AP Chemistry
Mr. Markic
Chapter 15 - Acids and Bases
Acids
Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid.
React with certain metals to produce hydrogen gas.
React with carbonates and bicarbonates to produce carbon dioxide gas.
Bases
Have a bitter taste.
Feel slippery. Many soaps contain bases.
A Brønsted acid is a proton donor
A Brønsted base is a proton acceptor
NH3(g) +

H2O(l)
NH4+
OH-
+
Sample Exercise
Identify the conjugate acid-base pairs in the reaction between ammonia and hydrofluoric acid in aqueous solution
NH3(aq) + HF(aq)  NH4+ (aq) + F-(aq)
CN- + H2O  HCN + OH-
Review of Concepts
Which of the following does not constitute a conjugate acid-base pair?
(a) HNO2 - NO1−
2
(b) H2CO3 - CO2−
3
(c) CH3NH3+1 - CH3NH2
Acid-Base Properties of Water
H2O (l)  H+ (aq) + OH- (aq)
autoionization of water
H
O
+
[
O
H
H
H
+
H
]
+
H
O
-
H
H
H2O
O
+
H2O

H3O+
+
OH-
The Ion Product of Water
H2O (l)  H+ (aq) + OH- (aq)
Kc =
[H+ ][OH− ]
[H2 O]
[H2O] = constant
Kc[H2O] = Kw = [H+][OH-]
Page 1 of 12
AP Chemistry
The ion-product constant (Kw) is the product of the molar concentrations of H+ and OH- ions at a particular
temperature.
At 250C
Kw = [H+][OH-] = 1.0 x 10-14
[H+] = [OH-]
[H+] > [OH-]
[H+] < [OH-]
Mr. Markic
Solution Is
neutral
acidic
basic
Sample Exercise
The concentration of OH- ions in a certain household ammonia cleaning solution is 0.0025 M. Calculate the
concentration of H+ ions.
What is the concentration of OH- ions in a HCl solution whose hydrogen ion concentration is 1.3 M?
pH – A Measure of Acidity
Solution Is
neutral
acidic
basic
+
-
[H ] = [OH ]
[H+] > [OH-]
[H+] < [OH-]
pH = -log [H+]
At 250C
[H+] = 1.0 x 10-7
[H+] > 1.0 x 10-7
[H+] < 1.0 x 10-7
pH = 7
pH < 7
pH > 7
pH 
[H+] 
Other important relationships
pOH = -log [OH-]
[H+][OH-] = Kw = 1.0 x 10-14
-log [H+] – log [OH-] = 14.00
pH + pOH = 14.00
Sample Exercise
The concentration of H+ ions in a bottle of table wine was 3.2 x 10-4 M right after the cork was
removed. Only half of the wine was consumed. The other half, after it had been standing open
to the air for a month, was found to have a hydrogen ion concentration equal to 1.0 x 10-3 M.
Calculate the pH of the wine on these two occasions.
Nitric acid (HNO3) is used for the production of fertilizer, dyes, drugs, and explosives. Calculate the pH of a HNO3
solution having a hydrogen ion concentration of 0.76 M
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AP Chemistry
Mr. Markic
The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82.
Calculate the H+ ion concentration of the rainwater.
The pH of a certain orange juice is 3.33. Calculate the H+ ion.
In a NaOH solution [OH-] is 2.9 x 10-4 M. Calculate the pH of the solution.
The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood?
