Scheelite is a calcium tungstate mineral with the chemical formula CaWO₄. Pyrochlorite (CaNaNb₂O₆) is an important ore/mineral of niobium.

Ilmenite (Fe.TiO₃) is an oxide ore of titanium. Hausmannite (Mn₃O₄) is an oxide ore of manganese. Linnaeite (Co₃S₄) is the sulphide ore of cobalt. Hence, only ilmenite (Fe.TiO₃) is the oxide ore of titanium.

(P) Manganese is an important additive in steel making and acts as a scavenger as it has the property of removing both oxygen and S and thus preventing bubbles and brittleness. (Q) Technetium (⁹⁹Tc) was the first man-made element with radioactive property. All known isotopes of technetium (includes ⁹⁷Tc, ⁹⁸Tc) have radioactive property. It is one of the fission products of uranium. It obtained by spent fuel rods in nuclear reactors. (R) Rhenium is used in making electric furnace windings and spectrophotometer filaments due to its very high melting point (3180^oC).

₂₆Mn²⁺has the electronic configuration of 3d⁵4s⁰. 3d⁷4s⁰ is the electronic configuration of ₂₇Co²⁺.

1. The floatation process is the concentration method of sulphide ores. Zinc ores mainly ZnS are concentrated by floatation method, then roasted in air to give ZnO and SO₂.
   1. Zn is used extensively for coating for the appliances made up of corrosive metals like Fe to prevent them from rusting.
   2. Cd metal absorbs neutrons very well, and is used to make control rods in nuclear reactors.
   3. The largest use of mercury is in electrolytic cells for the production of sodium hydroxide and chlorine gas at commercial level.

Wilkinson’s catalyst ([PH₃P]₃.Rh.Cl) is used in hydrogenation of alkenes. Adams catalyst (Pt/PtO) is used in various reduction reactions. Lazier catalyst (2CuO.Cr₂O₃) is an effective catalyst for the reduction of aldehydes, ketones and esters to the corresponding alcohols, and aliphatic double bonds. Ziegler-Natta catalyst (TiCl₄/Al(C₂H₅)₃) is used in the synthesis of polymers of alpha-olefins. Hence, the representation of codes A - 2, B - 1, C - 4, D - 3 is correct.

Chalcocite (CuS) is a sulphide ore of copper. Franklinite (ZnO.Fe₂O₃) is an ore of zinc. Siderite (FeCO₃) is the carbonate ore of iron. Smaltite (CoAs₂) is the arsenide ore of cobalt. Hence, the correct answer is P - 2 - i, Q - 3 - iv, R - 4 - iii, S - 1 - ii.

₂₉Cu has the electronic configuration of 3d¹⁰, 4s¹. ₄₂ Mo has the electronic configuration of 4d⁵, 5s¹. ₄₄ Ru has the electronic configuration of 4d⁷, 5s¹. ₄₅Rh has the electronic configuration of 4d⁸, 5s¹.

Hence, the correct answer is P - 3, Q - 4, R - 1, S - 2.

H₃Ru₃NiCp(CO)₉ clusters contain Ru-Ni as core metals and catalyse the ammonia synthesis.

The mixture of iron ore, coke and limestone is called ‘charge’. Iron ore is not pure iron oxide. It also contains an assortment of rocky material. This wouldn't melt at the temperature of the furnace, and would eventually clog it up. The limestone is added to convert this into slag which melts and runs to the bottom. The heat of the furnace decomposes the limestone to give calcium oxide.

P. The central metal atom in potassium chromate and potassium dichromate is Cr. Chromium has +VI oxidation state in K₂CrO₄ and K₂Cr₂O₇. Q. Limonite (Fe₂O₃.3H₂O) is an ore of iron and willemite is an ore of zinc. R. Zinc oxide is an inorganic compound with the formula ZnO. ZnO is a white powder that is insoluble in water, and it is widely used as an additive in numerous materials and products including rubbers, plastics, ceramics, glass, cement, etc. White lead (PbCO₃).2Pb(OH)₂ is a complex chemical compound containing both a carbonate and a hydroxide portion and occurs naturally as a mineral for lead. White lead compounds were used as pigmenting materials like cement, lubricants, etc. but due to its toxicity, it is not used frequently.  Hence, both the statements Q and R are incorrect.

In octahedral complexes, CFSE is given by the formula: CFSE = value of t²g (n) + value of eg (n) Dq, Now, CFSE for low-spin d⁴ = -4 (4) + 6 (0) = -16 + 0 = -16 Dq CFSE for high-spin d⁴ = -4 (3) + 6 (1) = -12 + 6 = -6 Dq CFSE for low-spin d⁵ = -4 (5) + 6 (0) = -20 + 0 = -20 Dq CFSE for high-spin d⁵ = -4 (3) + 6 (2) = -12 + 12 = 0 Dq CFSE for low-spin d⁷ = -4 (5) + 6 (2) = -20 + 12 = -8 Dq Hence, low-spin d⁵ will give maximum CFSE in an octahedral complex.

In a weak field ligand, the filling of electrons is in accordance with Hund’s rule. The CFSE is given by the formula  CFSE = value of t²g (n) + value of eg (n) Dq, Fe³⁺ corresponds to the d⁵ configuration. Now, CFSE for Fe³⁺ (d⁵) = - 4 (3) + 6 (2) = - 12 + 12 = 0 Dq CFSE for Fe²⁺ (d⁶) = - 4 (4) + 6 (2) = - 16 + 12 = - 4 Dq CFSE for Cr²⁺ (d⁴) = - 4 (3) + 6 (1) = - 12 + 6 = - 6 Dq CFSE for Mn⁺ (d⁶) = - 4 (4) + 6 (2) = - 16 + 12 = - 4 Dq CFSE for Mn³⁺ (d4) = - 4 (3) + 6 (1) = - 12 + 6 = - 6 Dq Hence, the metal ion with zero CFSE is Fe³⁺ when it is associated with weak field ligand.

Hafnium is used to make control rods for regulating the free neutron levels in the nuclear reactors employed in submarines. The mixture of AlEt₃ and TiCl₄ is called Zeigler-Natta catalyst and is used extensively in polymerising alkenes (like ethylene to polyethylene). VC₂ (vanadium carbide) on treated with water produce ethyne and thus it is used in the production of ethyne. Pt-Re alloys are used as catalysts for making low-lead or lead free petrol. Hence, the correct sequence is A - 3, B - 1, C - 4, D - 2.

P. Iron (Fe) is used as a catalyst in the manufacture of ammonia by Haber process. CuCl₂ is used as catalyst in Deacon process for the preparation of Cl₂ from HCl. Q. CuCl in aqueous solution form Cu⁺ ions, which possess no colour due to the presence of completely filled d-orbitals. R. Zone melting is a group of techniques used to purify an element or a compound/metal or control its composition by melting a short region (i.e. zone) and causing this liquid zone to travel slowly through a relatively long ingot, or charge of the solid. As the zone travels, it redistributes impurities along the charge. The final distribution of the impurity depends on its distribution in the starting charge of material. This method is based on fractional crystallisation and so it is also called as zone-refining. Hence, both the statements P and Q are false.