A solution is 5.0 times 10^-5 M in Sn^+2 and 0.10 M in H_2S. At what pH will Sn^

Question

A solution is 5.0 times 10^-5 M in Sn^+2 and 0.10 M in H_2S. At what pH will Sn^+2 first begin to precipitate form solution as SnS? If included in the qualitative analysis scheme, would Sn^+2 precipitate with Group II or Group III? (SnS: K_SPA^1 = 1 times 10^-5) An aqueous solution contains the metal cations Bi^+3 belonging to Groups II and III, respectively. Thioacetamide, which produces H_2S, is added, and the solution is buffered to a pH of 8 by addition on NH_3/NH_4CI. a. Under these Group III conditions, what compounded will premeditated from solution? b. If Group II conditions are imposed by lowering the solution pH to 1, what compound(s) (if any) will remain precipitated?

Explanation / Answer

1.  The group 11 cations are Cu2+, Bi3+, Hg2+, Cd2+, Sn2+, Sn4+, Sb3+

This group is separated by using 0.1M H2S at pH of 0.5

Sn 2++ H2S SnS (brown) + 2H+

Sn 4+ + 2H2S SnS2 (yellow) + 4H+

So sn2=will precipitate in group 11

when the solution with a little H2O2 in order to oxidize Sn2+ to Sn4+before the Group II sulfides are precipitated by H2S. Once the Group II cations are precipitated, SnS can be removed from the bulk by dissolving it in ammonium sulfide (NH4)2S as it is the only Group II sulfide that is soluble in ammonium sulfide.

2. Bismuth sulfide is precipitated with smaller Ksp from group 2 and nickel sulfide will precipitated with greater Ksp from group 3

3. Group 2 Cations do not precipitate as Chlorides but will precipitate upon treatment with Hydrogen Sulfide (H2S). In an aqueous environment, Hydrogen Sulfide dissociates as a weak acid.

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