UPSC Combined Geo-Scientist Main 2024 Chemistry Paper-I

 

5x10=50

1. (a) Match the following :

(i) Dolomite 1. Alumina

(ii) Corundum 2. MgCO₃·CaCO₃

(iii) Sphalerite 3. CuFeS₂

(iv) Silicon 4. ZnS

(v) Chalcopyrite 5. Semiconductor

(b) The observed magnetic moment for K₃[TiF₆] is 1.70 μ_B.

(i) Calculate μ (spin only) for this complex.

(ii) Why is there a difference between calculated and observed values? 5

(c) Match the following :

Titrations involving

A B

(i) Strong acid vs. Weak base 1. Methylene blue pK_a = 3.8

(ii) Weak acid vs. Strong base 2. Bromothymol blue pK_a = 7.2

(iii) Strong acid vs. Strong base 3. No indicator

(iv) Weak acid vs. Weak base 4. Phenolphthalein pK_a = 9.3

(v) Redox reactions 5. Methyl orange pK_a = 3.5

(d) A sample of slag from a blast furnace is analyzed for SiO₂ by decomposing a 0-5003 g sample with HCl leaving a residue with a mass of 0-1414 g. After treating with HF and H₂SO₄, and evaporating the volatile SiF₄, a residue with a mass of 0-0183 g remains. Determine the % w/w SiO₂ in the sample. 5

(e) Given that the standard electrode potentials for the reactions

2Fe²⁺(aq) ⇌ 2Fe³⁺(aq) + 2e⁻ (i)

H₂O₂ + 2H⁺(aq) + 2e⁻ ⇌ 2H₂O (ii)

are −0.77 V and 1.77 V respectively. Determine whether the oxidation of Fe²⁺ to Fe³⁺ is spontaneous or not. 5

UPSC Previous year question Combined Geo Scientist Main examinantion 2024 Chemistry 1st

(f) MnO₄⁻ is a stronger oxidizing agent than ReO₄⁻. Explain and relate the oxidizing abilities of these ions with the relative positions of their charge- transfer absorptions. 5

(g) What pH is better for the effective use of CN⁻ as a masking agent? Explain the reason for this pH dependency. 5

(h) The complex [Fe₂(Cp)₂(CO)₄] has one strong IR band at 1800 cm⁻¹ and other bands appeared at ~1980 cm⁻¹ and 2020 cm⁻¹ for CO. Account for each band with proper reason. 5

(i) Which of the curves represent nuclear decay? Justify your answer with mathematical logic: 5

[graph: Activity (A), cpm vs Time, h]

(j) Explain why stable and readily isolable carbonyl complexes are unknown for the lanthanoids. 5

SECTION-B

Attempt any one question

2. (a) (i) Differentiate between fluorite and antifluorite.

(ii) Differentiate between Schottky defect and Frenkel defect.

(iii) Differentiate between spinel and inverse spinel. 5+5+5=15

(b) Discuss each of the following observations: 10

(i) The [CoCl₄]²⁻ ion is a regular tetrahedron but [CuCl₄]²⁻ has a flattened tetrahedral structure.

(ii) The electronic spectrum of [CoF₆]³⁻ contains two bands with maxima at 11500 cm⁻¹ and 14500 cm⁻¹.

(c) What is polymorphism in crystals? Give an example. 5

3. (a) The titration curve for a weak acid vs. strong base (NaOH) has the following shape:

[graph: pH vs Vol. of OH⁻ added]

(i) Identify the weak acid :

I. CH₃COOH (K_a = 1.69×10⁻⁵)

II. HCN (K_a = 5×10⁻¹⁰)

III. Benzoic acid (K_a = 6.3×10⁻⁵)

IV. Pyridinium ion (K_a = 5.9×10⁻⁶)

(ii) Explain the process and species involved in the regions marked as (1), (2), (3) and (4) that contribute to the pH at near regions. 15

(b) What are the general criteria of choosing a suitable indicator for a given volumetric titration? 5

(c) Give the salient features of estimation of sodium carbonate and bicarbonate mixtures by acidimetry. Which two acid-base indicators are used and why? 10

SECTION-C

Attempt any one question

4. (a) Oxymethoglobin is bright red, whereas deoxyhemoglobin is purple. Explain the difference in colour qualitatively. 10

(b) Given the reactants PPh₃, NH₃ and [PtCl₄]²⁻, propose efficient routes to both cis and trans [PtCl₂(NH₃)(PPh₃)]. 10

(c) Iron(II) oxide, FeO crystal has a cubic structure and each edge of the unit cell is 5.0 Å. Taking density of the oxide as 4.0 g cm⁻³, calculate the number of Fe²⁺ and O²⁻ ions present in each unit cell. 10

5. (a) The compound [Ni₃(C₂H₅)₃(CO)₂] has a single CO stretching absorption at 1761 cm⁻¹. The IR data indicate that all C₅H₅ ligands are pentaahapto and probably in identical environments.

(i) On the basis of these data, propose a structure.

(ii) Does the electron count for each metal in the structure agree with the 18e⁻ rule? 10

(b) Identify the required particles to produce the listed product nuclei: 10

[graph: ²⁷₁₃Al branching to ²⁸₁₄Si, ²⁸₁₃Al, ³¹₁₅P, ³⁵₁₇Cl, ³⁹₁₉K with ?]

