Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 398

Thus, when measuring contaminant concentrations, we must know the atmospheric temperature
and pressure under which the samples were taken. At standard temperature and pressure (STP),
1 g-mol of an ideal gas occupies 22.4 L. The STP are 0°C and 760 mmHg. If the temperature is
increased to 25°C (room temperature) and the pressure remains the same, 1 g-mol of gas occupies
24.45 L.
Sometimes it is necessary to convert milligrams per cubic meter (mg/m 3 )—a weight-per-volume
ratio—into a volume-per-unit weight ratio. If it is understood that 1 g-mol of an ideal gas at 25°C
occupies 24.45 L, the following relationships can be calculated:

24.45
Molecularweight

mg/m 3 ppm = (15.2)

Molecularweight
24.45

3 =

mg/m ppm (15.3)

15.5.2.1 Gas Laws
As mentioned, gases can be pollutants as well as the conveyors of pollutants. Air (which is mainly
nitrogen) is usually the main gas stream. To understand the gas laws, it is imperative that we have
an understanding of various terms:

• Ideal gas —An imaginary model of a gas that has a few very important properties. First,
the gases are assumed to be infinitely small. Second, the particles move randomly in
straight lines until they collide into something (another gas molecule or the side of what-
ever container they are in). Third, the gas particles do not interact with each other (they do
not attract or repel one another like real molecules do). Finally, the energy of the particles
is directly proportional to the temperature in Kelvin (in other words, the higher the tem-
perature, the more energy the particles have). These assumptions are made because they
make equations a lot simpler than they would be otherwise, and because these assumptions
cause negligible deviation from the ways that actual gases behave.
• Kelvin —A temperature scale in which the degrees are the same size as degrees Celsius but
where 0 is defined as absolute zero, the temperature at which molecules are at their lowest
energy. To convert from degrees Celsius to Kelvin (not degrees Kelvins), add 273.
• Pressure —A measure of the amount of force that a gas exerts on whatever container we
put it into. Units of pressure include atmospheres (1 atm is the average atmospheric pres-
sure at sea level), Torr (equal to 1/760 of an atmosphere), millimeters of mercury (1 mmHg
= 1 Torr = 1/760 atm), and kilopascals (101,325 kPa = 1 atm).
• Standard temperature and pressure —A set of conditions defined as 273 K and 1 atm.
• Standard conditions (SC) —More commonly used than STP, standard conditions are typi-
cal room conditions of 20°C (70°F) and 1 atm. SC units of volume are commonly given as
normal cubic meters, or standard cubic feet (scf).
• Temperature —A measure of how much energy the particles in a gas have. It is defined as
that property of a body which determines the flow of heat. Heat will flow from a warm
body to a cold body.
• Volume —The amount of space that some object occupies. The unit of volume can be cubic
centimeters (cc, cm 3 ), milliliters (mL; 1 mL = 1 cm 3 ), liters (L; 1 L = 1000 mL), or cubic
meters (m 3 ; 1 m 3 = 1 million cm 3 ).

There are several types of temperature scales in general use. These scales depend on the freezing
and boiling points of water as boundary markers for the scale. In a conventional laboratory ther-
mometer, the boundary points are conveniently selected to relate to the known properties of water.