Wastewater Calculations - Mathematics and Statistics for the Environment - page 746

■ EXAMPLE 24.141
Problem: Given the following primary effluent BOD test results, calculate the 7-day average:

June 1 200 mg/L

June 2 210 mg/L

June 3 204 mg/L

June 4 205 mg/L

June 5 222 mg/L

June 6 214 mg/L

June 7 218 mg/L

Solution:

( ) =

200 ++++++ 18

7

210 204 205 222 214 2

7-dayaverage BOD = 210 mg/L

24.15.4 m ole and m olarity C alCulations
Chemists have defined a very useful unit called the mole. Moles and molarity, a concentration term
based on the mole, have many important applications in water/wastewater operations. A mole is
defined as a gram molecular weight; that is, the molecular weight expressed as grams. For example,
a mole of water is 18 g of water, and a mole of glucose is 180 g of glucose. A mole of any com-
pound always contains the same number of molecules. The number of molecules in a mole is called
Avogadro's number and has a value of 6.022 × 10 23 .

Note: How big is Avogadro's number? An Avogadro's number of soft drink cans would cover the
surface of the Earth to a depth of over 200 miles.

Key Point: Molecular weight is the weight of one molecule. It is calculated by adding the weights
of all the atoms that are present in one molecule. The units are atomic mass units (amu). A mole is
a gram molecular weight—that is, the molecular weight expressed in grams. The molecular weight
is the weight of 1 molecule in daltons. All moles contain the same number of molecules (Avogadro's
number), equal to 6.022 × 10 23 . The reason all moles have the same number of molecules is because
the value of the mole is proportional to the molecular weight.

24.15.4.1 Moles
A mole is a quantity of a compound equal in weight to its formula weight; for example, the formula
weight for water can be determined using the periodic table of elements (see Figure 24.11):

Hydrogen (1.008) × 2 = 2.016

+ Oxygen = 16.000

Formula weight of H 2 O = 18.016

Because the formula weight of water is 18.016, a mole is 18.016 units of weight. A gram-mole is 18.016
grams of water. A pound-mole is 18.016 pounds of water. For our purposes in this text, the term mole
will be understood to mean gram-mole. The equation used to determine moles is shown below:

Gramsofchemical
Formulaweight of ch

Moles = (24.176)

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