A solution containing a mixture of metal cations was treated with dilute HCl and

Question

A solution containing a mixture of metal cations was treated with dilute HCl and a precipitate formed. The solution was filtered and H2S was bubbled through the acidic solution. A precipitate again formed and was filtered off. Then, the pH was raised to about 8 and H2S was again bubbled through the solution. This time, no precipitate formed. Finally, the solution was treated with a sodium carbonate solution, which resulted in formation of a precipitate. Which metal ions were definitely present, which were definitely absent, and which may or may not have been present in the original mixture?

A solution containing a mixture of metal cations was treated with dilute HCl and a precipitate formed. The solution was filtered and H2S was bubbled through the acidic solution. A precipitate again formed and was filtered off. Then, the pH was raised to about 8 and s was again bubbled through the solution. This time no precipitate formed. Finally, the solution was treated with a sodium carbonate solution, which resulted in formation of a precipitate. Which metal ions were definitely present, which were definitely absent, and which may or may not have been present in the original mixture? S Definitely absent Possibly present Ag 2+ Cd 2+ 2+. Fe The complete protocol and a list of possible 2+ precipitates can be found here Mn 2+ Cr K+ 4+ Sn 2+. Mg 3+. 2+. g2 2+ Hg 2+ Pb

Explanation / Answer

This is the systematic analysis of cations through group chemistry

HCl is the reagent of Group I cations and ppt formation indicates presence of Grp 1 cations (Ag+, Hg+, Pb2+)

After filtering the ppt, and assuming that the filtarte is acidic enough, H2S is passed which is Grp 2 reagent

A ppt here again indicates presence of Grp 2 cations (mercury(II), copper(II), bismuth(III), cadmium (II), arsenic(III), antimony(III) )

Obviously the color or solubility of the ppt would have helped to recognise the metal ions specifically.

When pH is raised with the filtrate and H2S is passed and no ppt is found it means Grp 3 is completely absent

So the ions surely absent are iron(II), iron(III), cobalt(II), nickel(II), manganese(II), chromium(III), aluminium(III), and zinc(II).

Further ppt with Na2CO3 tells us that Grp 4 is present (Sr 2+, Ba2+ and Ca2+)

So from all the analysis we can make list of possibly present cations and definitely absent cations

Possibly present: Ag+, Hg+, Pb2+, Hg2+, Cu2+, Bi3+, As3+, Sb3+, Cd2+, Sr 2+, Ba2+ and Ca2+ (and also Na+, K+ can be present hence no other information is given). The reaction of the ppt or their solubility would have helped to specifically recognise the cations.

Definitely absent: Fe2+, Fe3+, Al3+, Cr3+, Ni2+, Zn2+, Co2+, Mn2+.

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