Strong Electrolyte – 100% dissociation
H2 O
NaCl (s) →
Na+ (aq) + Cl- (aq)
Weak Electrolyte – not completely dissociated
CH3COOH  CH3COO- (aq) + H+ (aq)
Strong Acids are strong electrolytes
HCl (aq) + H2O (l)  H3O+ (aq) + Cl- (aq)
HNO3 (aq) + H2O (l)  H3O+ (aq) + NO3- (aq)
HClO4 (aq) + H2O (l)  H3O+ (aq) + ClO4- (aq)
H2SO4 (aq) + H2O (l)  H3O+ (aq) + HSO4- (aq)
Weak Acids are weak electrolytes
HF (aq) + H2O (l)  H3O+ (aq) + F- (aq)
HNO2 (aq) + H2O (l)  H3O+ (aq) + NO2- (aq)
HSO4- (aq) + H2O (l)  H3O+ (aq) + SO42- (aq)
H2O (l) + H2O (l)  H3O+ (aq) + OH- (aq)
Strong Bases are strong electrolytes
Weak Bases are weak electrolytes
H2 O
+
NaOH (s) →
H2 O
KOH (s) →
Na (aq) + OH (aq)
K+ (aq) + OH- (aq)
H2 O
Ba(OH)2 (s) →
-
F- (aq) + H2O (l)  OH- (aq) + HF (aq)
NO2- (aq) + H2O (l)  OH- (aq) + HNO2 (aq)
Ba2+ (aq) + 2OH- (aq)
Conjugate acid-base pairs:
• The conjugate base of a strong acid has no measurable strength.
• H3O+ is the strongest acid that can exist in aqueous solution.
• The OH- ion is the strongest base that can exist in aqueous solution.
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AP Chemistry
Strong Acid (HCl)
Mr. Markic
Weak Acid (HF)
Sample Exercise
Calculate the pH of a
(a) 1.0 x 10-3 M HCl solution
(b) 0.020 M Ba(OH)2 solution
What is the pH of a 1.8 x 10-2 M Ba(OH)2 solution?
Predict the direction of the following reaction in aqueous solution:
HNO2(aq) + CN-(aq)  HCN(aq) + NO2- (aq)
Predict whether the equilibrium constant for the following reaction is greater than or smaller than 1:
CH3COOH(aq) + HCOO-(aq)  CH3COO-(aq) + HCOOH(aq)
Review of Concepts
(a) List in order of decreasing concentration of all the ionic and molecular species in the following acid solutions:
(i)
HNO3
(ii)
HF
(b) List in order of decreasing concentration of all the ionic and molecular species in the following base solutions:
(i)
NH3
(ii)
KOH
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AP Chemistry
Weak Acids (HA) and Acid Ionization Constants
Mr. Markic
[H+ ][A− ]
HA (aq) + H2O (l)  H3O+ (aq) + A- (aq)
Ka = =
HA (aq)  H+ (aq) + A- (aq)
Ka is the acid ionization constant
[HA]
Ka weak acid
 strength
Solving weak acid ionization problems:
1. Identify the major species that can affect the pH.
• In most cases, you can ignore the
autoionization of water.
• Ignore [OH-] because it is determined by [H+].
2. Use ICE to express the equilibrium
concentrations in terms of single unknown x.
3. Write Ka in terms of equilibrium concentrations.
Solve for x by the approximation method. If
approximation is not valid, solve for x exactly.
4. Calculate concentrations of all species and/or pH
of the solution.
Sample Exercise
A 0.0560-g quantity of acetic acid is dissolved in
enough water to make 50.0 mL of solution.
Calculate the concentrations of H+, CH3COO-1, and
CH3COOH (60 g/mol) at equilibrium. (Ka for acetic acid = 1.8 x 10-5.)
Practice Exercise
Calculate the percent ionization and pH of 0.20 M benzoic acid:
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AP Chemistry
The pH of a 0.10 M solution of formic acid (HCOOH) is 2.39. What is the Ka of the acid?
Mr. Markic
The pH of a 0.060 M weak monoprotic acid is 3.44. Calculate the Ka of the acid.
percent ionization =
ionized acid concentration at equilibrium
initial concentration of acid
x 100%
For a monoprotic acid HA,
Percent ionization =
[𝐻 + ]
[𝐻𝐴]0
x 100%
[HA]0 = initial concentration
Weak Bases and Base Ionization Constants
NH3 (aq) + H2O (l)  NH4+ (aq) + OH- (aq)
Kb =
−
[NH+
4 ][OH ]
[NH3 ]
Kb weak base  strength
Solve weak base problems like weak acids except
solve for [OH-] instead of [H+].
Kb is the base ionization constant
Sample Exercise
What is the pH of a 0.40 M ammonia solution?
