A solution containing a mixture of metal cations was treated as follows. 1. Dilu

Question

A solution containing a mixture of metal cations was treated as follows. 1. Dilute HCl was added and a precipitate formed. The precipitate was filtered off. 2. H_2S was bubbled through the acidic solution. Again, a precipitate formed and was filtered off. 3. The pH was raised to about 9 and H_2S was again bubbled through the solution. No precipitate formed. 4. Finally, sodium carbonate was added. A precipitate formed and was filtered off. What can be said about the presence of each of these groups of cations in the original solution? At least one of these ions was present. None of these ions were present. All of these Ions were present. unknown

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+)..... at least one present

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) ) At least one of these ions were present

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+).... one from these present

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+ and NH4+ 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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