McKay, Donald. "Front matter" Multimedia Environmental Models ...

give the derived units directly with no conversion factors. For example, energy
(joules) is variously the product of force (newtons) and distance (metres), or pressure
(pascals) and volume (cubic metres), or power (watts) and time (seconds). Thus, the
foot-pound, the litre-atmosphere, and the kilowatt-hour become obsolete in favor of
the single joule. Some key aspects of the SI system are discussed below. Conversion
tables from obsolete or obsolescent unit systems are available in scientific handbooks.
Length (metre, m)
This base unit is defined as the specified number of wavelengths of a krypton
light emission.
Area
Square metre (m 2 ). Occasionally, the hectare (ha) (an area 100 ¥ 100 m or 10 4 m 2 )
or the square kilometre (km 2 ) is used. For example, pesticide dosages to soils are
often given in kg/ha.
Volume (cubic metre, m 3 )
The litre (L) (0.001 m 3 ) is also used because of its convenience in analysis, but
it should be avoided in environmental calculations. In the United States, the spellings
“meter” and “liter” are often used.
Mass (kilogram, kg)
Kilogram (kg). The base unit is the kilogram (kg), but it is often more convenient
to use the gram (g), especially for concentrations. For large masses, the megagram
(Mg) or the equivalent metric tonne (t) may be used.
Amount (mole abbreviated to mol)
This unit, which is of fundamental importance in environmental chemistry, is
really a number of constituent entities or particles such as atoms, ions, or molecules.
It is the actual number of particles divided by Avogadro’s number (6.0 ¥ 10 23 ),
which is defined as the number of atoms in 12 g of the carbon-12 isotope. When
reactions occur, the amounts of substances reacting and forming are best expressed
in moles rather than mass, since atoms or molecules combine in simple stoichiometric
ratios. The need to involve atomic or molecular masses is thus avoided.
Molar Mass or Molecular Mass (or Weight) (g/mol)
This is the mass of 1 mole of matter and is sometimes (wrongly) referred to as
molecular weight or molecular mass. Strictly, the correct unit is kg/mol, but it is
often more convenient to use g/mol, which is obtained by adding the atomic masses
(weights). Benzene (C6H6) is thus approximately 78 g/mol or 0.078 kg/mol.
©2001 CRC Press LLC