FARINER\'S NAME: LAB ROOM NO DATE LAB DAY. (if any) DESK NO: LAB START TIME: Exp
ID: 486882 • Letter: F
Question
FARINER'S NAME: LAB ROOM NO DATE LAB DAY. (if any) DESK NO: LAB START TIME: Experiment 8 Pre Lab 1. During qualitative analysis, what factor is used for the preliminary separation of metallic cations into groups by addition of four different group reagents? 2. An aqueous solution of a salt one cation and one anion) is thought to contain the Group III metal cation Mn". To test this hypothesis, a student added His in NHMNH Cl buffer to 1 mL. of the salt solution and obtained a precipitate. The student then concluded that the aqueous solution contained a Group III metal cation. a. What is wrong with this student's reasoning? To what groups could the cation belong? What is the proper procedure for testing for the presence of Group III metal cation?Explanation / Answer
1. The factor that is employed in the preliminary separation of cations into four different groups is the solubility of the cations in the group reagents. For example, a qualitative analysis starts by dissolving a salt in dilute HCl. Group I cations are insoluble in HCl while cations of Groups II, III, IV and V are all soluble in HCl. Therefore, solubility plays an excellent role in separating cations of one group from those of another.
Group II cations separate out when H2S is passed through the acidic solution of the filtrate. Here also, the key determining factor is the solubility of the salts. Sulfides of Group II cations are insoluble in dilute HCl while all other metal sulfides (except Group I which we have already removed) are soluble in HCl. therefore, solubility in an acid or base decides the course of separation.
2 a) The student assumed that his solution has a Group III cation which should precipitate as sulfide when H2S is passed through the alkaline solution of the sample (the sample was made alkaline with NH3/NH4Cl). However, the student failed to recognize the anion present. It may be possible that the solution may actually contain carbonate of Ca, Mg, Ba or Sr which precipitate out in ammoniacal medium.
b) The student worked with aqueous solution and added NH3/NH4Cl directly. The student didn’t check for the presence of Group II cations which precipitate sulfide in HCl medium. Since the student started with an aqueous solution, chances of a Group II cation being present is less likely.
The student more likely had a Group III cation or Group IV cation. Group IV cations like Ca, Mg, Ba and Sr are usually present as carbonate and carbonates are insoluble in alkaline medium.
c) In order to rightly test for the presence of a Group III cation, the student must acidify the aqueous extract with HCl. If a precipitate is formed, the precipitate must be separated and H2S must be passed through the acidified filtrate to check for the presence of Group II cations. In case, no precipitate forms, the solution must be made alkaline with NH3/NH4Cl and then NH3 must be passed to precipitate out Group III cation either as sulfide or hydroxide. In case no precipitate forms, solid ammonium carbonate must be added to the filtrate to check for Group IV cations.
3) Upon addition of dilute HCl to the aqueous solution of the sample, a precipitate was formed. The solution clearly contained a Group I cation like Pb2+, Hg22+ or Ag+.
The precipitate was separated and H2S was passed through the acidified supernatant. The absence of a precipitate indicates no Group II cations were present.
Next, the solution was made alkaline with NH3/NH4Cl and H2S was passed. The formation of no precipitate indicates the absence of Group III cations.
Ammonium carbonate was next added to the alkaline filtrate. The appearance of a precipitate indicates the presence of Group IV cation like Ca2+, Ba2+ or Sr2+.
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