for engineering calculations because one of them is significant, and we will use moles to count atoms and molecules throughout this text.
Table 2‐1. S.I. Basic Units
The meter is the length of the path
traveled by light in vacuum during a
time interval of 1/299,792,458 of a second.
The kilogram is the unit of mass equal to
the international prototype of the
The second is the duration of 9,192,631,770
periods of the radiation corresponding to
the transition between the two hyperfine
levels of the ground state of the cesium 133
The ampere is that constant current which,
if maintained in two straight parallel
conductors of infinite length, of negligible
circular cross section, and placed 1 meter
apart in vacuum, would produce between
these conductors a force equal to 2x10‐7
newton per meter of length.
The Kelvin, unit of thermodynamic
temperature, is the fraction of 1/273.16 of
the thermodynamic temperature of the
triple point of water.
The mole is the amount of substance of a
system which contains as many elementary
entities as there are atoms in 0.012
kilogram of carbon‐12.
The candela is the luminous intensity, in a
given direction, of a source that emits
monochromatic radiation of frequency
540x1012 hertz and that has a radiant
intensity in that direction of 1/683 watt per
Sometimes chemical engineers make use of the “pound‐mole” as a unit of measure; however, in this text we will be consistent with chemists, physicists and biologists and use only the mole as a unit of measure.
2.1.1 Molecular mass
Here we follow the SI convention concerning the definition of molecular mass which is:
mass of the substance
amount of the substance
Continuing with the SI system, we represent the mass of a substance in kilograms and the amount of the substance in moles. For the case of carbon‐12 identified in Table 2‐1, this leads to
molecular mass of carbon‐12
Using the compact notation indicated in Table 2‐1, we express this result as 0.012 kg
in which the symbol MW is based on the historical use of molecular weight to describe the molecular mass. The molecular mass of carbon‐12 can also be expressed in terms of grams leading to
While Eq. 2‐3 represents the molecular mass in the preferred SI system of units, the form given by Eq. 2‐4 is extremely common, and we have used this form to list atomic masses and molecular masses in Tables A1 and A2 of Appendix A.
Energy can be described in units of kg m2/s2; however, the thermodynamic temperature represents an extremely convenient unit for the description energy and many engineering calculations would be quite cumbersome without it. The same comment applies to the luminous intensity which is an observable that can be assigned a numerical value in terms of the four fundamental standards of length, mass, time and electric charge. One of the attractive features of the SI system is that alternate units are created as multiples and submultiples of powers of 10, and these are indicated by prefixes such as giga for 9
10 , centi for
10 , nano
10 , etc. Some of these alternate units are listed in Table 2‐2 for the meter.
NIST (National Institute of Standards and Technology) provides a more extensive list of prefixes. In other systems of units, multiples of 10 are not