Salt Analysis and Qualitative Analysis Solved Examples

SOLVED EXAMPLES Ex. 1 Pink colour of acidified KMnO4 is decolourised but there is no evolution of any gas. This may happen with the compound containing the following acid radical. (A) SO32– (B) NO 2– (C) S2– (D) All of these Ans. Sol. (D) (A) 5SO32– + 2MnO4– + 6H+ 2Mn2+ + 5SO42– + 3H2O (B) 2MnO4– + 5NO2– + 6H+ 2Mn2+ + 5NO3– + 3H2O (C) 2MnO4– + H2S + 6H+ 2Mn2+ + 5S + 8H2O Ex. 2 Which of the following gives a precipitate with Pb(NO3)2 but not with Ba(NO3)2? (A) Sodium chloride (B) Sodium acetate (C) Sodium nitrate (D) Disodium hydrogen phosphate (A) (A) Pb2+ + 2Cl– PbCl2(white) ; Ba2+ + 2Cl– BaCl2(water soluble) Ans. Ans. (B) (CH3COO)2 Pb and (CH3COO)2Ba both are water soluble salts. (C) Nitrates are mostly soluble in water (D) 3Pb2+ + 2HPO42– Pb3(PO4)2 (white) + 2H+ ; Ba2+ + HPO42– BaHPO4 (white) Ex. 3 Ans. Sol. Ex. 4 Colour of cobalt chloride solution is : (A) pink (B) black (C) colourless (D) green (A) Anhydrous Co(II) salts are blue in colour while hydrated Co(II) salts are pink/red. Ans. BaCl2 solution gives a white precipitate with a solution of a salt, which dissolves in dilute hydrochloric acid with the evolution of colourless, pungent smelling gas. The gas as well as the salt both are used as bleaching agent in the textile industries. The salt contains: (A) sulphite (B) sulphide (C) acetate (D) carbonate (A) Sol. Ba2+ + SO32–  BaSO3  (white) BaSO3 + 2HCl  BaCl2 + SO2(colourless pungent smelling gas) + H2O SO32– and SO2 both act as bleaching agent. Ex. 5 Ans. Which of the following precipitate(s) does / do not dissolve in excess of ammonia solution ? (A) Zn(OH)2 (B) Ni(OH)2 (C) Al(OH)3 (D) (B) and (C) both (C) Sol. (A) Zn(OH)2 + 4 NH3  [Zn(NH3)4]2+ (colourless solution) + 2OH– (B) Ni(OH)2  + 6NH3  [Ni(NH3)6]2+ (deep blue solution) + 2OH– (C) Al(OH)3 + NH3  No reaction. Ex. 6 Ans. Sol Chocolate brown precipitate is formed with : (A) Cu2+ ions and [Fe (CN)6]3– (C) Fe2+ ions and [Fe (CN)6]4– (B) (A) Cu3 [Fe(CN)6]2 (green) ; (C) Fe4[Fe(CN)6]3 (Prussian blue) ; (B) Cu2+ ions and [Fe(CN)6]4– (D) Fe2+ ions and dimethylglyoxime (B) Cu2 [Fe(CN)6]  ( chocolate brown) (D) red solution of iron(II) dimethylglyoxime. SALT ANALYSIS AND QUALITATIVE ANALYSIS Ex. 7 Ans. Sol. Ex. 8 Ans. Sol. Ex. 9 Ans. Sol. A red colouration or precipitate is not obtained when : (A) Fe3+ reacts with potassium thiocyanate (B) Fe2+ reacts with dimethylglyoxime. (C) Hg2+ reacts with potassium iodide. (D) None (D) (A) Fe3+ + 3SCN–  Fe(SCN)3 (red solution) (B) Red solution of iron(II) dimethylglyoxime. (C) Hg2– + 2I–  HgI2  (red). When H2S gas is passed through an ammoniacal salt solution X, a slightly white precipitate is formed. The X can be (A) a cobalt salt (B) a lead salt (C) a zinc salt (D) a silver salt (C) Zn2+ + H2S  ZnS  (white) + 2H+ Consider the following statement : S1 : Cu2+ ions are reduced to Cu+ by potassium iodide and potassium cyanide both, when taken in excess S2 : H2S will precipitate the sulphide of all the metals from the solutions of chlorides of Cu, Zn and Cd if the solution is aqueous. S3 : The presence of magnesium is confirmed in qualitative analysis by the formation of a white crystal line precipitate of MgNH4 PO4. S4 : Calomel on reaction with potassium iodide gives red precipitate. and arrange in the order of true /false. (A) TTFF (B) TFTF (C) TTTT (D) TTTF (D) S1, S2 and S3 are correct statements. S4 : Hg22+ + 2I–  Hg2I2 (green) Ex. 10 Ans. Sol. Which of the following pair (s) of ions would be expected to form precipitate when dilute