Page 6 of 12
AP Chemistry
Calculate the pH of a 0.26 M methylamine solution (see Table 15.4)
Mr. Markic
Ionization Constants of Conjugate Acid-Base Pairs
HA (aq)

H+ (aq) + A- (aq)
Weak Acid and Its Conjugate Base
Ka
Ka =
𝐾𝑤
𝐾𝑏
Kb =
𝐾𝑤
𝐾𝑎
A- (aq) + H2O (l)  OH- (aq) + HA (aq) Kb
H2O (l)  H+ (aq) + OH- (aq) Kw
KaKb = Kw
Review of Concepts
Consider the following two acids and their ionization constants:
HCOOH
HCN
Ka = 1.7 x 10-4
Ka = 4.9 x 10-10
Which conjugate base (HCOO- or CN-) is stronger?
Diprotic and Triprotic Acids
• May yield more than one hydrogen ion per molecule.
• Ionize in a stepwise manner; that is, they lose one proton at a time.
• An ionization constant expression can be written for each ionization stage.
• Consequently, two or more equilibrium constant expressions must often be used to calculate the concentrations of
species in the acid solution.
H2CO3(aq)  H+(aq) + HCO3-(aq)
Ka1 =
[𝐻 + ][𝐻𝐶𝑂3− ]
[𝐻2 𝐶𝑂3 ]
HCO3-(aq)  H+(aq) + CO3-2(aq)
Ka2 =
[𝐻 + ][𝐶𝑂3−2 ]
[𝐻𝐶𝑂3− ]
H2CO3
HCO3-
CO3-2
Sample Exercise
Calculate the concentrations of H+, HCO3-, and CO32- in a 0.025 M H2CO3
Page 7 of 12
AP Chemistry
Calculate the concentrations of all species in a 0.100 M H3PO4
Mr. Markic
Review of Concepts
Which of the following diagrams shown here represents a
solution of sulfuric acid? Water molecules have been
omitted for clarity.
Molecular Structure and Acid Strength
The
stronger
the bond
The
weaker
the acid
Acidity Increases
H – X  H+ + X
HF << HCl < HBr < HI
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AP Chemistry
Molecular Structure and Oxoacid Strength
Z – Oδ- – Hδ+

Mr. Markic
Z – O- + H+
The O-H bond will be more polar and easier to break if:
• Z is very electronegative or
• Z is in a high oxidation state
1. Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number.
Acid strength increases with increasing electronegativity of Z
O
Cl is more electronegative than Br
••
••
••
••
••
O
••
••
H O Br O
••
••
HClO3 > HBrO3
••
••
••
••
H O Cl O
•• •• ••
2. Oxoacids having the same central atom (Z) but different numbers of attached groups.
Acid strength increases as the oxidation number of Z increases.
HClO4 > HClO3 > HClO2 > HClO
Sample Exericse
Predict the relative strengths of the oxoacids in each of the following
groups:
(a) HClO, HBrO, and HIO
(b) HNO3 and HNO2
Which of the following acids is weaker: HClO2 or HClO3?
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AP Chemistry
Mr. Markic
Acid-Base Properties of Salts
Neutral Solutions:
Salts containing an alkali metal or alkaline earth metal ion (except Be2+) and the conjugate base of a strong acid (e.g. Cl-,
Br-, and NO3-).
H2 O
NaCl (s) →
Na+ (aq) + Cl- (aq)
Basic Solutions:
Salts derived from a strong base and a weak acid.
H2 O
CH3COONa (s) →
Na+ (aq) + CH3COO- (aq)
CH3COO- (aq) + H2O (l) 
CH3COOH (aq) + OH- (aq)
Sample Exericse
Calculate the pH of a 0.15 M solution of sodium acetate (CH3COONa). What is the percent hydrolysis?
Calculate the pH of a 0.24 M sodium formate solution (HCOONa).
Acid-Base Properties of Salts
Acid Solutions:
Salts derived from a strong acid and a weak base.
