93 CHM-153 Advance Study Program CATIONS QUALITATIVE ANALYSIS OF GROUP NAME DATE
ID: 546154 • Letter: 9
Question
93 CHM-153 Advance Study Program CATIONS QUALITATIVE ANALYSIS OF GROUP NAME DATE 101131 1. What is the reagent responsible for the formation of hydrogen sulfide (H2S)? 2. When the cations are separated into subgroups, what is the purpose of the potassium hydroxide, KOH? 3. Explain the function of the aluminum wire and the mercury (l) chlonide when testing for tin. 4. When testing for antimony, what reagent prevents tin from reacting with the H2S to form SnS2? 5. What reagent is used to identify copper and explain its function.Explanation / Answer
1) In Lab, H2S is generated in Kipp's apparatus by treating Ferrous sulphide with HCl
Reaction will be
FeS + 2HCl --> FeCl2 + H2S
It can also be genrated by thioacetamide as shown below
CH3C(S)NH2 + H2O ---> CH3C(O)NH2 + H2S
2) KOH is used
a) It helps in identification of a group of cation as the sulphides of copper group [Hg+2 ,Cu+2 , Pb+2 etc]are insoluble in KOH while sulphide of arsenic subgroups [As+3 , Sb+3 ,Sn+4] are soluble in KOH
b) Similarly lead chromate is soluble in KOH while Bismuth salt is insolulbe
c) stannic compounds are boiled with KOH to convert them into metastannic forms.
3) While testing Tin Aluminium wire act as reducing agent and reduces Sn+4 to Sn+2. This Sn+2 will react with HgCl2 and reduce it to Hg2Cl2 (white ppt).
4) Ammonia is the reagent which prevents tin from reacting with H2S to forms SnS2
5) The confirmatory test for copper ion is
Potassium ferrocyanide test:
Here the Cu+2 reaction with H2S it will form black ppt. The black ppt of CuS will get dissolved in nitric acid. The dissolved salt will give blue solution in treatment with excess of NH4OH. to this blue solution we will add acetic acid and potassium ferrocyanide solution. This will give Chocolate brown precipitate confirming Cu+2.
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