Salt of a strong acid and a weak base:

The solution of such a salt is acidic in nature. The cation of the salt which has come from weak base is reactive. It reacts with water to form a weak base and H+ ions.

B+  +   H2O  ↔  BOH  +  H+

                         Weak base

Consider, for example, NH4Cl. It ionises in water completely into NH4 and CF ions. ions react with water to form a weak base (NH4OH) and H+ ions.

    NH+4  +  H2O   ↔   NH4OH  +  H+

    C(1-x)                   Cx         Cx

Thus, hydrogen ion concentration increases and the solution becomes acidic.

Applying law of mass action,

Kh = [Hx ][NH4 OH]/[NH4+ ]=(Cx.Cx)/C(1-x) = (x2 C)/((1-x))     ...... (i)

where C is the concentration of salt and x the degree of hydrolysis.

Other equilibria which exist in solution are

NH4OH ↔ NH+4 + OH-,    Kb = [NH+4][OH-]/[NH4Oh]    .... (ii)

H2O ↔  H+ + OH-,          Kb = [H+][H-]            ..... (iii)

From eqs. (II) and (iii)

Kw/Kb =[H+ ][NH4 OH]/[NH4+ ] =Kh      .... (iv)

[H+] = [H+ ][NH4+]/[NH4OH] = Kw/Kb ×[NH4+ ]/[NH4 OH]

log [H+] = log Kw - log Kb + log[salt]/[base]

-pH = -pKw + pKb + log[salt]/[base]

pKw - pH = pKb + log[salt]/[base]

pOH = pKb + log[salt]/[base]


Relation between Hydrolysis constant and Degree of hydrolysis

The extent to which hydrolysis proceeds is expressed as the degree of hydrolysisand is defined as the fraction of one mole of the salt that is hydrolysed when the equilibrium has been attained. It is generally expressed as h or x.

h = (Amount of salt hydrolysed)/(Total salt taken)

Considering again eq. (i),

Kh = x2C/(1-x)   or     Kh = h2C/(1-h)

        When h is very small (1-h) → 1,

        H2 = Kh × 1/c

  or   h = √(Kh/C)

        = √(Kw/Kb * C)

        [H+] = h × C = √(C*Kh)/Kb

        log [H+] =  1/2 log Kw + 1  1/2log C - 1/2log Kb

        pH = 1/2pKw - 1/2 log C - 1/2 pKb

        = 7 - 1/2 pKb -  1/2log C

Related Resources
Relative Strength of Acids and Bases

Relative Strength of Acids and Bases: According to...

Solubility Product

SOLUBILITY PRODUCT: If to a given amount of...

Hydrolysis of Amphiprotic Anion

Hydrolysis of Amphiprotic Anion: Let us consider...

Salt of Weak Acid and Weak Base

Salt of a weak acid and a weak base: Maximum...

Salt of Weak acid and Strong Base

Salt of a Weak Acid and a Strong Base: The...

Common ION Effect

COMMON ION EFFECT: Let AB to the weak electrolyte....

Acids and Bases

Acids and Bases: The earliest criteria for the...

Periodic Variations of Acidic and Basic Properties

Periodic variations of acidic and basic...

Salt Hydrolysis

Salt Hydrolysis: Pure water is a weak electrolyte...

Buffer Capacity

Buffer capacity: The property of buffer solution...

pH OF Weak Acids and Bases

pH OF Weak Acids and Bases: Weak acids and bases...

Buffer Solutions

1 | 2 BUFFER SOLUTIONS: For several purposes, we...

Relationship-Solubility and Solubility Product

1 | 2 | 3 | 4 | 5 Relationship between solubility...

Types of Electrolytes

Types of Electrolytes: The compounds which give...

Theory of Indicators

Ionic Equilibrium Types of electrolytes...

Ostwald s Dilution Law

Ostwald’s Dilution Law: According to...

Lewis Concept of Acids and Bases

Lewis Concept of Acids and Bases: This concept was...

Hendersons Equation

Henderson's Equation (pH of a buffer): (i) Acidic...

Acid-Base Neutralization

1 | 2 | 3 ACID-BASE NEUTRALISATION-SALTS When...

Hydrogen Ion Concentration pH Scale

HYDROGEN ION CONCENTRATION-pH SCALE: It is clear...

Applications of Solubility Product

Applications of Solubility Product (i)...

Ionic Product of Water

Ionic Product of Water: Pure water is a very weak...