Water/Wastewater Laboratory Calculations - Basic Mathematics for Water and Wastewater Operators - page 237

June 4 205 mg/L

June 5 222 mg/L

June 6 214 mg/L

June 7 218 mg/L

Solution:

7-day average BOD = (200 + 210 + 204 + 205 + 222 + 214 + 218) ÷ 7 = 210 mg/L

MOLES AND MOLARITY
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 compound always con-
tains 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.

Note: Molecular weight is the weight of one molecule. It is calculated by add-
ing the weights of all of 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
one molecule in daltons. All moles contain the same number of molecules
(Avogadro's number), equal to 6.022 × 10 23 . The reason why all moles have the
same number of molecules is because the value of the mole is proportional to
the molecular weight.

M oles
A mole is a quantity of a compound equal in weight to its formula weight; for exam-
ple, the formula weight for water (see Figure 16.1) can be determined using the
Periodic Table of Elements:

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:

Moles = Grams of chemical ÷ Formula weight of chemical (16.9)