1. Chapter 14 Acids and Bases
14.5 Ionization of Water
Learning Goal Use the ion product of water to calculate the [H3O+] and [OH−] in an aqueous solution.
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2. H2O(l) + H2O(l) H3O+(aq) + OH−(aq)
Ionization of Water
In pure water,
a forward reaction between two water molecules transfers an H+ from one water molecule to the other
H2O(l) + H2O(l) H3O+(aq) + OH−(aq)
H3O+ and OH− react in the reverse direction to re-form two water molecules
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3. Water, Conjugate Acid−Base Pairs
In the reaction between two water molecules, equilibrium is reached between the conjugate acid–base pairs.
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4. Writing the Ion Product Constant of Water
Using the equation for the reaction in water at equilibrium, we can write its equilibrium constant expression.
H2O(l) + H2O(l) H3O+(aq) + OH−(aq)
Omitting the constant concentration of pure liquid water to give the ion product constant for water, we obtain the Kw.
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5. Writing the Ion Product of Water
Experiments have determined that in pure water, the concentration of H3O+ at 25oC is 1.0 × 10−7 M.
Pure water [H3O+] = 1.0 × 10−7 M
Kw = [H3O+] [OH−]
and [H3O+] = [OH−] = 1.0 × 10−7 M
Kw = [1.0 × 10−7] [1.0 × 10−7] = 1.0 × 10−14
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6. Neutral, Acidic, and Basic Solution
When
H3O+ = OH−, the solution is neutral
H3O+ > OH−, the solution is acidic
H3O+ < OH−, the solution is basic
In neutral, basic, and acidic solutions,
Kw = 1.0 × 10−14
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7. Neutral, Acidic, and Basic Solution
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8. Examples of Neutral, Acidic, and Basic Solutions
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9. Using Kw to Calculate [H3O+] and [OH−]
If we know the [H3O+] of a solution, we can use the Kw to calculate the [OH−].
Kw = [H3O+] [OH−]
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10. Guide to Calculating [H3O+] and [OH−]
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11. What is the [H3O+] if [OH−] is 5.0 × 10−8 M?
Learning Check
What is the [H3O+] if [OH−] is 5.0 × 10−8 M?
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12. What is the [H3O+] if [OH−] is 5.0 × 10−8 M?
Solution
What is the [H3O+] if [OH−] is 5.0 × 10−8 M?
Step 1 State the given and needed quantities.
Given
Need
Know
[OH−] = 5.0 × 10−8 M
[H3O+]
Kw = [H3O+] [OH−]
Kw = 1.0 × 10−14
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13. What is the [H3O+] if [OH−] is 5.0 × 10−8 M?
Solution
What is the [H3O+] if [OH−] is 5.0 × 10−8 M?
Step 2 Write the Kw for water and solve for the unknown [H3O+] or [OH−]
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14. What is the [H3O+] if [OH−] is 5.0 × 10−8 M?
Solution
What is the [H3O+] if [OH−] is 5.0 × 10−8 M?
Step 3 Substitute known [H3O+] or [OH−] into the equation and calculate
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