a solution containing four cations was analyzed by a student, using the procedur
ID: 1016800 • Letter: A
Question
a solution containing four cations was analyzed by a student, using the procedure described in this experiment. Based on the following results, identify cations present in the solution. Explain which tests confirm your identifications.
The student added 6M NH3 and 6M HNO3 to the solution. A precipitate formed. The student centrifuged the mixture and saved the supernatant liquid in test tube A. The student added 1M NaOH to the precipitate, centrifuged the mixture, then added 2M HC2H3O2 and aluminon (with heating) to the decanted supernatant liquid. No reaction occurred. The student then dissolved the precipitate in 6M HNO3. He equally divided the resulting solution into two test tubes. He added KBiO3 to one of the test tubes, which resulted in a dark purple solution. He added KSCN to the solution in the other test tube. No reaction occurred.
Next, the student added 1M (NH4)2CO3 to the original supernatant liquid in test tube A. No precipitate formed. He then added 1M Na2HPO4 to test tube A, and a white precipitate formed. He decanted the supernatant liquid, then heated it to dryness. This yielded a dark residue that turned pale gray upon additional heating. He then dissolved the residue in 1M H3PO4 AND 1M NA2HPO4. He equally divided the resulting solution among three test tubes. He added KI to one test tube; the result was a red-brown solution and a pale tan precipitate. He added dimethylglyoxime to the second test tube and obtained a red precipitate. He added 1-nitroso-2-napthol to the third test tube, and there was no reaction.
Explanation / Answer
As addition of 6M NH3 & 6M HNO3 to the solution gives precipitate that means the solid may contain Al(OH)3, Fe(OH)3, and Mn(OH)2.
Addition of KSCN gives no red coloured solution. Thus Fe3+ is not there. But addition of KBiO3 gives dark purple coloured solution which is a confirmatory test for the presence of Mn2+. The bismuthate ion is reduced to Bi3+ accompanied by the oxidation of Mn2+ to form the purple permanganate ion, MnO4 – .
Addition of NaH2PO4 gives white precipitate indicates the presence of magnesium ion (Mg2+) due to the formation of MgHPO4.
Mg+2(aq) + HPO4-2 (aq) MgHPO4(s)
The dark residue which turned pale gray upon additional heating indicates the presence of Co2+, Cu2+, Ni2+. But addition of KI solution gives red-brown solution as well as a pale tan ppt, that indicates that Cu2+ is there.
Addition of dimethylglyoxime produces red precipitate which is a confirmatory test for Ni2+.
Ni(NH3)6+2(aq) + 3DMG- (aq) Ni(DMG)3 - (aq) + 6NH3(g)
Thus the 4 cations are Mn2+,Mg2+, Cu2+, Ni2+.
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