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1EXPERIMENTC6: QUALITATIVEANALYSIS OFCATIONSLearning OutcomesUpon completion of this lab, the student will be able to:1)Analyze a given sample of an ionic compound and identify which of thefollowing cations are present: silver, lead (II), bismuth (III), iron (III),manganese (II), aluminum, chromium (III), barium, strontium, calcium,nickel (II), copper (II), magnesium, zincIntroductionThe cations being tested in the qualitative analysis scheme are organized intogroups labeled A through D (Table 1)GroupCationsAAg+, Pb2+BBi3+, Fe3+, Mn2+, Al3+, Cr3+CBa2+, Sr2+, Ca2+DNi2+, Cu2+, Mg2+, Zn2+TABLE1This grouping of ions was arrived at based on the differences in the solubility ofthese ions.GROUPA: Ag+, Pb2+The KSPof the chloride salts of the two ions in this group is given in Table 2.SaltKSPAgCl1.7 × 10-10PbCl21.6 × 10-5TABLE2The chloride salts of all the other cations that will be analyzed in this experiment arehighly soluble. Therefore, addition of aqueous HCl will result in a precipitatecontaining chlorides of the Group A cations.Ag+(aq)+ Cl−(aq)!AgCl(s)Pb2+(aq)+ 2Cl−(aq)!PbCl2(s)
2The chlorides of all other cations will remain in solution. When this mixture iscentrifuged: the precipitate will contain the group A cations and the supernatantwill contain the cations from the other groups.Of the two chloride precipitates, PbCl2is soluble in hot water and AgCl is soluble inaqueous ammonia.Test for Pb2+When the chloride precipitates of the Group A cations are mixed with hot water,PbCl2will dissolve. This is separated from AgCl by centrifuging the mixture. ThePbCl2in the supernatant can be confirmed by reacting the supernatant with asolution of KI, which results in a yellow precipitate of PbI2.PbCl2(s)+ H2O(l)+ heat!Pb2+(aq)+ 2Cl−(aq)Pb2+(aq)+ 2KI(aq)!PbI2(s)+ 2K+(aq)Test for Ag+The AgCl precipitate dissolves in aqueous ammonia due to the formation of acomplex ion.AgCl(s)+ 2NH3(aq)!Ag(NH3)2(aq)+ Cl−(aq)Addition of HNO3results in the decomposition of the silver ammonium complex.The free silver ions combine with the chloride ions from the above reaction to formthe AgCl precipitate again.Groups A, B, C, DAdd 6M HClPrecipitate: Group A-AgCl, PbCl2Supernatant: GroupsB, C, D
3GROUPB: (Bi3+, Fe3+, Mn2+, Al3+, Cr3+)The hydroxides of the cations in this group are almost completely insoluble. The KSPof the hydroxide salts are given in Table 3.SaltKSPMn(OH)24.5 × 10-14Fe(OH)31.0 × 10-36Bi(OH)3InsolubleAl(OH)33.7 × 10-15Cr(OH)36.7 × 10-31TABLE3The hydroxide salts of the Group C cations are far more soluble. Although, thehydroxide salts of the Group D cations are also quite insoluble, the distinctionbetween the hydroxide salts of Group B cations vs. Group D cations is that, thehydroxide salts of the Group D cations are soluble in aqueous ammonia, whereasthose of the Group B cations are not.