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2.2: Untitled Page 14

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    18147
  • 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.

    Units

    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

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