2. An aqueous solution of a salt (one cation and one anion) is thought to contai

Question

2. An aqueous solution of a salt (one cation and one anion) is thought to contain the Group III metal cation Mn^+2. To test this hypothesis, a student added H2S in NH3/ NH4Cl 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 ? b. To what groups could the cation belong? c. What is the proper procedure for testing for the presence of Group III metal cation?

Explanation / Answer

a) The precipitate may well be due to Group II hydroxides, as Group II cations form insoluble hydroxides in basic solution.

b) As mentioned earlier, it could be a Group II cations.

c) Before precipitating Group III cations as sulfides, they must be separated from other group cations. By treating in dil. HCl, Group I cations can be separated as chloride precipitate and by treating with H2S in acidic medium, Group II cations can be precipitated as sulfides and then Group III cations should be tested with H2S in NH3/NH4Cl buffer which gives sulfide precipitates.

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