The Mole
145
relative atomic masses. Because these relative masses represent ratios, they are
dimensionless. The atomic masses of the different elements can be expressed in
various units, but the ratios of the masses will always be the same as the ratios
of these relative masses.
With an established scale of relative atomic weights, it is simple to compute
the relative molecular weights on the same scale. All you have to do is take the
sum of the relative masses of the atoms that make up the molecule. Thus, the
relative molecular weight of H 2 SO 4 is (2 x l.O) + (I x 32.1) + (4 x 16.0) = 98.1.
With more accurate values of the relative atomic weights, the value is found to
be 98.07. Regardless of the mass units used, the masses of the H atom, the
0 2 molecule, and the H 2 SO 4 molecule will stand in the ratio of
1.0079:31.9984:98.07.
Ever since instrumentation has been sophisticated enough to determine the
masses of individual atoms in an indirect fashion, it has been possible to have a
list of absolute values of the atomic masses. The mass of an atom is incredibly
small; expressed in grams, these values are cumbersome to use in day-to-day
work and conversation. In recent years, this difficulty has been surmounted by
using a special mass unit called adttlton (d): 1 d = 1.6604 x 10~4 g. This unit is
chosen so that the mass of the most common isotope of the carbon atom equals
12.00000 d. Thus, the absolute atomic weight of H is 1.0079 d, and the absolute
molecular weights of O 2 and H 2 SO 4 are 31.9984 d and 98.07 d, respectively. The
absolute atomic and molecular weights expressed in daltons (sometimes called
the Dalton atomic and molecular weights) have the same numerical values as
the relative atomic and molecular weights, but they represent iheactnalmasw,
of the atoms and molecules involved; we can convert them to grams by multiplying by 1.6604 x 10~
24 g/d.
THE MOLE
In the laboratory, we normally work with very large numbers of atoms or
molecules, usually with weights conveniently expressed in grams. Suppose we
have a certain weight of O 2 and another certain weight of H 2 SO 4 ; if the ratio of
these two weights is 31.9984:98.07, we know that each weight contains the
same number of molecules (because the weight ratio is the same as that of
individual molecules from the relative molecular weights). It is convenient to
use the relative atomic weight scale with values expressed in grams. Thus, the
gram atomic weight of H is 1.0079 g; the gram molecular weight of O 2 is 31.9984
g; and the gram molecular weight of H 2 SO 4 is 98.07 g. These gram atomic or
molecular weights are also called mole \\eii>hf>. One mole weight of any substance must contain the same number of atoms or molecules as one mole weight
of any other substance. This quantity is usually called one mole of the substance. Note that the same numerical scales are used for relative, Dalton, and
gram molecular weights; only the units are different.
145
relative atomic masses. Because these relative masses represent ratios, they are
dimensionless. The atomic masses of the different elements can be expressed in
various units, but the ratios of the masses will always be the same as the ratios
of these relative masses.
With an established scale of relative atomic weights, it is simple to compute
the relative molecular weights on the same scale. All you have to do is take the
sum of the relative masses of the atoms that make up the molecule. Thus, the
relative molecular weight of H 2 SO 4 is (2 x l.O) + (I x 32.1) + (4 x 16.0) = 98.1.
With more accurate values of the relative atomic weights, the value is found to
be 98.07. Regardless of the mass units used, the masses of the H atom, the
0 2 molecule, and the H 2 SO 4 molecule will stand in the ratio of
1.0079:31.9984:98.07.
Ever since instrumentation has been sophisticated enough to determine the
masses of individual atoms in an indirect fashion, it has been possible to have a
list of absolute values of the atomic masses. The mass of an atom is incredibly
small; expressed in grams, these values are cumbersome to use in day-to-day
work and conversation. In recent years, this difficulty has been surmounted by
using a special mass unit called adttlton (d): 1 d = 1.6604 x 10~4 g. This unit is
chosen so that the mass of the most common isotope of the carbon atom equals
12.00000 d. Thus, the absolute atomic weight of H is 1.0079 d, and the absolute
molecular weights of O 2 and H 2 SO 4 are 31.9984 d and 98.07 d, respectively. The
absolute atomic and molecular weights expressed in daltons (sometimes called
the Dalton atomic and molecular weights) have the same numerical values as
the relative atomic and molecular weights, but they represent iheactnalmasw,
of the atoms and molecules involved; we can convert them to grams by multiplying by 1.6604 x 10~
24 g/d.
THE MOLE
In the laboratory, we normally work with very large numbers of atoms or
molecules, usually with weights conveniently expressed in grams. Suppose we
have a certain weight of O 2 and another certain weight of H 2 SO 4 ; if the ratio of
these two weights is 31.9984:98.07, we know that each weight contains the
same number of molecules (because the weight ratio is the same as that of
individual molecules from the relative molecular weights). It is convenient to
use the relative atomic weight scale with values expressed in grams. Thus, the
gram atomic weight of H is 1.0079 g; the gram molecular weight of O 2 is 31.9984
g; and the gram molecular weight of H 2 SO 4 is 98.07 g. These gram atomic or
molecular weights are also called mole \\eii>hf>. One mole weight of any substance must contain the same number of atoms or molecules as one mole weight
of any other substance. This quantity is usually called one mole of the substance. Note that the same numerical scales are used for relative, Dalton, and
gram molecular weights; only the units are different.