Buffer Solutions and Hydrolysis
Henderson equation and salt hydrolysis of the four salt types. (Chemistry › Equilibrium, NEET UG syllabus.)
What is Buffer Solutions and Hydrolysis?
A solution that resists changes in pH upon the addition of small amounts of acid or base.
Key formula / rule: Henderson-Hasselbalch Equation (Acidic Buffer)
Key points
- Define buffer solutions and explain their mechanism of action.
- Differentiate between acidic and basic buffers and provide examples.
- Apply the Henderson-Hasselbalch equation to calculate the pH of buffer solutions.
- Explain the concept of buffer capacity.
Common exam trap
Confusing pKa with Ka and pKb with Kb in calculations.
Definitions
- Term
Buffer Solution
- Meaning
A solution that resists changes in pH upon the addition of small amounts of acid or base.
- Term
Salt Hydrolysis
- Meaning
The reaction of a salt's cation or anion (or both) with water, producing H₃O⁺ or OH⁻ ions and thereby changing the solution's pH.
- Term
Buffer Capacity
- Meaning
The amount of acid or base that can be added to a buffer solution before its pH changes significantly.
- Term
pKa
- Meaning
The negative logarithm (base 10) of the acid dissociation constant (Ka); indicates the strength of a weak acid.
- Term
pKb
- Meaning
The negative logarithm (base 10) of the base dissociation constant (Kb); indicates the strength of a weak base.
- Term
Hydrolysis Constant (Kh)
- Meaning
The equilibrium constant for a hydrolysis reaction, indicating the extent to which a salt ion reacts with water.
Learning objectives
Define buffer solutions and explain their mechanism of action.
Differentiate between acidic and basic buffers and provide examples.
Apply the Henderson-Hasselbalch equation to calculate the pH of buffer solutions.
Explain the concept of buffer capacity.
Define salt hydrolysis and classify salts based on their parent acid and base.
Predict the nature (acidic, basic, neutral) of salt solutions based on hydrolysis.
Calculate the hydrolysis constant (Kh) and pH for different types of salt solutions.
Solve numerical problems involving buffer solutions and salt hydrolysis.
Formulae
- Name
Henderson-Hasselbalch Equation (Acidic Buffer)
- Note
Used for calculating pH of an acidic buffer solution.
- Expression
pH = pKa + log([Salt]/[Acid])
- Name
Henderson-Hasselbalch Equation (Basic Buffer)
- Note
Used for calculating pOH of a basic buffer solution. pH = 14 - pOH.
- Expression
pOH = pKb + log([Salt]/[Base])
- Name
Hydrolysis Constant (SA-WB Salt)
- Note
For salts formed from a strong acid and a weak base (e.g., NH₄Cl).
- Expression
Kh = Kw / Kb
- Name
Hydrolysis Constant (WA-SB Salt)
- Note
For salts formed from a weak acid and a strong base (e.g., CH₃COONa).
- Expression
Kh = Kw / Ka
- Name
Hydrolysis Constant (WA-WB Salt)
- Note
For salts formed from a weak acid and a weak base (e.g., CH₃COONH₄).
- Expression
Kh = Kw / (Ka × Kb)
- Name
pH of SA-WB Salt Solution
- Note
Where C is the initial concentration of the salt.
- Expression
pH = 7 - (1/2)pKb - (1/2)logC
- Name
pH of WA-SB Salt Solution
- Note
Where C is the initial concentration of the salt.
- Expression
pH = 7 + (1/2)pKa + (1/2)logC
- Name
pH of WA-WB Salt Solution
- Note
Independent of salt concentration, assuming dilute solution.
- Expression
pH = 7 + (1/2)pKa - (1/2)pKb
Prerequisites
Basic concepts of acids and bases (Arrhenius, Brønsted-Lowry).
Chemical equilibrium and equilibrium constant (Kc, Kp).
Ionic product of water (Kw).
Strong and weak electrolytes.
Logarithms and their properties.
Common mistakes
Confusing pKa with Ka and pKb with Kb in calculations.
Incorrectly identifying the type of salt and thus the nature of hydrolysis.
Applying Henderson-Hasselbalch equation to strong acids/bases or non-buffer solutions.
Forgetting to consider the common ion effect when calculating pH of buffers.
Assuming all salts are neutral; only SA-SB salts are neutral.
Incorrectly using concentrations (moles vs. molarity) in buffer calculations.
Keywords
Buffer
Acidic Buffer
Basic Buffer
Henderson-Hasselbalch
pH
pKa
pKb
Salt Hydrolysis
Hydrolysis Constant
Kw
Conjugate Acid-Base Pair
Buffer Capacity
Practice preview
An acidic buffer solution contains 0.1 M CH3COOH (Ka = 1.8 x 10^-5) and 0.1 M CH3COONa. What is the pH of the buffer solution?…
easy
Calculate the pH of a 0.1 M solution of sodium acetate (CH3COONa). Given Ka for CH3COOH = 1.8 x 10^-5 and Kw = 1.0 x 10^-14.…
medium
To prepare an acidic buffer solution with a pH of 5.00, what ratio of [CH3COONa]/[CH3COOH] is required? (Given Ka for CH3COOH = 1.8 x 10^-5)…
hard