Group III cations (Fe, Al, Cr)

46 questions on Group III cations (Fe, Al, Cr) from the Qualitative Analysis: Cations chapter: 10 NEET level, 18 JEE Main, 18 JEE Advanced. Each one opens with its answer and solution.

• The precipitate of Fe(OH)3 obtained in group III is NEET level
• Group III cations (Fe^3+, Al^3+, Cr^3+) are precipitated as hydroxides by the group reagent, which is NEET level
• A white gelatinous precipitate that dissolves in excess NaOH to give a clear solution is most likely NEET level
• On adding potassium thiocyanate solution to a solution of Fe^3+, the colour obtained is NEET level
• The colour of freshly precipitated Cr(OH)3 is NEET level
• In the group III analysis, NH4Cl is added before NH4OH mainly to NEET level
• A pale-green solution gives a deep blue precipitate with K3[Fe(CN)6] and no blood-red colour with KSCN. The cation present is JEE Main level
• Before adding NH4Cl and NH4OH for group III, the filtrate from group II is boiled with a few drops of concentrated HNO3. The main purpose is to JEE Main level
• Grey-green Cr(OH)3 is warmed with NaOH and H2O2, the yellow solution is acidified with acetic acid and lead acetate is added. The change in the oxidation number... JEE Main level
• Which reagent--observation pair is correct for the confirmatory test of Al^3+ (after precipitating Al(OH)3 in ammoniacal medium)? JEE Main level
• Assertion (A): In group III, Mg^2+ is not precipitated although Mg(OH)2 is only sparingly soluble. Reason (R): NH4Cl lowers [OH^-] so that the ionic product of ... JEE Main level
• Match the compound in List I with its colour in List II. List I: (P) Fe(OH)3, (Q) Al(OH)3, (R) Cr(OH)3, (S) Fe4[Fe(CN)6]3. List II: (1) white, gelatinous; (2) d... JEE Main level
• Prussian blue, Fe4[Fe(CN)6]3, is formed when Fe^3+ reacts with K4[Fe(CN)6]. Which description is correct? JEE Main level
• A precipitate containing Fe(OH)3, Al(OH)3 is treated with excess NaOH and filtered. The filtrate, on boiling with solid NH4Cl, gives a white gelatinous precipit... JEE Main level
• Which of the following statements about group III are correct? JEE Main level
• Which of the following are yellow? JEE Main level
• A solution contains 3.36 g of Fe^3+. It is treated with excess K4[Fe(CN)6] and the whole of the iron(III) is converted to Prussian blue, Fe4[Fe(CN)6]3 (4Fe^3+ +... JEE Main level
• A solution is 0.010 M in Mg^2+ and 0.010 M in Fe^3+. It is made 0.10 M in NH3 and 1.00 M in NH4Cl. Take K_b(NH3) = 1.8x10^-5, K_sp[Mg(OH)2] = 1.2x10^-11, K_sp[F... JEE Advanced level
• Base is added slowly to a solution that is 1.0x10^-3 M in each of Fe^3+ and Al^3+. Take K_sp[Fe(OH)3] = 1.0x10^-38 and K_sp[Al(OH)3] = 3.0x10^-34. Which hydroxi... JEE Advanced level
• A green solution of salt X gives a grey-green precipitate with NH4Cl/NH4OH. The precipitate, warmed with NaOH and H2O2, gives a yellow solution that yields a ye... JEE Advanced level
• A brown precipitate from group III dissolves in dilute HCl giving a yellow solution that gives a blood-red colour with KSCN and a deep blue precipitate with K4[... JEE Advanced level
• Acidified chromate solution is shaken with H2O2 and ether, giving a blue colour in the ether layer, due to CrO5 (one oxo, two peroxo groups). The oxidation stat... JEE Advanced level
• Which statement correctly explains why Al^3+ is NOT detected as a sulphide when H2S is passed through the ammoniacal solution (group IV step) containing it? JEE Advanced level
• A solution of FeCl3 in dilute HCl is saturated with H2S. If 0.20 mol of Fe^3+ is present, the observation and mass of sulphur formed (S = 32) are JEE Advanced level
• A mixture of Fe(OH)3, Al(OH)3 and Cr(OH)3 is boiled with excess NaOH and Na2O2 and filtered. Which description is correct? JEE Advanced level
• Select the correct statements about the preparation of the filtrate for group III. JEE Advanced level
• Which of the following statements about chromium species are correct? JEE Advanced level
• Select the correct statements (high-spin unless a strong-field ligand is stated). JEE Advanced level
• The group reagent for group III cations (Fe^3+, Al^3+, Cr^3+) is NEET level
• The precipitate obtained when ammonium hydroxide is added to FeCl3 solution (in presence of NH4Cl) is NEET level
• Addition of NH4OH (with NH4Cl) to AlCl3 solution gives NEET level
• Potassium thiocyanate added to a solution of Fe^3+ gives NEET level
• Assertion (A): In group III, solid or aqueous NH4Cl is added before NH4OH, and Mg^2+ then stays in solution. Reason (R): NH4Cl suppresses ionisation of NH4OH by... JEE Main level
• Which combinations give an intense blue precipitate immediately (air excluded) in the cation tests? JEE Main level
• Aluminium hydroxide, formed as a white precipitate with a little NaOH, dissolves in excess NaOH. On adding solid NH4Cl to this clear solution and boiling, a whi... JEE Main level
• Which cations give a precipitate on adding NH4OH in presence of excess NH4Cl? JEE Main level
• In alkaline medium, 2[Cr(OH)4]- + 3H2O2 + 2OH- -> 2CrO4^2- + 8H2O. The number of moles of H2O2 required to oxidise one mole of Cr^3+ (as [Cr(OH)4]-) is JEE Main level
• Before precipitating group III, the filtrate from group II is boiled with a few drops of conc. HNO3. The main purpose of the nitric acid is JEE Main level
• A few drops of blue litmus are added to a solution containing Al^3+, which is then made slightly alkaline with NH4OH. The observation that indicates Al^3+ (lake... JEE Main level
• A pale green solid dissolves in water to give a pale green solution that gives a deep blue precipitate with K3[Fe(CN)6]. The cation present, and the moles of KM... JEE Advanced level
• A green solution of a chromium(III) salt is treated with excess NaOH and H2O2 and warmed; the yellow solution is acidified with acetic acid and lead acetate is ... JEE Advanced level
• A buffer is 0.10 M in NH4OH and 1.0 M in NH4Cl (K_b = 1.8x10^-5). A solution containing 0.010 M each of Mg^2+ and Al^3+ is made up in this buffer. Take K_sp: Mg... JEE Advanced level
• A red solution formed by Fe^3+ and thiocyanate, [Fe(SCN)(H2O)5]^2+, is decolourised on adding excess sodium fluoride: [Fe(SCN)(H2O)5]^2+ + 6F- -> [FeF6]^3- + SC... JEE Advanced level
• The residue of group III hydroxides is treated with NaOH and H2O2 and boiled. The mixture is filtered, and the yellow filtrate (containing [Al(OH)4]- and CrO4^2... JEE Advanced level
• Excess aqueous Fe^3+ is treated with K4[Fe(CN)6] to give insoluble Prussian blue, 4Fe^3+ + 3[Fe(CN)6]^4- -> Fe4[Fe(CN)6]3. The moles of K4[Fe(CN)6] required to ... JEE Advanced level
• Which statements about the hydroxides of group III cations are correct? JEE Advanced level