1 Chapter 14 Acids and Bases 14.5 Dissociation of Water Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

2 A few water molecules ionize when H + is transferred from one H 2 O molecule to another. One water molecule acts as an acid, while another acts as a base. H 2 O + H 2 O H 3 O + + OH − :O: H + :O:H H:O:H + + :O:H − H HH water molecules hydronium hydroxide ion (+) ion (-) Ionization of Water

3 Pure Water is Neutral In pure water, The ionization of water molecules produces small, but equal quantities of H 3 O + and OH − ions. Molar concentrations are indicated as [H 3 O + ] and [OH − ]. [H 3 O + ] = 1.0 x 10 −7 M [OH − ] = 1.0 x 10 −7 M Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

4 Acidic Solutions Adding acid to pure water Increases the [H 3 O + ]. Causes the [H 3 O + ] to exceed 1.0 x M. Decreases the [OH − ]. Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

5 Basic Solutions Adding base to pure water, Increases the [OH − ]. Causes the [OH − ] to exceed 1.0 x 10 − 7 M. Decreases the [H 3 O + ]. Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

6 Comparison of [H 3 O + ] and [OH − ] Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

7 The ion product constant, K w, for water Is the product of the concentrations of the hydronium and hydroxide ions. K w = [ H 3 O + ] [ OH − ] Can be obtained using the concentrations in pure water. K w = [ H 3 O + ] X [ OH − ] K w = [1.0 x 10 − 7 M] X [ 1.0 x 10 − 7 M] = 1.0 x 10 − 14 Ion Product Constant of Water, K w

8 K w in Acids and Bases In neutral, acidic, and basic solutions, the K w is equal to 1.0 x 10 −14. Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings Table 14.7

9 Guide to Calculating [H 3 O + ] and [OH - ] Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

10 Calculating [H 3 O + ] What is the [H 3 O + ] if [OH - ] is 5.0 x M? STEP 1 Write the K w for water. K w = [H 3 O + ][OH − ] = 1.0 x 10 −14 STEP 2 Rearrange the K w to solve for [H 3 O + ] [H 3 O + ] = 1.0 x [OH − ] STEP 3 Substitute [OH − ] [H 3 O + ] = 1.0 x = 2.0 x M 5.0 x

11 If lemon juice has [H 3 O + ] 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 Learning Check

12 3) 5 x 10 −12 M Rearrange the K w to solve for [OH - ] K w = [H 3 O + ][OH − ] = 1.0 x 10 −14 [OH − ] = 1.0 x = 5 x 10 −12 M 2 x Solution

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

14 3) 2.5 x 10 −13 M [ H 3 O + ] = 1.0 x 10 − x 10 −2 Solution