Geoscience Reference
In-Depth Information
H
+
O
-
H
+
FIGURE 24.11
A molecule of water.
■
EXAMPLE 24.142
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:
Gramsofchemical
Formulaweight of ch
66 g
35 g/mol
Moles
=
=
=
1.9 moles
emical
The molarity of a solution is calculated by taking the moles of solute and dividing by the liters of
solution:
Molesofsolute
Liters of solution
Molarity
=
(24.177)
■
EXAMPLE 24.143
Problem:
What is the molarity of 2 moles of solute dissolved in 1 L of solvent?
Solution:
2moles
1L
Molarity
=
=
M
2
Key Point:
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).
24.15.4.2 Normality
The
molarity
of a solution refers to its concentration (the solute dissolved in the solution). The
nor-
mality
of a solution refers to the number of
equivalents
of solute per liter of solution. The definition
of chemical equivalent depends on the substance 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 amounts. Keeping in mind that normality is a
measure of the reacting power of a solution (i.e., 1 equivalent of a substance reacts with 1 equivalent
of another substance), we use the following equation to determine normality:
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