1.8 Units 23
as required. It follows that when equation (1.1) is written in the formV 1 = 1 nRT 2 p, as in
Example 1.1, the dimensions of Vare energy 2 pressure, with SI unit
for the volume.
0 Exercise 95
Large and small units
Decimal multiples of SI units have names formed from the names of the units and
the prefixes listed in Table 1.3. For example, a picometre ispm 1 = 110
− 12m, a decimetre
isdm 1 = 110
− 1m. These units of length are frequently used in chemistry; molecular
bond lengths in picometres, and concentrations in moles per decimetre cube,
mol dm
− 31 = 110
3mol m
− 3.
Table 1.3 SI prefixes
Multiple Prefix Symbol Multiple Prefix Symbol
10 deca da 10
− 1
deci d
10
2
hecto h 10
− 2
centi c
10
3
kilo k 10
− 3
milli m
10
6
mega M 10
− 6
micro μ
10
9
giga G 10
− 9
nano n
10
12
tera T 10
− 12
pico p
10
15
peta P 10
− 15
femto f
10
18
exa E 10
− 18
atto a
10
21
zetta Z 10
− 21
zepto z
10
24
yotta Y 10
− 24
yocto y
0 Exercises 96–103
The quantities that are of interest in chemistry often have very different magnitudes
from those of the SI units themselves, particularly when the properties of individual
atoms and molecules are considered. For example, the mole is defined as the amount
of substance that contains as many elementary entities (atoms or molecules) as there
are atoms in12 g(0.012 kg)of
12C. This number is given by Avogadro’s constant,
N
A1 ≈ 1 6.02214 1 × 110
23mol
− 1. The mass of an atom of
12Cis therefore
m(
12C) 1 = 1122 (6.02214 1 × 110
23) g 1 ≈ 121 × 110
− 26kg
or m(
12C) 1 = 1 12 u, where
u 1 = 112 (6.02214 1 × 110
23) g 1 ≈ 1 1.66054 1 × 110
− 27kg
is called the unified atomic mass unit(sometimes called aDalton, with symbol Da).
J
Pa
N m
N m
==m
− 23