An unknown solution possibly contains ions from Groups I, IV, and V. Two drops o

Question

An unknown solution possibly contains ions from Groups I, IV, and V. Two drops of 6 M HCI are added to 1.0 ml. of this unknown solution. A white precipitate forms. After centrifuging, the precipitate is separated from the supernatant solution. Water is added to the precipitate and it is heated After centrifuging, the supernatant is remove Next the precipitate is dissolved in 15M NH_4OH. When this solution is acidified with HNO_3, a white precipitate appears. Adding K_2C_1O_4(aq) to the supernatant yields a yellow solution. Next, the supernatant solution from paragraph I above is adjusted to buffered basic conditions by the addition of NH4CI and/or NH_4OH. When (NH_4)CO_3 is added, a white precipitate forms. After separation, the precipitate is dissolved in HC_2H_3O_2. This solution is made basic with NH_4OH. When (NH_4)_2C_2O_4 is added, a white precipitate forms. Lastly, when this precipitate is re-dissolved and subjected to a flame test, a red-orange flame is observed.

Explanation / Answer

Group 1 cations 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.

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

Pb+2 ion :

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

Ag+:

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 .

So Ag+ and Pb+2 ions are present .

Mg+2 :

Mg2+ ions react with ammonium phosphate in presence of NH4Cl and NH4OH to form white precipitate of magnesium ammonium phosphate.

Ca+2 :

The white precipitate of calcium carbonate formed in the group analysis dissolves in hot dil. acetic acid due to the formation of soluble calcium acetate.

Calcium acetate (formed by dissolving calcium carbonate in dil. acetic acid) reacts with ammonium oxalate to form a white precipitate o calcium oxalate.

Calcium imparts brick red colour to the flame.

By above confirmations we can say Ca+2 and Mg+2 are present in the solution .

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