Solubility Product (K_sp) in Chemical Equilibrium

Solubility Product (Ksp) is a special equilibrium constant that applies to sparingly soluble salts. It helps predict solubility, precipitation, and ion concentration in solutions.

Concept of Solubility Product

When a salt dissolves, it establishes a dynamic equilibrium between solid and ions:

AB_m(s) ⇌ Aⁿ⁺(aq) + mB^z-(aq)

At equilibrium, the solubility product is:

K_sp = [Aⁿ⁺] × [B^z-]^m

Example: Silver Chloride (AgCl)

Dissolution of AgCl:

AgCl(s) ⇌ Ag⁺(aq) + Cl^–(aq)

K_sp = [Ag⁺] × [Cl^–]

Relation Between Solubility and Ksp

If molar solubility = s:

• [Aⁿ⁺] = s
• [B^z-] = m × s

Thus, K_sp = s × (ms)^m = m^m s^m+1

Factors Affecting Solubility

• Temperature: Higher T increases solubility for most salts.
• Common Ion Effect: Common ions decrease solubility.
• pH: Affects salts with acidic/basic ions.

Applications of Ksp

• Predicting precipitation in ionic reactions.
• Calculating solubility of sparingly soluble salts.
• Qualitative and quantitative analysis in chemistry labs.
• Water treatment and pharmaceutical formulations.

Summary

• Ksp = product of equilibrium ion concentrations.
• Relates solubility and precipitation.
• Affected by temperature, pH, and ion concentration.

Quiz: Test Your Knowledge

1. Define solubility product and explain its role in equilibrium.
2. Write the Ksp expression for BaSO₄.
3. How does common ion effect alter solubility?
4. Relate molar solubility and Ksp for AB₂.
5. List factors affecting solubility product.

Quiz Answers

1. It is the equilibrium constant for dissolution of sparingly soluble salts.
2. Ksp = [Ba²⁺] × [SO₄^2-]
3. It reduces solubility by shifting equilibrium to solid.
4. Ksp = 4s³
5. Temperature, pH, and common ions.

MCQs

1. Ksp applies to:
   • a) All ionic compounds
   • b) Sparingly soluble salts ✔
   • c) Gases
   • d) Strong electrolytes
2. For CaF₂:
   • a) [Ca²⁺][F^–]
   • b) [Ca²⁺][F^–]² ✔
   • c) [Ca²⁺]²[F^–]
3. Common ion effect decreases solubility by:
   • a) Adding a common ion salt ✔
   • b) Increasing T
   • c) Dilution
4. Units of Ksp:
   • a) mol/L
   • b) Depends on salt’s formula ✔
   • c) atm

MCQ Explanations

• Ksp is defined only for sparingly soluble salts.
• Stoichiometry of CaF₂ gives [Ca²⁺] [F^–]².
• Common ion reduces ionization, lowering solubility.
• Units vary depending on exponents in expression.

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