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Common-Ion Effect

The common-ion effect refers to the decrease in solubility of an ionic compound when a solution already contains one of the ions present in the compound.

Key Concepts​

  • Occurs in solutions of weak electrolytes when a common ion is added
  • The presence of the common ion shifts equilibrium according to Le Chatelier's Principle
  • Reduces the ionization of the weak electrolyte

Example​

For AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq):

  • Adding NaCl increases [Cl⁻]
  • Equilibrium shifts left → less AgCl dissolves

Calculating Effects​

  • Use ICE tables to account for the additional common ion
  • New solubility: adjust K_sp = [Ag⁺][Cl⁻] including the common ion concentration

Importance​

  • Predicts solubility in mixed solutions
  • Essential for precipitation reactions and qualitative analysis
  • Helps in designing selective precipitation and separation processes

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