Ch.15 - Acid and Base EquilibriumSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

Associated with any weak acid or weak base is a Ka or Kb value respectively. 

Acid and Base Dissociation Constants.

Concept #1: Understanding Ka and Kb Expressions.

Weak acids possess a Ka value less than 1, while weak bases possess a Kb value less than 1. The equilibrium expressions of Ka and Kb are the same as other equilibrium constants we’ve seen. 

Concept #2: Understanding Ka and Kb. 

The greater the Ka value then the stronger the acid, while the greater the Kb the stronger the base. Ka and Kb are connected by the following equation: 

Practice: If the Kb of NH3 is 1.76 x 10-5, determine the acid dissociation constant of its conjugate acid. 

Example #1: Knowing that HF has a higher Ka value than CH3COOH, determine, if possible, in which direction the following equilibrium lies.

 

HF (aq) + CH3COO (aq) ⇌ F (aq) + CH3COOH (aq)

 

a)  Equilibrium lies to the left.

b)  Equilibrium lies to the right.

c)  Equilibrium is equal and balanced.

d)  Not enough information given. 

Example #2: What is the equilibrium constant for the following reaction and determine if reactants or products are favored. 

 

       HCN (aq)  +  ClO2 (aq)  ⇌ CN (aq)  +  HClO2 (aq)

 

The acid dissociation constant of HCN is 4.9 x 10-10 and the acid dissociation of HClO2 is 1.1 x 10-2.

       HCN (aq)  +  H2O (aq)  ⇌ CN (aq)  +  H3O+ (aq)

       HClO2 (aq)  +  H2O (aq)  ⇌ ClO2 (aq)    +  H3O+ (aq)

Example #3: Which of the following solutions will have the lowest pH?

  1. 0.25 M HC2F3O2
  2. 0.25 M HIO4
  3. 0.25 M HC3H5O3
  4. 0.25 M H2CO3
  5. 0.25 M HSeO4

Practice: Which Bronsted-Lowry base has the greatest concentration of hydroxide ions? 

Practice: Which Bronsted-Lowry acid has the weakest conjugate base?

Additional Problems
Given the list of weak acids and their K a values, which is the  strongest weak acid? a. HCN; 4.9 x 10 -10 b. HClO; 2.9 x 10-8 c. HIO; 2.3 x 10-11 d. HCO2H; 1.8 x 10-4 e. CH3CO2H; 1.8 x 10-5  
Which of the following bases is the STRONGEST? The base is followed by its K b. A) (CH3CH2)2NH, 8.6 × 10-4 B) CH3NH2, 4.4 × 10-4 C) C6H5NH2, 4.0 × 10-10 D) NH3, 1.76 × 10-5 E) C5H5N, 1.7 × 10-9
Which of the following compounds has the strongest conjugate acid? a) C6H5NH2        (Kb = 3.9 x 10 -10) b) (CH3)3N        (Kb =  6.4 x 10 -5) c)  NH3               (Kb =  1.75 x 10 -5) d)  CO32-           (Kb =  1.8 x 10 -4)
Which of the following statements is/ are correct. i. The ethylammonium ion, C2H5NH3+, has a pKa = 10.75 ii. The hypobromite ion, BrO - has a pKb = 8.55 iii. The weaker the weak acid, the stronger its conjugate base. A. i only B. ii only C. iii only D. i and ii E. i and iii
The equilibrium constant for autoionization of water (Kw) is always 1.0 X 10 -14.  a. True b. False
The base-dissociation constant, K b, for pyridine (C 5H 5N) is 1.4 × 10 −9.  The acid-dissociation constant, K a, for the pyridinium ion (C 5H 5NH +) is __________. A) 1.4 × 10 −23      B) 1.0 × 10 −7      C) 7.1 × 10 −4     D) 7.1 × 10 −6      E) 1.4 × 10 −5
Calculate the value of Kb for the conjugate of the weak acid HNO 2. A. 4.6 x 10-4 B. 4.2 x 1010 C. 2.2 x 10-11 D. 4.6 x 10-18 E. 1.2 x 10-17
Which of the following bases is the  STRONGEST? The base is followed by its Kb. A) (CH3CH2)2NH, 8.6 x 10-4 B) CH3NH2, 4.4 x 10-4 C) C6H5NH2, 4.0 x 10-10 D) NH3, 1.76 x 10-5 E) C5H5N, 1.7 x 10-9
Determine the Kb for CN1- at 25°C. The Ka for HCN is 4.9 x 10-10. A) 4.9 x 10-14 B) 2.3 x 10-9 C) 1.4 x 10-5 D) 2.0 x 10-5 E) 3.7 x 10-7
Which of the following solutions has the highest pH? A) 0.40 M CH3COOH B) 0.40 M HClO4 C) 0.40 M HCOOH
In a triprotic acid, which Ka has the highest value? A) Ka1 B) Ka2 C) Ka3 D) Kb1 E) Kb2
Which below correctly describes the relationship amongst the successive p Ka and Kb values for most polyprotic acids?  a) pKa1 > pKa2 > pKa3 b) pKa3 > pKa2 > pKa1 c) Kb1 > Kb2 > Kb3 d) Ka1 < Kb1 e) cannot be predicted
Consider two aqueous solutions of equal concentration: chlorous acid (HClO2, Ka = 1.1 x10 -2 ) and phenol (HC6H5O, Ka = 1.3 x10 -10) Which statement is true? A) HClO2 produces more [H3O+] than HC6H5O B) HClO2 is basic compared with HC6H5O C) HClO2 produces less [H3O+ ] than HC6H5O D) HClO2 is a strong acid E) ClO2– produces more [OH−] than C6H5O–
Which of the following is the strongest base? A) CH3NH2 , Kb = 4.4 x 10-4 B) C5H5N, Kb = 1.7 x 10-9 C) H2NCONH2, Kb = 1.5 x 10-14 D) NH3, Kb = 1.8 x 10-5 E) C2H5NH2, Kb = 5.6 x 10-4
At 0 degrees C, the ion product constant of water is 1.2x10 –15. The pH of pure water at this temperature is: 6.88 7.00 7.46 7.56
The ion-product constant for water, Kw, is 1.0 x 10 -14 at 25 oC and 3.8 x 10 -14 at 40 oC. Is the auto ionization of water endothermic or exothermic?
The following equation represents what property of water? H2O ⇌ H + + OH –   a. it is oxidizing b. it is reducing c. it auto-ionizes d. it is both oxidizing and reducing e. it disproportionates
In a triprotic acid which Ka value is largest in value? a) Ka1 b) Ka2 c) Ka3 d) They are all equal. 
Given the Ka values (in parentheses) for the following acids: HCl (Ka is very large) HNO2 (Ka = 4.0 x 10 –4 ); HF (Ka = 7.2 x 10 –4 ) CH3COOH (Ka = 1.8 x 10 –5 ) HCN (Ka = 6.2 x 10 –10) C6H5COOH (Ka = 6.3 x 10 –5 ) Rank (or list) these acids in order of decreasing strength (strongest first). A. HCl > HF > HNO2 > C6H5COOH > CH3COOH > HCN B. HCl > HF > HNO2 > CH3COOH > C6H5COOH > HCN C. HCN > CH3COOH > C6H5COOH > HNO2 > HF > HCl D. HCl > HNO2 > HF > CH3COOH > C6H5COOH > HCN E. HCN > C6H5COOH > CH3COOH > HF > HNO2 > HCl
Which of the following relationships is incorrect? Assume 25° C.  
Which of the following statements about the autoprotolysis of water is not true? A. The concentration of protons and hydroxides in pure water is always equal B. The pH of neutral water is about 7 at room temperature C. The process is endothermic D. The measured proton concentration is the same in hot and cold water
Which substance is the strongest acid in the above table? a) HClO b) C6H5COOH c) HNO2 d) C6H5OH e) HCO3–
Which of the following is the strongest base? A) CH3NH2, Kb = 4.4 x 10-4 B) C5H5N, Kb = 1.7 x 10-9 C) H2NCONH2, Kb = 1.5 x 10-14 D) NH3, Kb = 1.8 x 10-5 E) C2H5NH2, Kb = 5.6 x 10-4
What is the value of K b for the formate anion, HCOO-? Ka (HCOOH) = 2.1 x 10-4 A) -2.1 x 10-4 B) 2.1 x 10-4 C) 6.9 x 10-6 D) 4.8 x 10-11
The Ka for HC2H3O2 is 1.8 x 10-5. What is the value of  Kb for C2H3O2-?
Of the following acids, __________ is the strongest acid.  A) HCHO2 B) HClO C) HF D) HOAc E) HOAc and HCHO2
Hydrofluoric acid, HF, is often used to etch glass. What is the value of K b for the fluoride ion, F-?A. 3.5 x 10-18B. 3.5 x 1010C. 1.0 x 10-14D. 7.1 x 10-19E. 2.9 x 10-11
The pka of acetic acid is 4.76 and the pka of trichloroacetic acid, which is used to remove warts, is 0.7. Calculate the dissociation constant of each acid. Which is the stronger acid?
Given the following acids and Ka values:  What is the order of increasing base strength?a. CN-, ClO4-, F-, OAC-b. ClO4-, OAC, CN-, F-c. CN-, F-, OAC-, ClO4-d. ClO4-, F-, OAC-, CN-e. CN-, OAC-, F-, ClO4
The Ka of HCN is 6.2 x 10 -10. What is the Kb value for CN - at 25°C
Which of the following is the correct equilibrium expression for the autoionization of water?
What is the Kb for C2H3O2- if the Ka for HC2H3O2 is 1.8 x 10-5?
What is the correct equation for the ion product of water at 25 °C?  
Given that Ka for HClO is 4.0 x 10-8 at 25°C, What is the value of Kb for ClO- at 25°C? Kb =____? 
a. Given that Ka for HClO is 4.0 × 10-8 at 25°C, what is the value of Kb for ClO– at 25°C? b. Given that Kb for CH3CH2NH2 is 6.3 × 10-4 at 25°C, what is the value of Ka for CH3CH2NH3+ at 25°C?
Ka for HC2H3O2: 1.8 x 10 -5 Ka for HCO3-: 4.3 x 10 -7 Using the Ka's for HC2H3O2 and HCO3, calculate the Kb's for the C2H3O2- and CO32- ions.
The Ka for HCN is 4.9 x 10-10. What is the value of  Kb for CN-?
If you know Kb for ammonia, NH3, you can calculate the equilibrium constant, Ka, for this reaction by the equation: NH4 +  ⇌  NH3  +  H + a) Ka = KwKb b) Ka = Kw / Kb c) Ka = 1 / Kb d) Ka = Kb / Kw
The pKa of formic acid is 3.75. What is the K a of formic acid?
Given: acetic acid, pKa = 4.75 HClO2, pKa = 2.00 HF, pKa = 3.45 The order of these acids from strongest to weakest is a) HClO2 > HF > acetic acid b) acetic acid > HF > HClO 2 c) HF > HClO 2 > acetic acid d) HClO2 > acetic acid > HF e) HF > acetic acid > HClO 2
a. Given that Ka for HCN is 6.2x10-10 at 25°c, what is the value of Kb for CN- at 25°c?b. Given that Kb for NH3 is 1.8x10-5 at 25°c, what is the value of Ka for NH4+ at 25°c?
Which of the following is the correct equilibrium expression for the autoionization of water?A) K = [H2O]2B) K = [OH][H3O]/[H2O]2C) Kw = [OH][H3O]D) Kw = [OH-][H3O+]E) Kw = [H3O+]/[OH-]
a. What is the K a reaction of HCN? The Ka of HCN is 6.2 × 10-10.b. What is Kb value for CN– at 25°C?
Given that Kb for (CH3)3N is 6.3 × 10-5 at 25°C, what is the value of Ka for (CH3)3NH+ at 25 °C?
To find the pKa of X-281, you prepare a 0.084 M test solution of X-281 at 25.0  ∘C. The pH of the solution is determined to be 2.70.a. What is the pKa of X-281?b. What is pKb of the conjugate base of X-281? (Assume 25  ∘C.)
Which of the following is the STRONGEST? The base is followed by its pKb.A) (CH3CH2)2NH,      3.07B) CH3NH2,               3.36C) C6H5NH2,             9.40D) NH3,                      4.75E) C5H5N,                  8.77
The pKa of methanol (CH3OH) is 15.5 and the pKa of ammonia (NH3) is 36. Which is a stronger base CH3O- or NH2-? Briefly explain your answer.
Rank these acids according to their expected p Ka values. From highest to lowest
Rank these acids according to their expected p Ka values.
Rank the basicity of the amine groups in these amino acids according to their expected pKb values.
What is the kb of water?
Formic acid has a pKa of 3.75. What is the ratio of concentrations of formate (the conjugate base) to formic acid (the conjugate acid) at pH 8.25?A) 32,000B) 10,000C) 100,000D) 3,200E) none of these
The pKa of acetic acid, CH3COOH, is 4.76. What is the value of the equilibrium constant Keq, for the following equilibrium? The concentration of water in a dilute aqueous solution is 55 M.CH3COOH + H2O ⇌ H3O+ + CH3COO-
An acid with a low pKa: (a) is a weak acid (b) is a strong acid (c) has a weak conjugate base (d) both b and c  
Given the following acids and Ka values: HClO4                  HOAc                     HCN                    HF 1 x 107                 1.76 x 10-5             4.93 x 10-10         3.53 x 10-4 What is the order of increasing base strength?
The Kb for a weak base is 6.7 times 10 -13. What will be the Ka for its conjugate acid? 
For the simple hydrides, MHn, pKa values decrease in the order: 
a) The Ka for acetic acid (H3CCO2H) is 1.737 times 10^-5. What is the pKa for this acid? b) use the pKa calculated above to determine the pH at which [H3CCO2H] = [H3CCO2]
Given that Ka for HF is 6.3 x 10 at 25 C, what is the value of Kb for F at 25°C?
A monoprotic acid, HA, is dissolved in water: HA ⇌ H+ + A- The equilibrium concentrations of the reactants and products are [HA] = 0.240 M [H+] = 2.00 x 10-4 M [A-] = 2.00 x 10-4 M Calculate the value of pKa for the acid HA.
What does it mean to say that water autoionizes? Water will cause many ionic compounds to dissociate without additional input of energy. Water is polar, like an ion is polar. Therefore it is autoionic. Water is a good conductor of electricity, and when it conducts, it ionizes. A water molecule can donate a proton to another water molecule, forming H 2O+ and OH- in solution. 
What is the Ka reaction of HCN?The Ka of HCN is 6.2 x 10-10. What is the Kb, value for CN- at 25°C? Kb = 
Which of the following organic compounds has the strongest conjugate base? 
Given that Ka for HCN is 6.2 x 10-10 at 25°C, what is the value of Kb for CN- at 25°C? Given that Kb for C6H5NH2 is 1.7 x 10-9 at 25°C, what is the value of Ka for C6H5NH3+ at 25°C?
Complete the Kw expression for the autoionization of water.Kw = 1.00 x 10-14 =  
A monoprotic acid, HA, is dissolved in water HA ⇌ H+ + A- The equilibrium concentrations of the reactants and products are HA 0.150 M [H+] = 3.00 × 10-4 M[A-] = 3.00 × 10-4 M Calculate the value of pka for the acid HA.