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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Ionic Equilibria Page 3

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Common Ion Effect


The suppression of the degree of dissociation of a weak electrolyte by the addition of a strong electrolyte having an ion in common with the weak electrolyte is called the common ion effect.


  1. Purification of NaCl:
    • Addition of HCl, Cl- is a common ion.
    • Concentration of Cl- ions increases.
    • Suppresses solubility of NaCl and it precipitates.
    • NaCl Na+ + Cl-
      HCl H+ + Cl
      Precipitation takes place when ionic product > solubility product.
  2. Salting Out of Soap:
    • Saturated solution of NaCl is added to soap solution.
    • Concentration of Na ions increases.
    • Due to common ion Na+, solubility of soap decreases.
    • Results in precipitation of soap.
    • RCOONa RCOO- + Na+
      NaCl Na+ + Cl
  3. Gravimetric Analysis:
    • Precipitate obtained is washed with solution of strong electrolyte having common ion with precipitated solid.
    • Solubility of precipitate decreases.
    • Minor losses due to dissolution are avoided.
      Example. BaSO4 washed with dilute H2SO4

BaSO4 Ba+2+ SO42-
2H+ + SO42-




Solubility Product


The product of the ionic concentration of the ions of a sparingly soluble electrolyte present in its saturated solution at a given temperature is called a solubility product of the electrolyte.
For a salt,
AB A+ + B
- Ksp = [A+] [B-]
AB2 A2+ + 2B
- Ksp = [A2+] [B-]2
A2B 2A+ + B2
- Ksp = [A+]2 [B2-].
In qualitative analysis:

  • Group II metal cations (Hg+2, Cu+2, Bi+3, Ph+2, etc.)
    are precipitated as sulphides by passing H2S in presence of HCl.
  • Group III B metal cations (Co+2, Mn+2, etc.) are precipitated by passing H2S in presence of NH4OH.
  • Group III A cations (Al+3, Cr+3, Fe+2, Fe+3) are precipitated in the form of their hydroxides using NH4OH.

Prediction of precipitation:

  • For a salt when ionic product exceeds solubility product, precipitation occurs.



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