Water/Wastewater Laboratory Calculations - Basic Mathematics for Water and Wastewater Operators - page 238

H +

H 2 O

O -

H +

FIGURE 16.1 A molecule of water.

■ Example 16.9
Problem: The atomic weight of a certain chemical is 66. If 35 g of the chemical are
used in making up a 1-L solution, how many moles are used?

Solution:

Grams of chemical ÷ Formula weight of chemical = 66 g ÷ 35 g/mole = 1.9 moles

The molarity of a solution is calculated by taking the moles of solute and dividing
by the liters of solution:

Molarity = Moles of solute ÷ Liters of solution (16.10)

■ Example 16.10
Problem: What is the molarity of 2 moles of solute dissolved in 1 L of solvent?

Solution:

Molarity = 2 moles ÷ 1 L = 2 M

Note: Measurement in moles is a measurement of the amount of a substance.
Measurement in molarity is a measurement of the concentration of a substance—
the amount (moles) per unit volume (liters).

NORMALITY
The molarity of a solution refers to its concentration (the solute dissolved in the
solution). The normality of a solution refers to the number of equivalents of solute
per liter of solution. The definition of chemical equivalent depends on the sub-
stance or type of chemical reaction under consideration. Because the concept of
equivalents is based on the reacting power of an element or compound, it follows
that a specific number of equivalents of one substance will react with the same
number of equivalents of another substance. When the concept of equivalents is
taken into consideration, it is less likely that chemicals will be wasted as excess