Hydrolysis of salts
Understanding the hydrolysis of different types of salts (e.g., strong acid-weak base, weak acid-strong base) and calculating the pH of their solutions.
What is Hydrolysis of salts?
The reaction of the ions of a salt with water molecules, leading to the formation of acidic or basic solutions.
Key formula / rule: Hydrolysis Constant (SA-WB)
Key points
- Understand the phenomenon of salt hydrolysis.
- Differentiate between the four types of salt hydrolysis.
- Calculate the hydrolysis constant for different types of salts.
- Determine the pH of aqueous solutions of various salts.
Common exam trap
Confusing hydrolysis with dissolution.
Definitions
- Term
Salt Hydrolysis
- Meaning
The reaction of the ions of a salt with water molecules, leading to the formation of acidic or basic solutions.
- Term
Hydrolysis Constant (K<0xE2><0x82><0x95>)
- Meaning
An equilibrium constant that quantifies the extent to which the ions of a salt react with water.
- Term
Dissociation Constant (K<0xE2><0x82><0x90>, K<0xE2><0x82><0x99>)
- Meaning
A measure of the strength of a weak acid or weak base, indicating the extent of its ionization in water.
- Term
Ionic Product of Water (K<0xE1><0xB5><0xA1>)
- Meaning
The product of the concentrations of hydrogen ions and hydroxide ions in pure water at a given temperature (approximately 1.0 x 10⁻¹⁴ at 25°C).
Learning objectives
Understand the phenomenon of salt hydrolysis.
Differentiate between the four types of salt hydrolysis.
Calculate the hydrolysis constant for different types of salts.
Determine the pH of aqueous solutions of various salts.
Relate the pH of salt solutions to the strength of their parent acids and bases.
Formulae
- Name
Hydrolysis Constant (SA-WB)
- Note
K<0xE1><0xB5><0xA1> = Ionic product of water, K<0xE2><0x82><0x99> = Dissociation constant of weak base
- Expression
K<0xE2><0x82><0x95> = K<0xE1><0xB5><0xA1> / K<0xE2><0x82><0x99>
- Name
Hydrolysis Constant (WA-SB)
- Note
K<0xE1><0xB5><0xA1> = Ionic product of water, K<0xE2><0x82><0x90> = Dissociation constant of weak acid
- Expression
K<0xE2><0x82><0x95> = K<0xE1><0xB5><0xA1> / K<0xE2><0x82><0x90>
- Name
Hydrolysis Constant (WA-WB)
- Note
K<0xE1><0xB5><0xA1> = Ionic product of water, K<0xE2><0x82><0x90> = Dissociation constant of weak acid, K<0xE2><0x82><0x99> = Dissociation constant of weak base
- Expression
K<0xE2><0x82><0x95> = K<0xE1><0xB5><0xA1> / (K<0xE2><0x82><0x90> * K<0xE2><0x82><0x99>)
- Name
pH for SA-WB salt
- Note
C = concentration of salt, pKa = -log K<0xE2><0x82><0x90>
- Expression
pH = 7 + 1/2 (pKa + log C)
- Name
pH for WA-SB salt
- Note
C = concentration of salt, pKb = -log K<0xE2><0x82><0x99>
- Expression
pH = 7 + 1/2 (pKb + log C)
- Name
pH for WA-WB salt
- Note
pKa = -log K<0xE2><0x82><0x90>, pKb = -log K<0xE2><0x82><0x99>
- Expression
pH = 7 + 1/2 (pKa - pKb)
Prerequisites
Concept of acids, bases, and neutralisation.
Strength of acids and bases (dissociation constants K<0xE2><0x82><0x90> and K<0xE2><0x82><0x99>).
Ionic product of water (K<0xE1><0xB5><0xA1>).
pH and pOH calculations.
Logarithm rules.
Common mistakes
Confusing hydrolysis with dissolution.
Incorrectly applying the hydrolysis constant formula.
Assuming all salts hydrolyse.
Not considering the relative strengths of weak acids and bases in salt hydrolysis.
Calculation errors in pH and pOH.
Keywords
Salt Hydrolysis
Acidic Salt
Basic Salt
Neutral Salt
Hydrolysis Constant
pH Calculation
Weak Acid
Weak Base
Strong Acid
Strong Base
Ionic Product of Water
Practice preview
Which of the following salts will produce a basic solution when dissolved in water?…
easy
A salt of a strong acid and a weak base is dissolved in water. The resulting solution will be:…
easy
For a salt of weak acid and weak base, which of the following conditions will lead to an acidic solution?…
medium