JEE MainChemistryPractical Chemistry
A student is given an aqueous mixture containing Fe ³⁺ and Mg ²⁺ ions. Upon adding solid NH₄Cl followed by excess NH₄OH solution, a brown precipitate forms, but Mg ²⁺ remains in the solution. The combined thermodynamic and equilibrium reason why Mg ²⁺ does not precipitate under these specific conditions is that :
Options
- Athe common ion effect of NH₄^+ keeps [ OH ⁻] low, which fails to exceed the higher K_ sp of Mg(OH)₂
- BMg(OH)₂ has a lower K_ sp than Fe(OH)₃ , preventing its precipitation in basic medium
- Cthe common ion effect of Cl ⁻ suppresses the dissociation of the mixture, preventing the precipitation of MgCl
- DMg ²⁺ forms a highly soluble ammine complex [ Mg(NH₃)₄ ]²⁺ with excess ammonia
Correct answer
A. the common ion effect of NH₄^+ keeps [ OH ⁻] low, which fails to exceed the higher K_ sp of Mg(OH)₂
Step-by-step solution
Ammonium hydroxide ( NH₄OH ) is a weak base, and its ionization is suppressed by the addition of the strong electrolyte ammonium chloride ( NH₄Cl ) due to the common ion effect of NH₄^+ ions. NH₄OH NH₄^+ + OH ⁻ NH₄Cl NH₄^+ + Cl ⁻ This suppression keeps the concentration of OH ⁻ ions very low in the solution. The solubility product ( K_ sp ) of Fe(OH)₃ is much lower (around 10⁻³⁸ ) compared to that of Mg(OH)₂ (around 10⁻¹¹ ). The low [ OH ⁻] is sufficient to make the ionic product of Fe(OH)₃ exceed its K_ sp , leadi