solutions are mixed? (A) NH4+, [Co(NO2)6]3– (B) NH4+, CO32– (C) Fe3+, OH– (D) Ba2+, SO42– (A,C,D) (A) NH4+ + [Co(NO2)6]3–  (NH4)3 [Co(NO2)6]  (yellow) (B) Ammonium and alkali metal carbonates are water soluble. (C) Fe3+ + OH–  Fe(OH)3(reddish - brown) (D) Ba2+ + SO42–  BaSO4 (white) Comprehension (Q.11 to Q.13) Aqueous solution of salt (A) NaOH(aq)/warm Gas(B) Colourless gas which is alkaline in nature Solution of salt (C) FeSO4 conc. H2SO4 Zn/NaOH /heat dil.HCI White fumes Brown ring (D) Gas(B) at the junction of the two layers Salt (A) on heating gives a colourless neutral gas which supports combustion. From the aforesaid, flow diagram, answer the following questions. Ex. 11 Ans. Sol. The compound (A) contains the following acid radical. (A) NO2– (B) NO3– (C) Br– (D) SO32– (B) NO3– and NO2– both give brown ring test and reduction of NO3– and NO2– both give ammonia which with dilute HCl gives dense white fumes. if NH4NO3  N2O + 2H2O ; N2O supports the combustion  but NH4NO2 N2 + 2H2O ; Nitrogen does not supports combustion. – Hence, the anion is NO3 . Ex. 12 Ans. The basic radical of salt (A) and gas B both gives brown precipitate with Nessler’s reagent. The composition of the brown precipitate is : (A) (NH4)2 [HgI4] (B) Hg(NH2) NO3 (C) HgO. Hg (NH2)I (D) (NH4)3[Co(NO2)6] (C) Sol. NH4+ + 2[HgI4]2– + 4OH–  HgO. Hg (NH2)I + 7I– + 3H2O Hence the cation is NH4+. Ex. 13 Ans. 1 Which of the following statement is correct ? (A) Salt (A) gives yellow precipitate with chloroplatinic acid as well as with sodium cobaltinitrite. (B) The brown ring is formed due to the formation of nitroso ferrous sulphate [Fe(NO)]2+SO4–. (C) Salt ‘C’ reacts with silver nitrate solution to form white precipitate. (D) (A) and (B) both. (D) Sol. (A) 2NH4+ + [PtCI6]4–  (NH4)2 [PtCl6]  (yellow) 3NH4+ + [Co(NO2)]3–  (NH4)3 [Co(NO2)6]  (yellow) (B) 2NO3– + 4H2SO4 + 6Fe2+  6Fe3+ + 2NO  + 4SO42– + 4H2O SO42– + Fe2+ + NO  [Fe(NO)]2+ SO42– (C) Ag NO3 + NaNO3  No reaction. If the anion is NO2– then Ag+ + NO2–  Ag NO2  (white) Reactions : NH4NO3 + NaOH  NH3 + NaNO3 (A) (B) (C) NH3+ HCI  NH4CI (White) NO3– + 4Zn + 7OH– + 6H2O  NH3 + 4[Zn(OH)4]2– Ex. 14 Ans. Sol. Statement - 1 : Addition of NH4OH to an aqueous solution of BaCl2 in presence of NH4Cl (excess) precipitates Ba(OH)2 . Statement - 2 : Ba(OH)2 is water soluble. (A) Both Statement-1 and Statement-2 are true and Statement-2 is the correct explanation of Statement-1. (B) Both Statement-1 and Statement-2 are true but Statement-2 is not correct explanation of Statement-1. (C) Statement-1 is true but Statement-2 is false. (D) Statement-1 is false but Statement-2 is true (D) Ba2+ ions does not give any precipitate with NH4OH solution in excess of NH4Cl because product formed, Ba(OH)2 is soluble in water SALT ANALYSIS AND QUALITATIVE ANALYSIS Ex. 15 Ans. Statement - 1 : Sodium meta aluminate on boiling with ammonium chloride produces white gelatinous precipitate. Statement - 2 : Aluminium hydroxide is formed which is not soluble in water (A) Both Statement-1 and Statement-2 are true and Statement-2 is the correct explanation of Statement-1. (B) Both Statement-1 and Statement-2 are true but Statement-2 is not correct explanation of Statement-1. (C) Statement-1 is true but Statement-2 is false. (D) Statement-1 is false but Statement-2 is true (A) Sol. [Al(OH)4]– Ex. 16 Ans. Which of the following statement(s) is (are) incorrect? (A) Fe2+ ions give a dark blue precipitate with potassium hexacyanidoferrate (III) solution. (B) Fe3+ ions give intense blue precipitate with potassium