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MHT CET Medical202625 April 2026Morning ShiftChemistryIonic EquilibriumActual

Which of the following buffer solutions is used for precipitation of cations of the III A group?

Options

  1. AHF + NaF in water.
  2. BHCOOH + HCOONa in water
  3. CNH ₄ OH + NH ₄ Cl in water.
  4. DCH ₃ COOH + CH ₃ COONa in water.

Correct answer

C. NH ₄ OH + NH ₄ Cl in water.

Step-by-step solution

In qualitative analysis, the cations of group III A (such as Fe ³⁺ , Al ³⁺ , and Cr ³⁺ ) are precipitated as their hydroxides. The group reagent used for this purpose is a mixture of ammonium hydroxide ( NH ₄ OH ) and ammonium chloride ( NH ₄ Cl ). The addition of NH ₄ Cl suppresses the ionization of the weak base NH ₄ OH due to the common ion effect. This maintains a low concentration of hydroxide ions ( OH ⁻ ), which is just sufficient to exceed the solubility product ( K_ sp ) of group III A hydroxides and preci

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