JEE Advanced Chemistry: Qualitative Inorganic Analysis questions with solutions

Q1. In the confirmatory test for phosphate ions, a canary yellow precipitate is formed. What is the correct chemical formula of this precipitate?

1. (NH4)3PO4 * 12MoO3
2. (NH4)2MoO4
3. (NH4)3PO4 * 12MoO3 * xH2O
4. MoS2

Answer: (NH4)3PO4 * 12MoO3

When a phosphate solution is treated with ammonium molybdate in excess nitric acid and warmed, a characteristic canary yellow precipitate of ammonium phosphomolybdate, (NH4)3PO4 * 12MoO3, is formed, confirming the presence of phosphate ions.

Q2. In the qualitative analysis of cations, in which groups are the cations precipitated as sulphides?

1. Group II and III
2. Group II and IV
3. Group II and V
4. Group VI and III

Answer: Group II and IV

In the standard scheme of qualitative cation analysis: Group II (Cu2+, Pb2+, Bi3+, Cd2+, As3+, Sb3+, Sn2+) are precipitated as insoluble sulphides by passing H2S in dilute HCl. Group III (Fe3+, Al3+, Cr3+) are precipitated as hydroxides by NH4OH. Group IV (Co2+, Ni2+, Mn2+, Zn2+) are precipitated as sulphides in ammoniacal solution (NH4OH + NH4Cl + H2S). So sulphide precipitation occurs in Group II and Group IV.

Q3. In the systematic qualitative analysis of cations using group reagents, cations of which groups are precipitated in the form of sulfides?

1. Group II and III
2. Group II and IV
3. Group II and V
4. Group VI and III

Answer: Group II and IV

In qualitative analysis, Group II cations (Cu2+, Pb2+, Bi3+, etc.) are precipitated as sulfides by passing H2S in dilute HCl medium. Group IV cations (Co2+, Ni2+, Zn2+, Mn2+) are precipitated as sulfides by ammonium sulfide ((NH4)2S) in ammoniacal medium.