hexacyanidoferrate (II) solution. (C) Fe3+ ions give a brown colouration with potassium hexacyanidoferrate (III) solution. (D) Fe2+ ions give a deep red colouration with ammonium thiocyanate. (D) Sol. (A) NH4Cl Boil Al(OH)3  + OH– Fe2+ + [Fe(CN)6]3–  Fe3+ + [Fe(CN)6]4– 4Fe3+ + 3 [Fe(CN)6]4–  Fe4[Fe(CN)6]3(turnbull's blue) (B) 4Fe3+ + 3[Fe(CN)6]4–  Fe4[Fe(CN)6]3 (intense blue) (C) Fe3+ + [Fe(CN)6]3–  Fe[Fe(CN)6] (brown colouration) (D) Fe3+ + 4SCN–  Fe (SCN)3 (deep red colouration) Fe2+ + 4 SCN–  No reaction Subjective : Ex. 17 What happens when ? (A) Aqueous solution of CrCl3 is added to ammonia solution. (B) Ammonium carbonates reacts with MgCl2 (i) in absence of ammonium salts and (ii) in presence of ammonium salts Sol. (A) Cr3+ + 3NH3 + 3H2O  Cr(OH)3  (green) + 3NH4+ Cr(OH)3 precipitate formed becomes slightly soluble in excess of precipitant in cold forming a violet or pink solution containing [Cr(NH3)6]3+ complex ions. Cr(OH)3  + 6 NH3  [Cr(NH3)6]3+ + 3OH– (B) (i) 5Mg2+ + 6 CO23 + 7H2O  4MgCO3. Mg(OH)2. 5 H2O  + 2HCO3– White precipitate of basic magnesium carbonate is formed. (ii) In presence of ammonium salts no precipitation occurs, because the equilibrium NH4+ + CO32–  NH3+ HCO3– is shifted towards the formation of HCO3– ions. Ex. 18 Ans. Salts given in column (I) reacts with the excess of reagents given in column (II) and form white /coloured precipitates. Select the correct options for the salts given in column (I) with the reagent(s) given in the column (II) Column - I Column - II (A) Zn (NO3)2 (p) Sodium hydroxide (B) Cu (NO3)2 (q) Ammonia solution (C) Fe(NO3)3 (r) Disodium hydrogen phosphate (D) Ag(NO3)2 (s) Potassium ferrocyanide (A - r, s) ; (B - p, r, s) ; (C - p, q, r) ; (D - p, q, r, s) Sol. (A) Zn2+ + 2OH–  Zn(OH)2 (white) ; Zn (OH)2 + 2OH–  [Zn(OH)4]2– (colourless soluble complex) Zn2+ + 2NH3 + 2H2O  Zn (OH)2  (white) + 2NH4+ Zn(OH)2  + 4NH3  [Zn(NH3)4]2+ (colour less soluble complex) + 2OH– 3Zn2+ + 2HPO42–  Zn3 (PO4)2  (white) + 2H+ 3 Zn2+ + 2K+ + 2[Fe(CN)6]4–  K2 Zn3 [Fe(CN)6]2  (bluish white) (B) Cu2+ + 2OH–  Cu(OH)2  (blue) Cu2+ + 4NH3  [Cu(NH3)4]2+ (deep blue soluble complex) Cu2+ + 2HPO42–  Cu3(PO4)2 (blue) + 2H+ Cu2+ [Fe(CN)6]4–  Cu2[Fe(CN)6]  (chocolate brown) (C) Fe3+ + 3OH–  Fe (OH)3  (reddish brown) Fe3+ + 3NH3 + 3H2O  Fe(OH)3  (reddish brown) + 3NH4+ Fe3+ + HPO42–  FePO4  (yellowish-white) + H+ excess 4Fe3+ + 3 [Fe(CN)6]4–  Fe4 [Fe(CN)6]3  (intense blue) K [Fe(CN) ] KFe [Fe(CN)6]. 4 6 (soluble prussian blue) (D) 2Ag + 2OH  Ag2O  (brown) + H2O + – 2Ag+ + 2NH3 + H2O  Ag2O (brown) + 2NH4+ 4Ag+ + HPO42–  Ag3PO4  (yellow) + H+ 4Ag+ + 3 [Fe(CN)6]4–  Ag4[Fe(CN)6]  (white) True/False : Ex. 19 Sol. Magnesium is precipitated from its salt solution as only magnesium ammonium phosphate by adding disodium hydrogen phosphate solution in absence of ammonium chloride and aqueous ammonia. (False) Precipitation is carried out in presence of ammonium chloride and aqueous ammonia as they prevent precipitation of magnesium hydroxide. Mg2+ + NH3 + HPO42–  Mg (NH4) PO4  (white) Ex. 20 Sol. When a solution of nitrite acidified with dilute hydrochloric acid is treated with solid urea, the nitrite is decomposed, and nitrogen and carbon dioxide are evolved. (True) CO(NH2)2 + HNO2  2N2  + CO2  + 3H2O. Ex. 21 Solution of alkali metal cyanide containing freshly prepared iron (II) sulphate solution and dilute H2SO4 on exposure to air produces prussian blue precipitate Sol. (True) Fe2+ + 2 CN–  Fe(CN)2  ; Fe(CN)2  + 4CN–  [Fe(CN)6]4– 4Fe2+ + O2 + 4H+  4Fe3+ + 2H2O ; Fe3+ [Fe(CN)6]4–  Fe4[Fe(CN)6]3 