Question

2)A solution.may contain any of the three group I cations. A white precipitate if formed upon the addition of HCl. This precipitate is partially soluble in hot water and any residue dissolve upon the addition of NH4OH. Which group I ions are present? which are absent? which are in doubt? How would you remove all doubt?

Answer 1

The most important cations in 1st group are Ag⁺, Hg₂⁺², and Pb²⁺. The solution may contain any of these cations.

The chlorides of these elements cannot be distinguished from each other by their colour - they are all white solid compounds.

The given salt is only partially soluble and dissolve upon addition of NH₄OH. So the precipitate may be AgCl. The salt present in the solution could be Ag⁺. AgCl dissolves in ammonia due to the formation of the complex ion [Ag(NH₃)₂]⁺.

Furthermore, AgCl is reduced to silver under light, which gives samples a violet colour.

Hg₂Cl₂ gives a black precipitate consisting of a mixture of chloro-mercuric amide and elemental mercury on addition of NH₄OH.

PbCl₂ is soluble in hot water, and can therefore be differentiated easily.

So the salts of Hg₂⁺², and Pb²⁺ are absent.

Salt of Ag⁺ is in doubt.

Confirmation test for Silver ion:

Ag⁺ + KI → AgI + K⁺

2Ag⁺ + K₂CrO₄ → Ag₂CrO₄ + 2 K⁺