Common Ion Effect
Ionic Equilibrium
6
JEE Qs
8%
Hard
60
min
Master the approximation techniques for equilibrium calculations in the presence of a common ion, as they are crucial for speed and accuracy in JEE problems.
🧮 Key Formulas
✅ Key Points for JEE
- 1The Common Ion Effect describes the suppression of dissociation of a weak electrolyte (acid or base) or the solubility of a sparingly soluble salt when a strong electrolyte containing a common ion is added.
- 2It is a direct application of Le Chatelier's Principle: the system shifts to relieve the stress of added common ion, thus reducing the concentration of other ions from the weak electrolyte.
- 3In calculations, the contribution of the common ion from the weak electrolyte's dissociation/solubility is often negligible compared to that from the strong electrolyte, simplifying equilibrium calculations (e.g., x << C).
- 4For weak acids/bases, the common ion effect reduces the degree of dissociation (alpha) and changes the pH/pOH towards neutrality relative to the weak electrolyte alone.
- 5For sparingly soluble salts, the common ion effect significantly decreases the molar solubility (s) of the salt, which can lead to precipitation.
⚠️ Common Mistakes
- ✕Failing to identify the common ion source and its concentration correctly, especially when the common ion is produced from a strong electrolyte (which dissociates completely).
- ✕Incorrectly setting up the ICE (Initial, Change, Equilibrium) table, particularly by not accounting for the initial concentration of the common ion from the added strong electrolyte.
- ✕Neglecting to apply appropriate approximations (e.g., assuming x is very small compared to initial concentrations) or applying them when they are not valid, leading to complex quadratic equations unnecessarily.
- ✕Confusing the common ion effect with buffer action; while related, common ion effect is the underlying principle, while buffer action describes the resistance to pH change.
📝 Practice Questions
See allQ74.If 1 mM solution of ethylamine produces pH = 9, then the ionization constant (Kb) of ethylamine is 10−x . The value of x is ______ (nearest integer). [The degree of ionization of ethylamine can be neglected with respect to unity.]
Q61. Ksp for Cr(OH)3 is 1.6 × 10−30 . What is the molar solubility of this salt in water? (1) 1.8×10−30 (2) 5√1.8 × 10−30 27 × 10−30 (3) 4√1.6×10−3027 (4) 2√1.6
Q58.The molar solubility(s) of zirconium phosphate with molecular formula (Zr4+)3(PO3−4 )4 is given by relation : (1) Ksp 13 (2) Ksp 17 ( 9612 ) ( 6912 ) (3) Ksp 17 (4) Ksp 16 ( 8435 ) ( 5348 )
Q62.Which of the following happens when NH4OH is added gradually to the solution containing 1 M A2+ and 1MB3+ ions? Given : Ksp [A(OH)2] = 9 × 10−10 and Ksp [B(OH)3] = 27 × 10−18 at 298 K . (1) Both A(OH)2 and B(OH)3 do not show (2) A(OH)2 will precipitate before B(OH)3 precipitation with NH4OH (3) B(OH)3 will precipitate before A(OH)2 (4) A(OH)2 and B(OH)3 will precipitate together excess HCHO alkali−
Q68.pH of water is 7 at 25∘C. If water is heated to 80∘C., it's pH will : (1) Decrease (2) H+ concentration increases, OH− concentration decreases (3) Remains the same (4) Increase 2025 (23 Jan Shift 2) JEE Main Previous Year Paper
Q58.A weak acid HA has degree of dissociation x . Which option gives the correct expression of ( pH pKa )? (1) 0 (2) log(1 + 2x) (3) log ( 1−xx ) (4) log ( 1−xx )
NCERT Chapters
- Class 11 Chemistry Part I Ch 7: Equilibrium