H2 O
NH4Cl (s) →
NH4+ (aq) + Cl- (aq)
NH4+ (aq)  NH3 (aq) + H+ (aq)
Acid-Base Properties of Salts
Solutions in which both the cation and the anion hydrolyze:
• Kb for the anion > Ka for the cation, solution
will be basic
• Kb for the anion < Ka for the cation, solution
will be acidic
• Kb for the anion  Ka for the cation, solution
will be neutral
Page 10 of 12
AP Chemistry
Sample Exercise
Predict whether the following solutions will be acidic, basic, or nearly neutral:
(a) NH4I
(c) FeCl3
(b) NaNO2
(d) NH4F
Mr. Markic
Predict whether the following solutions will be acidic, basic, or nearly neutral:
(a) LiClO4
(c) Bi(NO3)2
(b) Na3PO4
(d) NH4CN
Review of Concepts
The diagrams shown here represent solutions of three salts NaX (X = A, B, or C).
(a) Which X- has the weakest conjugate acid?
(b) Arrange the three X- anions in order of increasing base
strength. The Na+ ion and water molecules have been
omitted for clarity.
Oxides of the Representative Elements in Their Highest Oxidation States
Na2O (s) + H2O (l)  2NaOH (aq)
CO2 (g) + H2O (l) 
H2CO3 (aq)
N2O5 (g) + H2O (l)  2HNO3 (aq)
Page 11 of 12
AP Chemistry
Mr. Markic
Big Idea 6: Any bond or intermolecular attraction that can be formed can be broken. These two processes are in
dynamic competition, sensitive to initial conditions and external perturbations.
Learning Objectives: 1.20, 3.7, 6.11, 6.12, 6.13, 6.14, 6.15, 6.16, 6.17, 6.18, 6.19, 6.20
Duration: Middle March/Early April
Textbook Chapter: 15
Enduring Understanding
Essential Knowledge
6.C: Chemical equilibrium plays an
6.C.1: Chemical equilibrium reasoning can be used to describe the
important role in acid-base chemistry and
proton-transfer reactions of acid-base chemistry.
in solubility.
6.C.2: The pH is an important characteristic of aqueous solutions that
can be controlled with buffers. Comparing pH to pKa allows one to
determine the protonation state of a molecule with a labile proton.
Learning Objectives
1.20 The student can design, and/or interpret data from, an experiment that uses titration to determine the
concentration of an analyte in a solution.
3.7 The student is able to identify compounds as Bronsted-Lowry acids, bases, and/or conjugate acid-base pairs,
using proton-transfer reactions to justify the identification.
6.11 The student can generate or use a particulate representation of an acid (strong or weak or polyprotic) and a
strong base to explain the species that will have large versus small concentrations at equilibrium.
6.12 The student can reason about the distinction between strong and weak acid solutions with similar values of pH,
including the percent ionization of the acids, the concentrations needed to achieve the same pH, and the amount of
base needed to reach the equivalence point in a titration.
6.14 The student can, based on the dependence of Kw on temperature, reason that neutrality requires [H+] = [OH–]
as opposed to requiring pH = 7, including especially the applications to biological systems.
6.15 The student can identify a given solution as containing a mixture of strong acids and/or bases and calculate or
estimate the pH (and concentrations of all chemical species) in the resulting solution.
6.16 The student can identify a given solution as being the solution of a monoprotic weak acid or base (including
salts in which one ion is a weak acid or base), calculate the pH and concentration of all species in the solution,
and/or infer the relative strengths of the weak acids or bases from given equilibrium concentrations.
6.17 The student can, given an arbitrary mixture of weak and strong acids and bases (including polyprotic systems),
determine which species will react strongly with one another (i.e., with K >1) and what species will be present in
large concentrations at equilibrium.
6.18 The student can design a buffer solution with a target pH and buffer capacity by selecting an appropriate
conjugate acid-base pair and estimating the concentrations needed to achieve the desired capacity.
6.19 The student can relate the predominant form of a chemical species involving a labile proton (i.e.,
protonated/deprotonated form of a weak acid) to the pH of a solution and the pKa associated with the labile
proton.
6.20 The student can identify a solution as being a buffer solution and explain the buffer mechanism in terms of the
reactions that would occur on addition of acid or base.
Page 12 of 12