Ch.15 - Acid and Base EquilibriumWorksheetSee 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
Sections
Identifying Acids and Bases
Arrhenius Acid and Base
Bronsted Lowry Acid and Base
Amphoteric
Lewis Acid and Base
pH and pOH
Ka and Kb
Ionic Salts
Diprotic Acid
Polyprotic Acid
Additional Practice
Conjugate Acids and Bases
Strong Acid-Base Calculations
Weak Acids
Weak Bases
Additional Guides
Strong Acids and Strong Bases
LearnAdditional Practice

A polyprotic acid possesses more than 2 hydronium ions (H+). 

Polyprotic Acids

Our understanding of diprotic acids and bases can be used to understand polyprotic acids and bases. 

Concept #1: The most common type of polyprotic acid is the triprotic acid, which contains 3 acidic hydronium ions. 

Concept #2: Triprotic bases contain 3 Kb values, so they would have 3 equilibrium equations. 

Concept #3: As a result of these equations for polyprotic acids and bases the relationship between Ka and Kb can be established. 

Example #1: Determine the pH of 0.300 M sodium hydrogen phosphate, Na2HPO4. Phosphoric acid, H3PO4, contains Ka1 = 7.5 x 10-3, Ka2 = 6.2 x 10-8 and Ka3 = 4.2 x 10-13. 

Example #2: Determine the pH of 0.300 M citric acid, H3C6H5O7 it possesses Ka1 = 7.4 x 10-4, Ka2 = 1.7 x 10-5 and Ka3 = 4.0 x 10-7. 

Previous SectionDiprotic Acid
Additional Problems
Identify the triprotic acid. A) HNO3 B) H3PO4 C) H2CO3 D) HFO4 E) H2SO4
Determine the pH of a 0.03 M solution of NaH 2PO4? Assume H3PO4 has a pKa1 of 2.1 and a pKa2 of 7.2 and a pKa3 of 12.7.  1. 4.36 2. 7.11 3. 4.65 4. 7.40 5. 9.95 6. 1.81
Consider a solution that is 0.10 M in a weak triprotic acid which is represented by the general formula H3A with ionization constants K1 = 1.0 × 10−3 , K2 = 1.0 × 10−8 , and K3 = 1.0 × 10−12. What is the pH of the solution? 1. 2.15 2. 2.25  3. 2.02 4. 2.35 5. 2.54
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
In a triprotic acid which Ka value is largest in value? a) Ka1 b) Ka2 c) Ka3 d) They are all equal. 
What are diprotic and triprotic acids? List an example of each.
Farmers who raise cotton once used arsenic acid, H3AsO4 , as a defoliant at harvest time. Arsenic acid is a polyprotic acid with K1 = 2.5 x 10-4 , K 2 = 5.6 x 10-8 , and K3 = 3 x 10-13 . What is the pH of a 0.500 M solution of arsenic acid? A) 0.85 B) 1.95 C) 3.90 D) 4.51
For the polyprotic acid H3PO4 (phosphoric acid), which of the following expressions would correspond to Ka3?1. H3PO4 ⇌ PO43− + 3 H+2. PO43− + 3 H+ ⇌ H3PO43. HPO42− ⇌ PO43− + H+4. H2PO4− ⇌ HPO42− + H+5. H3PO4 ⇌ H2PO4− + H+ 
What is the pH of a solution of 0.25 M K3PO4, potassium phosphate? GivenKa1 = 7.5 * 10-3Ka2 = 6.2 * 10-8Ka3 = 4.2 * 10-13
Phosphoric acid (H3PO4) is characterized by three reactions for stepwise dissociation. Arrange the following species in the order of decreasing concentration in an aqueous solution of H3PO4. Clearly indicate the order of concentration change. JUSTIFY your answer.H3PO4          H 2PO4-          HPO 42-          PO 43-          H 3O+  
Identify the weak triprotic acid.A) CH3COOHB) H3PO3C) H3PO4D) H2SO4E) H2SO3
Citric acid, which is present in citrus fruits, is a triprotic acid (Ka1 = 7.4x10-4, Ka2 = 1.7x10-5, Ka3 = 4.0x10-7). Calculate the pH for a 0.048 M { m ;{ m M}} solution of citric acid.
Citric acid, which is present in citrus fruits, is a triprotic acid (Ka1 = 7.4x10-4, Ka2 = 1.7x10-5, Ka3 = 4.0x10-7). Is the concentration of citrate ion (C6H5O73–) equal to, less than, or greater than the H+ ion concentration?
Citric acid, which is present in citrus fruits, is a triprotic acid (Ka1 = 7.4x10-4, Ka2 = 1.7x10-5, Ka3 = 4.0x10-7). Did you have to make any approximations or assumptions in completing your calculations?
What is the equilibrium associated with Ka3 for H3PO4?
Write out the stepwise Ka reactions for citric acid (H3C6H5O7), a triprotic acid.
Arsenic acid (H3AsO4) is a triprotic acid with K a1 = 5.5 X 10 -3, Ka2 = 1.7 X 10 -7, and Ka3 = 5.1 X 10-12. Calculate [H+], [OH-], [H3AsO4], [H2AsO4-], [HAsO42-], and [AsO43-] in a 0.20-M arsenic acid solution.
Citric acid has four hydrogens bonded to oxygen. How does the hydrogen that is not an acidic proton differ from the other three?
Write chemical equations for first ionization step of phosphoric acid.
Write corresponding equilibrium expressions for first ionization step of phosphoric acid.
Write chemical equations for second ionization step of phosphoric acid.
Write corresponding equilibrium expressions for second ionization step of phosphoric acid.
Write chemical equations for third ionization step of phosphoric acid.
Write corresponding equilibrium expressions for third ionization step of phosphoric acid.
Calculate the [H3O+] and pH of each polyprotic acid solution.a. 0.125 M H2CO3b. 0.125 M H3C6H5O7
Cysteine is an amino acid with R=-CH2SH, which ionizes according to: The three Pka's of cysteine are: pKA'=1.71, pKA''=8.33, and pKA=10.78 If one prepares 0.40 M solution of cysteine and the pH is adjusted to pH=8.75, calculate the molar concentrations, [Cys] and [Cys-], in the resulting solution.
Phosphoric acid is a triprotic acid (Ka1 = 6.9 x 10-3, Ka2 = 6.2x 10-8, and Ka3 = 4.8 x10-13). To find the pH of a buffer composed of H2PO4- (aq) and HPO42- (aq), which pKa value would you use in the Henderson-Hasselbalch equation?Calculate the pH of a buffer solution obtained by dissolving 13.0 g of KH2PO4(s) and 26.0 g of Na2HPO4(s) in water and then diluting to 1.00 L.