Chemistry Reference
In-Depth Information
substances, and it is more common for teaching the density of substances to use
g/ml or g/cm
3
for solids or liquids, but g/L for gases.
In the literature, one can find different symbols for liter and milliliter: ml and l
respectively mL and L. Since the capital L in the latter notation better contrasts with
the number 1, the notation mL and L is used in this topic.
Atomic mass.
In chemistry, there also exists another mass unit than the kg: the
unit u for the atomic mass. The former “atomic weight” was dimensionless, it was
first defined by comparison with the mass of a hydrogen atom, then by comparison
with the mass of an oxygen atom, the values were entered in first atomic mass
tables. Later, it was redefined to have an exact value and to make the atomic mass
comparable with other parameters: 1 u
1/12 of the mass of the isotope C-12. It
can also be connected to the unit g with the help of Avogadro's constant: 1 g
¼
¼
N
A
1u.
The molecular mass as well as the molar mass can be obtained from the addition
of atomic masses of participating atoms. Examples:
m
1C atom
¼
12 u
;
m
1mol C atoms
¼
12 g
;
M
(C)
¼
12 g/mol,
m
1C
2
H
5
OHmolecule
¼
46 u
;
m
1mol C
2
H
5
OHmolecules
¼
46 g
;
M
ð
C
2
H
5
OH)
¼
46 g/mol
:
Mole.
The unit for the amount of substance
n
is the mole with the symbol mol.
The meaning of this unit has changed drastically. Some decades ago, the definition
was: the mole equals the molecular weight in g, therefore molecular masses in u
were easily changed into the unit g. Today, the following IUPAC-definition is used:
“one mole is the amount of a substance that has as many particles as atoms in 12 g
of carbon, which only consists of C-12 isotopes. The particles of that amount of a
substance have to be further characterized: it can be atoms, molecules, ions,
electrons, and other particles” [
2
]. The advantage of this definition is that the
imprecise information “1 mol oxygen” is replaced by “1 mol O atoms,” or “1 mol
O
2
molecules,” or even “1 mol O
3
molecules”:
m
1mol O atoms
¼
16 g
;
m
1mol O
2
molecules
¼
32 g
;
m
1mol O
3
molecules
¼
48 g
;
¼
=
:
the sulfur portion m
32 g either consists of 1mol S atoms or 1
8 mol S
8
molecules
The molar mass
M
(g/mol), the molar volume
V
m
(L/mol), the molar quantity
N
A
(1/mol), the molar charge
F
(C/mol), molar concentration molarity (mol/L), or
molality (mol/kg) can be derived from the mole. The common use of the molarity
unit mol/L for concentrations allows precise values according to the amount of
particles. For example, the imprecise use of the term “1 molar sulfuric acid” should
be replaced by the precise use:
c
H
2
SO
4
¼
0
:
1mol/L means
c
H
þ
ð
aq
Þ
0
:
2mol/L resp
:
c
SO
2
4
ð
aq
Þ
0
:
1mol/L
:
