Ka and Kb

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

Acid and Base Dissociation Constants.

Concept: Understanding Ka and Kb Expressions.

Video Transcript

Hey guys, in this new video, we're going to take a look at the acid and base constants. As you guys can know from looking at the rules for identifying compounds as either acids or bases, you should realize that there's a lot more weak species out there than there are strong ones. What we need to realize is these weak species have constants associated with them. We're going to say here consider a weak monoprotic HA acid. Monoprotic just means it has one H positive. You were looking at how it ionizes in water. Basically, HA is the acid so it’s going to give away an H positive to the water. The water accepts the H positive so that’s how it becomes H3O+. The acid loses an H positive and that’s how it becomes A minus. What we're going to say here, the equilibrium expression equals products over reactants. Remember, we’ve talked about the equilibrium expression before. It looks at all phases of matter except for liquids and solids. Liquids and solids will not be included. Here we have a liquid, so we’re going to ignore it when we do the equilibrium expression. It's simply going to become A minus times H3O+ over HA. 

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: Understanding Ka and Kb. 

Video Transcript

Now, we're going to say that ka represents our acid dissociation constant and it basically measures how strong our weak acids are, and we're going to say weak acids use ka, weak bases don't use ka, instead they use Kb, so this is our base dissociation constant and we're going to say here is their relationship to each other ka and Kb can be expressed by this formula Kw equals ka times Kb and as you realize this Kw is the same Kw we saw earlier in the earlier times, we saw it as Kw equals h positive times OH minus, it's the same exact Kw and we're going to say in general, the higher the ka value then the stronger the acid, the stronger the acid the greater the concentration of H+. Now, we're going to say related to ka is pKa and remember, we've talked about what p means, p means negative log. So, pKa means negative log of ka, here we're going to say there's an inverse relationship, it's actually the lower the pKa the stronger the acid and the stronger the acid the greater the H+ concentration. Now, we're going to say here is when it comes to a weak acid, we're going to say weak acid tends to have Ka value less than 1 and a pKa value greater than 1 just realize, we said inverse relationship. So, k and pKa are opposites, if one is up the other one is down, strong acids though, strong acids tend to have pKa is much greater than 1, I mean not pK's, but Kas much greater than 1 and they tend to have pk's lower than 1, pKa's that are negative and remember, we should be able to establish an a relationship between ka and pKa in the same way we've been able to establish a relationship between H+ and pH because, here we can just simply say ka equals 10 to the negative pKa, we could also say the same thing for pkb, pkb equals negative log of kb and therefore Kb equals 10 to the negative pKa, just remember these relationships and how they affect the acidity of our compound, as long as you can remember these few quick easy calculations you'll be able to go from one to the other and one last thing, we can say here that the relationship from p to pKa and pKb can be established by looking at this equation here, by taking the negative log of Kw ka and Kb, we say that the equation becomes 14 equals pKa plus pKb. Now, that we've seen all these types of new variables and new types of equations attempt to do the practice question on the bottom here, I say if the Kb of NH3 is 1.76 times 10 to the negative 5, what is the acid dissociation constant of the conjugate acid, conjugate acid is a term we learned earlier a few videos back but here we were given Kb, we are asked to find the acid dissociation constant. So, it's a simple conversion from one unit to another. So, once you're done with that click back on the explanation button and see how I approach this problem good luck guys.

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: 

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


Example: 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: 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: 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

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


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

a) HCNO          (Ka = 2.0 x 10-4)

b)  HF                (Ka = 3.5 x 10-4)

c)  HN3             (Ka = 2.5 x 10-5)

d)  H2CO3       (Ka = 4.3 x 10-7)


Ka and Kb Additional Practice Problems

The Ka for HC2H3O2 is 1.8 x 10-5. What is the value of  Kb for C2H3O2-?

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The pKa of formic acid is 3.75. What is the K a of formic acid?

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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.

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The Ka for HCN is 4.9 x 10-10. What is the value of  Kb for CN-?

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0.1 mol of NH4CH3COO are dissolved in 1 Liter of water.   Is the solution acidic, basic, or neutral?   Note, Kb (CH3COO-) = 5.73 x 10-10 ) and    Kb, ammonia  = 1.8 *10-5). 

Nearly neutral

Impossible to determine

Substantially acidic

Substantially basic

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Rank these acids according to their expected p Ka values.

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Rank these acids according to their expected p Ka values. From highest to lowest

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What is the kb of water?

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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?

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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

Watch Solution

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

Watch Solution

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 10-18

B. 3.5 x 1010

C. 1.0 x 10-14

D. 7.1 x 10-19

E. 2.9 x 10-11

Watch Solution

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

Watch Solution

Which of the following is the correct equilibrium expression for the autoionization of water?

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The base-dissociation constant, b, for pyridine (C 55N) is 1.4 × 10 −9.  The acid-dissociation constant, a, for the pyridinium ion (C 55NH +) 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

Watch Solution

Of the following acids, __________ is the strongest acid. 





E) HOAc and HCHO2

Watch Solution

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

Watch Solution

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

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The equilibrium constant for autoionization of water (Kw) is always 1.0 X 10 -14

a. True

b. False

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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?

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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) ClO2produces more [OH] than C6H5O

Watch Solution

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)

Watch Solution

In a triprotic acid, which Ka has the highest value?

A) Ka1

B) Ka2

C) Ka3

D) Kb1

E) Kb2

Watch Solution

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

Watch Solution

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

Watch Solution

A beaker left on a lab bench contains 0.0926 M solution of an unknown weak base solution. Using a pH meter, a student measures the pH of this solution and determines it to be 11.48. Determine the base dissociation constant (Kb) of the base.

A. 9.12 x 10-6
B. 9.85 x 10-5
C. 1.02 x 10-4
D. 3.58 x 10-11
E. 1.08 x10-13

Watch Solution

Which of the following statements is/ are correct.

i. The ethylammonium ion, C2H5NH3+, has a pK= 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

Watch Solution

You are seeking to identify an unknown monoprotic acid by determining its Ka. A 6.05 x 10-2 M solution of this unknown monoprotic acid has a pH of 2.122. Determine the K  a of this unknown acid?

A. 4.47 x 10-4
B. 9.42 x 10-4
C. 2.85 x 10-2
D. 1.08 x 10-3
E. 3.58 x 10-1

Watch Solution

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


Watch Solution

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, K= 1.8 x 10-5

E) C2H5NH2, Kb = 5.6 x 10-4

Watch Solution

Butyric acid is responsible for the odor in rancid butter. A solution of 0.25 M butyric acid has a pH of 2.71. What is the Ka for the acid?

A) 0.36

B) 2.4 x 10-2

C) 7.8 x 10-3

D) 1.5 x 10-5

E) none of the above

Watch Solution

Which substance is the strongest acid in the above table?

a) HClO


c) HNO2

d) C6H5OH

e) HCO3

Watch Solution

What is the correct equation for the ion product of water at 25 °C?



Watch Solution

Which of the following solutions has the highest pH?

A) 0.40 M CH3COOH

B) 0.40 M HClO4

C) 0.40 M HCOOH

Watch Solution

A 0.040 M solution of a monoprotic acid is 14% ionized. What is the K a of the acid?

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At 0 degrees C, the ion product constant of water is 1.2x10 –15. The pH of pure water at this temperature is:

  1. 6.88
  2. 7.00
  3. 7.46
  4. 7.56
Watch Solution

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

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If you know Kb for ammonia, NH3, you can calculate the equilibrium constant, Ka, for this reaction by the equation:

NH+  ⇌  NH3  +  H +

a) Ka = KwKb

b) Ka = K/ Kb

c) Ka = 1 / Kb

d) Ka = K/ Kw

Watch Solution

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).






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In a triprotic acid which Ka value is largest in value?

a) Ka1

b) Ka2

c) Ka3

d) They are all equal. 

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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

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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

Watch Solution

Which of the following relationships is incorrect? Assume 25° C.


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