# 11.5 Dissociation of Water The equilibrium reached between 11.5 Dissociation of Water The equilibrium reached between the conjugate acidbase pairs of water produces both H3O+ and OH. + H2O(l) + H2O(l) + + -

H3O+(aq) + OH(aq) Learning Goal Use the water dissociation constant to calculate the [H3O+] and [OH] in an aqueous solution. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Dissociation Constant of Water, Kw Water is amphotericit can act as an acid or a base. In water, H+ is transferred from one H2O molecule to another. one water molecule acts as an acid, while another acts as a base.

equilibrium is reached between the conjugate acidbase pairs. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Writing the Dissociation Constant, Kw In the equation for the dissociation of water, there is both a forward and a reverse reaction. H2O(l) + H2O(l) Base H3O+(aq) + OH(aq)

Acid Conjugate acid Conjugate base In pure water, the concentrations of H3O+ and OH at 25 C are each 1.0 107 M. Kw [H3O+] = [OH] = 1.0 107 M Kw = [H3O+] [OH]

= (1.0 107 M) (1.0 107 M) = 1.0 1014 at 25 C General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Dissociation Constant, Kw The ion product constant for water, Kw, is defined as the product of the concentrations of H3O+ and OH. equal to 1.0 1014 at 25 C (the concentration units are omitted). When [H3O+] and [OH] are equal, the solution is neutral. [H3O+] is greater than the [OH], the solution is acidic.

[OH] is greater than the [H3O+], the solution is basic. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Using Kw to Calculate [H3O+] and [OH] If we know the [H3O+] of a solution, we can use the Kw to calculate the [OH]. If we know the [OH] of a solution, we can use the Kw to calculate the [H3O+]. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake

2016 Pearson Education, Inc. Pure Water Is Neutral In pure water, the ionization of water molecules produces small but equal quantities of H3O+ and OH ions. [H3O+] = 1.0 107 M [OH] = 1.0 107 M [H3O+] = [OH] Pure water is neutral. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc.

Acidic Solutions Adding an acid to pure water increases the [H3O+]. causes the [H3O+] to exceed 1.0 107 M. decreases the [OH]. [H3O+] > [OH] The solution is acidic. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc.

Basic Solutions Adding a base to pure water increases the [OH] causes the [OH] to exceed 1.0 107 M decreases the [H3O+] [H3O+] < [OH] The solution is basic. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Comparison of [H3O+] and [OH]

General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Neutral, Basic, and Acidic Solutions Core Chemistry Skill Calculating [H3O+] and [OH] in Solutions General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Guide to Calculating [H3O+] and [O] in

Aqueous Solutions General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Calculating [H3O+] What is the [H3O+] of a solution if [OH] is 5.0 108 M? STEP 1 State the given and needed quantities. ANALYZE Given Need Know THE

[OH] = 5.0 108 M [H3O+] Kw = [H3O+][OH] PROBLEM = 1.0 1014 STEP 2 Write the Kw for water and solve for the unknown [H3O+]. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Calculating [H3O+] What is the [H3O+] of a solution if [OH] is 5.0 108 M?

STEP 3 Substitute in the known [H3O+] or [OH] and calculate. Because the [H3O+] of 2.0 107 M is larger than the [OH] of 5.0 108 M, the solution is acidic. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Study Check If lemon juice has [H3O+] of 2.0 103 M, what is the [OH] of the solution? A. 2.0 1011 M

B. 5.0 1011 M C. 5.0 1012 M General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Solution If lemon juice has [H3O+] of 2.0 103 M, what is the [OH] of the solution? STEP 1 State the given and needed quantities. ANALYZE Given Need Know THE

[H3O+] = 2.0 103 M [OH] Kw = [H3O+][OH] PROBLEM = 1.0 1014 STEP 2 Write the Kw for water and solve for the unknown [H3O+] or [OH]. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Solution If lemon juice has [H3O+] of 2.0 103 M, what is the [OH] of the solution?

STEP 3 Substitute in the known [H3O+] or [OH] and calculate. Because the [H3O+] concentration of 2.0 103 M is greater than the [OH] of 5.0 1012 M, the solution is acidic. The answer is C. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc.