Question

during detection of group if first group is detected what care should be taken for next group​

Answer 1

Explanation:

One common task in analytical chemistry is the identification of the various ions present in a particular sample. For example, if you are an environmental chemist your job may be to recover soil or water samples in order to determine the presence of toxic ions such as Pb2+ or Hg2+2 . A common experimental method used to identify ions in a mixture is called qualitative analysis.

In qualitative analysis, the ions in a mixture are separated by selective precipitation. Selective precipitation involves the addition of a carefully selected reagent to an aqueous mixture of ions, resulting in the precipitation of one or more of the ions, while leaving the rest in solution. Once each ion is isolated, its identity can be confirmed by using a chemical reaction specific to that ion.

Cations are typically divided into Groups, where each group shares a common reagent that can be used for selective precipitation. The classic qualitative analysis scheme used to separate various groups of cations is shown in the flow chart below.

Screen Shot 2017-12-07 at 11.28.34 AM.png

Note that Ag+ , Pb2+ , and Hg2+2 are called the Group I cations since they are the first group separated from the larger mixture. Since these ions all form insoluble chlorides, their separation from all other ions may be accomplished by the addition of 6 M HCl (aq) resulting in the precipitation of AgCl (s), PbCl2 (s), and Hg2Cl2 (s):

Ag+(aq)+Cl−(aq)⟶AgCl(s)(6.1)

Hg2+2(aq)+2Cl−(aq)⟶Hg2Cl2(s)(6.2)

Pb2+(aq)+2Cl−(aq)⟶PbCl2(s)(6.3)

The sample must then be centrifuged (spun rapidly), which separates the solid precipitates from the ions still in solution. The solids settle to the bottom, and the solution containing the remaining ions (Groups 2 – 5) remains on top of the solid. This solution is called the supernatant solution, and must be carefully decanted (poured off without disturbing the solid), and saved for further study. The Group I cations contained within the collected precipitate must then be separated from each other in order for the presence of each ion to be confirmed.