Pre-Laboratory Assignment 1. Read an authoritative source for a discussion of th
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Pre-Laboratory Assignment 1. Read an authoritative source for a discussion of the techniques to be used In this experiment. . For a solution containing at least one of the cat- ons involved in this experiment, answer the following A solution is acidified to pH 0.5, and H2S is 2. Consult a reference book regarding the hazards questions. associated with H2S.On the basis of this information, added. A dark precipitate results. Which cation(s) explain the merits of generating H25 in the solution in may be present? which it is to be used. Briefly explain the chemistry in- volved in this generation. (1) (2) KOH is added to the precipitate from (1). The supernatant liquid is separated from the dark precipitate and tested with HCVH2S. A colorless solu- tion results, with no precipitate. Which cation(s) may be present? 3. Write equations for the following chemical reac- tions. State the color of the products for each reaction. (3) The remaining precipitate from (1) ls treated with dilute HCI. The supernatant liquid is tested with NH3 and H2S. A yellow precipitate results. Which cat lon(s) may be present? (1) Cd2 (aq)+S2 (aq) (2) Sns2(s)+OH(aq) (4) The precipitate left from the first step in (3) is dissolved in HNO3, and the resulting supernatant liq- uid is treated with NH3. A blue supernatant liquid re- sults, with no precipitate. Give the identity of all Group II cations present in the original solution. (3) Cu2 (aq) +NHg(aq)Explanation / Answer
1. The tests carried out here are qualitative analysis of cations.
2. H2S is a toxic gas. Inhalation of it may cause death. It is a string irritant when exposed to skin and respiratory tract. Immediate wash with lots of water is suggested. Proper medical help is suggested. Therefore proper care must be taken when H2S is generated in solution in the lab.
3. Chemical reactions
(1) Cd^2+(aq) + S^2-(aq) ---> CdS(s)
CdS precipitate is yellow in color
(2) SnS2(s) + 6OH-(aq) --> [Sn(OH)6]^2- + 2S^2-
SnS2 is pale yellow which turns into colorless clear solution after the reaction.
(3) Cu2+(aq) + 4NH3(aq) ---> [Cu(NH3)4]^2+
The color of the final solution would be dark blue due to the complex formation.
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4.
(1) a solution at pH 0.5, added H2S gas, forms dark precipitate of sulfide
cations may be present : Hg2+, Bi3+, Pb2+, Cu2+, Sn4+, Cd2+
(2) KOH added to precipitate from (1) and supernatant tested with HCl/H2S. No precipitate formed
Cations may be present : Hg2+, Bi3+, Pb2+, Cu2+, Cd2+
Sn4+ forms an insoluble precipitate of Sn(OH)4 with KOH.
(3) remaining precipitate from (1) treated with dil. HCl. Supernatant is tested with NH3 and H2S. yellow precipitate formed.
Cation may be present : Cd2+
(4) The precipitate in (1) treated with HNO3 and NH3 to form blue solution
cation present : Cu2+
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