Difficult chemistry question. Precipitates...?

Question

A solution may contain any of the 3 group I cations. A white precipitate is formed upon the addition of HCl. This precipitate is partially soluble in hot water and any residue dissolves upon the addition of NH4OH.

(A) Which group I ions are present?

(B) Which are absent?

(C) Which are in doubt?

(D) How would you remove all doubt?

Thank you very much for you help!!!

Answer 1

The group 1 metals are copper, silver, and gold.  Copper can form Cu2+ and Cu+ ions in solution.  Silver really only forms Ag+ (there is Ag2+, but it is not very stable), and gold forms Au+ and Au3+.

Of these, Cu2+ and Au3+ are soluble with chloride, while AuCl (Au+) decomposes when added to water (see link; it thus cannot be in the solution or precipitate).  Thus, it must be either Ag+ or Cu+.  However, this does not mean that the other cations are not present in the solution.  Cu2+ has a light blue color that will become dark blue when the ammonia is added.  If copper metal is added to the solution after the HCl is added and a precipitate is observed, Au3+ is present (this is metallic gold plating out).

The white precipitate is either AgCl or CuCl - both will dissolve in ammonia to form soluble complex ions.  Separate out the precipitate and dissolve it in ammonia.  If the solution turns dark blue when exposed to air, Cu+ is present.  If not, it must be Ag+.  Also, if a precipitate is observed from this solution when copper metal is added, this indicates Ag+.

So the answers for the individual questions:

a. Ag+ or Cu+

b. Au+ must be absent.  If nothing else was found in the liquid after precipitation, Cu2+ and Au3+ are also absent.

c. Cu+ and Ag+ are in doubt, as are any present in the liquid.

d. Do the tests as described above to remove doubt: check for gold and silver with copper metal, check for the dark blue color of the Cu2+ ammonia complex (tetraamminecopper(II)).

Answer 2

nope... this group I refers to the first group of cations, mostly easily precipitated in qualtitative analysis, & not a group 1 from the periodic chart

group 1 catrions are Ag+, Pb+2 , & (Hg2)+2

they are all precipitated as white precipitates upon the addition of HCl, as AgCl, PbCl2, & Hg2Cl2

PbCl2 is the only one of the three ppts that dissociates in hot water. so Lead is present

AgCl is the only one of the three that dissociates upon the addition of NH3 (aka NH4OH), it forms the soluble complex ion [Ag(NH3)2]+

there is not much doubt, mercury is not there. Hg2Cl2 would have turned black to grey upon the addition of NH3 ,  forming Hg(NH2)Cl a white ppt & Hg metal appearing black. Mercury is absent.

the only doubt I can think of is that a small trace of PbCl2 might be left behind after wasing with hot water, & someone might think it was silver chlorid

to remove any doubt any way, we do confirmatory tests:

we centrifuge & remove the hot water solution above the AgCl ppt, and to that hot solution we add a few drops of K2CrO4, which makes a yellow ppt of PbCrO4 as a confirmatory test

and we centrifuge & remove the ammonia solution  after stirring to allow the AgCl to react & dissociate into solution, and to that liquid we add HNO3 until it is acidic. when it is acidic , you will have destroyed the NH3, & allow this allows that AgCl to re-precipitate as a confirmatory test