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An “unknown” liquid was obtained
An “unknown” liquid was obtained. Several drops of 6 M HCl were added to this solution and a white precipitate was visible. The test tube is centrifuged, and the solution decanted. The decantate is set aside. 8 drops of DI water, was added the mixture stirred and centrifuged to wash the precipitate. After the tube is centrifuged, the wash water is discarded. 15 drops of DI were added to the precipitate and the tube was placed in a hot water bath for 3 minutes. The precipitate dissolved. Upon adding 4 drops of potassium chromate to the solution, a bright yellow precipitate formed.
To the liquid decantate, 4 drops of 1 M thioacetamide was added and the solution was mixed and heated in a hot water bath. A noticeable black precipitate formed. The test tube was removed from the hot water bath, centrifuged and the liquid decanted and set aside. 8 drops of 6 M HNO3 were added to the precipitate and the test tube was heated in a hot water for 2 minutes. The test tube was removed from the hot water bath, allowed to cool for a minute and 12 drops of concentrated NH4OH were added producing a deep wildcat blue color.
Highlight the cation that is definitely NOT PRESENT in the unknown.
Ag+ Pb2+ Cu2+ Fe3+ Co2+ Ni2+ Ca2+ Ba2+
Expert Solution
Answer:
Insoluble Chlorides:
On adding HCl , group 1 cation will precipitate as chloride. So Ag+ and Pb2+ may give prcipitate of AgCl and PbCl2.
Ag+, Pb2+, and are all quite insoluble in water. Because PbCl2 is much more soluble in hot water than are the other two chloride salts, so adding water to the precipitate and heating the resulting slurry will dissolve any PbCl2 present. Isolating the solution and adding a small amount of K2CrO4 solution to it will produce a bright yellow precipitate of PbCrO4 if Pb2+ were in the original sample.
We got a yellow colour on adding potassium chromate, so presence of Pb2+ is confirmed.
Acid-Insoluble Sulfides:
On adding H2S ( which is produced from thioactamide) , group 2 cation will get precipitated as sulphide.
So Cu2+ will precipitate as CuS on adding thiacetamide which has a blck colour. Black precipitate of CuS formed dissolves in nitric acid and a blue solution is obtained on addition excess of NH4OH .So presnce of Cu2+ is confirmed.
So the cations present are Pb2+ and Cu2+ .
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