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In the qualitative analysis of Group II cations, hydrogen sulfide ( H₂S ) gas is passed through the original solution in the presence of dilute hydrochloric acid ( HCl ). The primary chemical reason for adding dilute HCl before passing H₂S gas is to :

Options

  1. Aincrease the concentration of S ²⁻ ions for faster precipitation
  2. Bincrease the concentration of Cl ⁻ ions to form soluble chloride complexes
  3. Cdissolve any interfering phosphate ions present in the mixture
  4. Ddecrease the concentration of S ²⁻ ions by common ion effect

Correct answer

D. decrease the concentration of S ²⁻ ions by common ion effect

Step-by-step solution

HCl is a strong acid that completely dissociates to give H ⁺ and Cl ⁻ ions. HCl H ⁺ + Cl ⁻ Hydrogen sulfide ( H₂S ) is a weak diprotic acid that ionizes as: H₂S 2 H ⁺ + S ²⁻ The presence of H ⁺ ions from HCl suppresses the ionization of H₂S due to the common ion effect. This significantly decreases the concentration of S ²⁻ ions in the solution. The reduced [ S ²⁻] is sufficient to exceed the extremely low solubility product ( K_ sp ) of Group II sulfides, but it is too low to precipitate Group IV sulfides, thereby

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