Consider the following qualitative analysis scenario and then classify what type
ID: 500934 • Letter: C
Question
Consider the following qualitative analysis scenario and then classify what type of qualitative analysis principle is being used. A precipitate contains Bi2S3(s) and Sb2S3(s). The Sb3 can be separated from the Bi2S3(s) by adding excess S2– to form [SbS3]3–(aq).
Options: Competitive equilibrium solubilization: anion protonation, selective precipitation, fractional precipitation, Competitive equilibrium solubilization: cation complexation, Selective oxidation/reduction, and Amphoteric Precipitate Solubilization.
Explanation / Answer
We have two precipitates Bi2S3 (s) and Sb2S3 (s). Write down the equilibrium constants for the two precipitates:
Bi2S3 (s) <====> 2 Bi3+ (aq) + 3 S2- (aq); K1 = [Bi3+]2/[S2-]3/[Bi2S3] …..(1)
Sb2S3 (s) <====> 2 Sb3+ (aq) + 3 S2- (aq); K2 = [Sb3+]2[S2-]3/[Sb2S3] ….(2)
When excess S2- is added to the solution, Sb2S3 dissolves into the solution forming soluble [SbS3]3- as per the equation
Sb2S3 (s) + 3 S2- (aq) ------> 2 [SbS3]3-
This is clearly a case of competitive equilibrium solubilization where we increase the concentration of S2- to such an extent that expression (2) is no longer valid and Sb2S3 dissolves into the solution forming the soluble salt (ans).