# 2.2: Untitled Page 14

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## Chapter 2

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

*Quantity *

*Name *

*Symbol *

* Definition *

The meter is the length of the path

traveled by light in vacuum during a

length

meter

m

time interval of 1/299,792,458 of a second.

The kilogram is the unit of mass equal to

mass

kilogram

kg

the international prototype of the

kilogram.

The second is the duration of 9,192,631,770

periods of the radiation corresponding to

time

second

s

the transition between the two hyperfine

levels of the ground state of the cesium 133

atom.

The ampere is that constant current which,

if maintained in two straight parallel

conductors of infinite length, of negligible

electric

ampere

A

circular cross section, and placed 1 meter

current

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

temperature

kelvin

K

the thermodynamic temperature of the

triple point of water.

The mole is the amount of substance of a

elemental

mole

mol

system which contains as many elementary

entities

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

luminous

candela

cd

monochromatic radiation of frequency

intensity

540x1012 hertz and that has a radiant

intensity in that direction of 1/683 watt per

steradian.

15

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

molecular mass

(2‐1)

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

0.012 kilogram

molecular mass of carbon‐12

(2‐2)

mole

Using the compact notation indicated in Table 2‐1, we express this result as 0.012 kg

*MW*

12

(2‐3)

C

mol

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

12 g

*MW*

12

(2‐4)

C

mol

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

2

10 , *nano*

for

9

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