JEE Advanced Chemistry: Qualitative Analysis questions with solutions

Q1. An aqueous solution of CuSO4 is taken in a test tube. When which of the following gases is passed into it, a metal ion gets precipitated?

1. H2S(g)
2. HCl(g)
3. NH3(g) in excess
4. PH3(g)

Answer: H2S(g)

When H2S gas is bubbled into CuSO4 solution, the highly insoluble CuS (Ksp ~ 10⁻³⁶) precipitates as a black solid: CuSO4 + H2S -> CuS(s) + H2SO4. This is a standard Group II qualitative analysis reaction.

Q2. In qualitative analysis of cations using group reagents, 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

Group II cations (Cu²+, Pb²+, Bi³+, etc.) are precipitated as sulphides in dilute HCl medium with H2S. Group IV cations (Zn²+, Mn²+, Co²+, Ni²+) are precipitated as sulphides in ammoniacal medium. Hence Groups II and IV are precipitated as sulphides.

Q3. Match each reagent/test in Column I with its correct observation in Column II. Column I: (A) NO2- + dil. H2SO4 + FeSO4(aq), (B) S2- + Na2[Fe(CN)5NO], (C) Flame test of Ba2+, (D) Pb2+ + excess NaOH Column II: (P) Purple colour in solution, (Q) Brown ring formation, (R) Colourless solution formed, (S) Apple green colour flame, (T) A complex compound is the product

1. (A)-(Q), (B)-(P), (C)-(S), (D)-(T)
2. (A)-(R), (B)-(T), (C)-(S), (D)-(P)
3. (A)-(Q), (B)-(T), (C)-(S), (D)-(R)
4. (A)-(P), (B)-(Q), (C)-(R), (D)-(S)

Answer: (A)-(Q), (B)-(P), (C)-(S), (D)-(T)

A-Q (brown ring for NO2-), B-P (purple with nitroprusside and S2-), C-S (apple green flame for Ba2+), D-T (Pb(OH)2 dissolves in excess NaOH forming [Pb(OH)4]2-).