compound.