General, Organic, and
Biological Chemistry
Fourth Edition
Karen Timberlake
Chapter 10
Acids and Bases
10.3
Ionization of Water
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Lectures
Ionization of Water
In the ionization of water,
 H+ is transferred from one H2O molecule to another.
 one water molecule acts as an acid, while another
acts as a base.
H2O(l) + H2O(l)
H3O+(aq) + OH−(aq)
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Chapter 10 Section 3
2
Ion Product of Water, Kw
The ion product constant, Kw, for water at 25 C
 is the product of the concentrations of the hydronium
and hydroxide ions.
Kw = [ H3O+][ OH−]
 is obtained from the concentrations in pure water.
Kw = [ H3O+][ OH−]
Kw = [1.0 x 10−7 M][1.0 x
= 1.0 x 10−14
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Chapter 10 Section 3
3
Pure Water is Neutral
In pure water, the ionization of
water molecules produces
small but equal quantities of
H3O+ and OH− ions.
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Chapter 10 Section 3
4
Acidic Solutions
Adding an acid to pure water
 increases the [H3O+],
 decreases the [OH−], and
 causes the [H3O+] to
exceed 1.0 x 10−7 M.
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Chapter 10 Section 3
5
Basic Solutions
Adding a base to pure water,
 increases the [OH−],
 decreases the [H3O+], and
 causes the [OH−] to
exceed 1.0 x 10−7 M.
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Chapter 10 Section 3
6
Comparison of [H3O+] and [OH−]
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Chapter 10 Section 3
7
[H3O+] and [OH−] in Solutions
In neutral, acidic, or basic solutions, the Kw at 25 C is
always 1.0 x 10−14.
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Chapter 10 Section 3
8
Guide to Calculating [H3O+] and
[OH−]
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Chapter 10 Section 3
9
Calculating [H3O+]
What is the [H3O+] of a solution if [OH−] is 5.0 x 10−8 M?
Step 1 Write the Kw for water.
Kw = [H3O+ ][OH− ]
Step 2 Solve the Kw for the unknown [H3O+].
[H3O+] = 1.0 x 10−14
[OH−]
Step 3 Substitute the known [OH−] into the equation
and calculate.
[H3O+] = 1.0 x 10
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Chapter 10 Section 3
10
Learning Check
If lemon juice has [H3O+] of 2 x 10−3 M, what is the [OH−]
of the solution?
A. 2 x 10−11 M
B. 5 x 10−11 M
C. 5 x 10−12 M
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Chapter 10 Section 3
11
Solution
If lemon juice has [H3O+] of 2 x 10−3 M, what is the [OH−]
of the solution?
Step 1 Write the Kw for water.
Kw = [H3O+][OH−] = 1.0 x 10−14
Step 2 Solve the Kw for the unknown [OH−].
[OH−] = 1.0 x 10−14
[H3O+ ]
Step 3 Substitute the known [H3O+] into the equation
and calculate.
[2 x 10−3 ]
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Chapter 10 Section 3
Answer is C.
12