Solubility Equilibria
Solubility product, common-ion effect and precipitation conditions. (Chemistry › Equilibrium, NEET UG syllabus.)
What is Solubility Equilibria?
The equilibrium constant for the dissolution of a sparingly soluble ionic compound in a saturated solution at a given temperature.
Key formula / rule: Solubility Product Constant (Ksp)
Key points
- Define solubility product (Ksp) and write its expression for various sparingly soluble salts.
- Calculate Ksp from molar solubility (s) and vice-versa.
- Explain the common-ion effect and predict its impact on solubility.
- Determine conditions for precipitation by comparing ionic product (Qsp) and Ksp.
Common exam trap
Incorrectly writing the Ksp expression, especially regarding stoichiometric coefficients.
Definitions
- Term
Solubility Product (Ksp)
- Meaning
The equilibrium constant for the dissolution of a sparingly soluble ionic compound in a saturated solution at a given temperature.
- Term
Common-ion effect
- Meaning
The decrease in the solubility of an ionic precipitate when a soluble salt containing a common ion is added to the solution.
- Term
Ionic Product (Qsp)
- Meaning
The product of the molar concentrations of the ions in a solution, each raised to the power of its stoichiometric coefficient, at any given moment (not necessarily at equilibrium).
- Term
Molar Solubility (s)
- Meaning
The number of moles of solute that dissolve to form one litre of a saturated solution.
Learning objectives
Define solubility product (Ksp) and write its expression for various sparingly soluble salts.
Calculate Ksp from molar solubility (s) and vice-versa.
Explain the common-ion effect and predict its impact on solubility.
Determine conditions for precipitation by comparing ionic product (Qsp) and Ksp.
Understand how factors like temperature and pH influence solubility.
Formulae
- Name
Solubility Product Constant (Ksp)
- Note
For a sparingly soluble salt AxBy(s) ≤> xAy+(aq) + yBx-(aq). Concentrations are at equilibrium.
- Expression
Ksp = [A]x [B]y
- Name
Relationship for AB type salt
- Note
Where 's' is the molar solubility.
- Expression
Ksp = s2
- Name
Relationship for A2B or AB2 type salt
- Note
Where 's' is the molar solubility.
- Expression
Ksp = 4s3
- Name
Relationship for AxBy type salt
- Note
General relationship where 's' is the molar solubility.
- Expression
Ksp = xx * yy * s^(x+y)
- Name
Ionic Product (Qsp)
- Note
Same form as Ksp, but uses instantaneous (non-equilibrium) ion concentrations.
- Expression
Qsp = [A]instx [B]insty
Prerequisites
Basic understanding of chemical equilibrium and equilibrium constants.
Knowledge of stoichiometry and balancing chemical equations.
Understanding of ionic compounds and their dissociation in water.
Familiarity with Le Chatelier's Principle.
Common mistakes
Incorrectly writing the Ksp expression, especially regarding stoichiometric coefficients.
Confusing molar solubility (s) with the solubility product (Ksp).
Failing to apply the common-ion effect correctly in calculations.
Misinterpreting the conditions for precipitation (Qsp vs Ksp).
Ignoring the effect of pH on the solubility of certain salts (e.g., hydroxides, salts of weak acids).
Keywords
Solubility product
Ksp
Common-ion effect
Ionic product
Qsp
Precipitation
Saturated solution
Molar solubility
Equilibrium
Practice preview
What is the correct expression for the solubility product (Ksp) of a sparingly soluble salt MX2?…
easy
The solubility of silver bromide (AgBr) in water is 1.0 x 10^-6 mol/L. What is its solubility product (Ksp)?…
easy
The Ksp of AgI is 8.3 x 10^-17. What is the solubility of AgI in 0.05 M KI solution?…
medium