Add 10 drops of 6 M NH, to the precipitate from step G1 -2. The formation of a d

Question

Add 10 drops of 6 M NH, to the precipitate from step G1 -2. The formation of a dark gray precipitate indicates the presence of mercury. Centrifuge and decant the clear supernate into a clean test tube. Add 20 drops of 6 M HNO_3 to the decantate. Stir the solution and test its acidity. Continue to add HNO_3 dropwise until the solution is acidic. A white cloudiness confirms the presence of Ag^-. Before beginning Part A of this experiment in the laboratory, you should be able to answer the following questions: 1. What are the symbols and charges of the group 1 cations? 2. Which chloride salt is insoluble in cold water but soluble in hot water? 3. Which chloride salt dissolves in aqueous NH_3? 4. How could you distinguish: (a) BaCl_2 from AgCI? (b) HNO_3 from HCl? 5. Complete and balance the following equations: (a) AgCl(s) + NH_3 (aq) rightarrow (b) Pb^2+(aq) + CrO_4 ^2-(aq) rightarrow (c) Hg_2 Cl_2(s) + NH_3(aq) rightarrow (d) Ag(NH_3)_2^+ (ag) + H^+ (aq) + Cl^- (aq). 6. What can you conclude if no precipitate forms when HCl is added to an unknown solution? 7. Why are precipitates washed? 8. How do you decant supernatant liquids from small test tubes?

Explanation / Answer

1. Group I cations are : Pb2+, Ag+ and Hg2(2+) [Mercurous ion]

2.PbCl2 is insoluble in cold water but soluble in hot water.

3. AgCl is soluble in aq. NH3

4. (a) BaCl2 is soluble in water while AgCl is insoluble in water while AgCl is soluble in aq. NH3.

(b) HCl on treatment with Ammonia gives white fumes of NH4Cl while HNO3 does not give such white fumes.

5 (a) [Ag(NH3)2]+Cl- (soluble)

(b) PbCrO4 (yellow)

(c) Hg + Hg(NH2)Cl (dark brown ppt)

(d) AgCl + NH3

6. We can say given solution is devoid of group I cations.

7. So that any cations does not get washed away with ppt.

8. To proceed for cation analysis of other groups (II onwards)

  

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