Experiment 9 Post Lab l. A solution is 5.0 x 10 s M in sn\'a and o 10 M in H,s.

Question

Experiment 9 Post Lab l. A solution is 5.0 x 10 s M in sn'a and o 10 M in H,s. At what pH will sni first begin to precipitate from solution as sns? If included in the qualitative analysis scheme, sn'a precipitate with Group II or Group III? (sns: Ksmw' 1x 10 2. An aqueous solution contains the metal cations Bi sand Ni belonging to Groups II and III, respectively. which produces Has, is added, and the solution is buffered to a pH addition of NH,NHCI. a. Under these Group III conditions, what compounds will precipitate from solution b. If Group II conditions are imposed by lowering the solution pH to 1, what compound(s) (if any) will remain precipitated? Experiment 9 l Group Qualitative Analysis 117

Explanation / Answer

1. Ksp of SnS = [Sn2+][S^2-] = 1 x 10^-25

SnS <==> Sn2+ + S2^-

[S^2-] = 1 x 10^-25/5 x 10^-5 = 2 x 10^-21

For HS-,

Ka2 = [H+][S^2-]/[HS-] = 1 x 10^-19

[HS-] = (2 x 10^-21)^2/1 x 10^-19 = 4 x 10^-23 M

Ka1 = [H+][HS-]/[H2S] = 9.5 x 10^-8

[H+] = 9.5 x 10^-8 x 0.1/4 x 10^-23 = 2.37 x 10^14

pH = -log[H+] = 14.37

So at pH 14.37 SnS would precipitate

2. With thioacetamide reaction of Ni2+ and Bi3+ caions,

a. at pH 8, Ni2+ would precipitate out as NiS. Group III cations precipitate out as sulfides under basic medium.

b. When pH was lowered to 1, we would have Bi3+ precipittes out as Bi2S3 compound. Under these conditions the sulfides from group III remained in soluble form.

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