1. Chapter 10 Acids and Bases
10.3 Ionization of Water

2. 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 + H2O H3O+ + OH−
H:O: + H:O: H:O:H+ + :O:H−
H H H
water water hydronium hydroxide ion(+) ion(-)

3. Pure Water is Neutral In pure water,
the ionization of water molecules produces small but equal quantities of H3O+ and OH− ions
molar concentrations are indicated in brackets as [H3O+] and [OH−]
[H3O+] = 1.0 x 10−7 M
[OH−] = 1.0 x 10−7 M

4. Acidic Solutions Adding an acid to pure water increases the [H3O+]
causes the [H3O+] to
exceed 1.0 x 10−7 M
decreases the [OH−]

5. Basic Solutions Adding a base to pure water, increases the [OH−]
causes the [OH−] to exceed 1.0 x 10−7 M
decreases the [H3O+]

6. Comparison of [H3O+] and [OH−]

7. Ion Product of Water, Kw The ion product constant, Kw, for water
is the product of the concentrations of the hydronium and hydroxide ions
Kw = [ H3O+][ OH−]
is obtained from the concentrations in pure water
Kw = [1.0 x 10−7 M][1.0 x 10−7 M]
= x 10−14

8. [H3O+] and [OH−] in Solutions
In neutral, acidic, or basic solutions, the Kw at 25 °C is
always 1.0 x 10−14.

9. Guide to Calculating [H3O+]

10. Example of 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− ] = 1.0 x 10−14
STEP 2 Rearrange the Kw expression for [H3O+]:
[H3O+] = 1.0 x 10−14
[OH−]
STEP 3 Substitute the known [OH−] and calculate:
[H3O+] = 1.0 x 10−14 = 2.0 x 10−7 M
[5.0 x 10−8]

11. Learning Check
If lemon juice has [H3O+] of 2 x 10−3 M, what is the [OH−] of the solution?
1) 2 x 10−11 M
2) 5 x 10−11 M
3) 5 x 10−12 M

12. Solution 3) 5 x 10−12 M STEP 1 Write the Kw for water:
Kw = [H3O+][OH−] = 1.0 x 10−14
STEP 2 Rearrange the Kw expression for unknown [OH−]:
[OH−] = 1.0 x 10−14
[H3O+ ]
STEP 3 Substitute the known [H3O+] and calculate:
[OH−] = 1.0 x 10−14 = 5 x 10−12 M
[2 x 10−3 ]

13. Learning Check
The [OH−] of an ammonia solution is 4.0 x 10−2 M. What is the [H3O+ ] of the solution?
1) 2.5 x 10−11 M
2) 2.5 x 10−12 M
3) 2.5 x 10−13 M

14. Solution 3) 2.5 x 10−13 M STEP 1 Write the Kw for water:
Kw = [H3O+ ][OH− ] = 1.0 x 10−14
STEP 2 Rearrange the Kw expression for [H3O+]:
[H3O+] = 1.0 x 10−14
[OH−]
STEP 3 Substitute the known [OH−] and calculate:
[H3O+] = 1.0 x 10−14 = 2.5 x 10−13 M
4.0 x 10−2