(c) An ore containing magnetite, Fe₃O₄ was analyzed by dissolving a 1.500 g sample in conc. HCl giving a mixture of Fe²⁺ and Fe³⁺. After adding HNO₃ to oxidize any Fe²⁺ to Fe³⁺, the resulting solution was diluted with water and Fe³⁺ precipitated as Fe(OH)₃ by adding NH₃. After filtering and rinsing, the residue was ignited giving 0.8525 g of pure Fe₂O₃. Calculate the % w/w Fe₃O₄ in the solution. [Given : FW of Fe₂O₃ = 159.69 g/mol and FW of Fe₃O₄ = 231.54 g/mol] 10

SECTION-D

Attempt any one question

6. (a) A thermogram was recorded for calcium oxalate monohydrate, CaC₂O₄·H₂O. The original sample weighed 24.60 mg and was heated from room temperature to 1000°C at a rate of 5°C/min. The following changes in mass and corresponding temperature ranges were observed :

Loss of 3.03 mg from 100−250°C

Loss of 4.72 mg from 400−500°C

Loss of 7.41 mg from 700−850°C

Determine the solid residue at each step of the thermal decomposition. 15

(b) Match the following :

(i) Cupferron 1. Co²⁺

(ii) Cupron 2. Ni²⁺

(iii) 1- Nitroso- 2- naphthol 3. Al³⁺

(iv) Dimethylglyoxime 4. Cu²⁺

(v) 8- Hydroxyquinoline 5. Fe³⁺

6. Pb²⁺

7. Cr³⁺

(c) Discuss the following observations :

(i) ΔH° for the formation of [Ln(EDTA)(OH₂)ₓ]⁻ (x = 2 or 3) in aqueous solution is nearly constant for all Ln and is almost zero.

(ii) Many actinoid oxides are non- stoichiometric but few lanthanoid oxides are.

7. (a) Explain the terms oxidative addition and reductive elimination in the hydrogenation reactions and how the coordination numbers of the metal atom alter with an example. 10

(b) (i) Draw the two isomeric forms of Co₂(CO)₈.

(ii) Show the cluster structure of Ir₄(CO)₁₂ metal- metal bonds. 10

(c) Ordinary bottle glass appears nearly colourless when viewed through the wall of the bottle but visibly coloured (green) when viewed from the end so that light has a long path through the glass. The colour is associated with the presence of Fe³⁺ in the silicate matrix. Explain this observation. 10

SECTION-E

Attempt any one question

8. (a) Complete the following by giving the missing part on the right or left hand side of the equations and also identify their applications in daily life or industry : 15

(i) PbSO₄ + 2e⁻ →

(ii) PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ → ??

(iii) 6CO₂ + 6H₂O →(hν) ??

(iv) ?? → Cu

(b) Following are the standard redox potentials for the reactions in aqueous media :

Ag²⁺ + e⁻ ⇌ Ag⁺ +1.98 V

Mg²⁺ + 2e⁻ ⇌ Mg −2.37 V

Li⁺ + e⁻ ⇌ Li −3.04 V

Ca²⁺ + 2e⁻ ⇌ Ca −2.87 V

Na⁺ + e⁻ ⇌ Na −2.71 V

Arrange the above metals in the order of their increasing reducing power. 5

(c) Establish that the end point in a redox reaction occurs when

E = E°_(In_ox/In_red) − 0.05916/n

10

9. (a) Calculate the activity (in dpy) due to ¹⁴C in 1.00 kg of carbon found in a living specimen. The abundance of ¹⁴C in a carbon sample present-day specimen is 1.3×10⁻¹² (t₁/₂ of ¹⁴C is 5730 y, N_A = 6.023×10²³ atoms/mole, At. mass of carbon = 12.011 u). 10

(b) Identify the missing part in the following nuclear reactions : 5

⁷₃Li + ? → ¹⁰₅B + ¹₀n

¹²₆C + ¹₁H → ? + γ

¹²₆C + ? → ⁸₄Be + ⁴₂He

⁴¹₁₉K + ¹₀n → ⁴⁰₁₉K + ?

²⁷₁₃Al + ²₁d → ? + ¹₀n

(c) Explain the relation between elementary separation factor, α and enrichment factor, ε in terms of abundance of a particular isotope in a mixture during the separation. 10

(d) A sample of radioactive isotope with t₁/₂ of 1600 y has an activity of 20 mCi (Ci = curie). How long it would take the activity to be reduced to almost 1 mCi (≈1.25 mCi)? 5

10. (a) The EDTA titration of mixtures of Ca²⁺ and Mg²⁺ can be followed thermometrically. Sketch the thermometric titration curve for a mixture of 5.00×10⁻³ M Ca²⁺ with 5.00×10⁻³ M Mg²⁺ with 0.0100 M EDTA. The heats of formation for CaY²⁻ and MgY²⁻ are respectively −23.9 kJ/mol and −23.0 kJ/mol. (Given: log K₁ for Ca²⁺- EDTA complex = 10.69 and log K₁ for Mg²⁺- EDTA complex = 8.79) 10

(b) Show that F⁻ is an effective masking agent in preventing a reaction of Al³⁺ with EDTA. Assume that only significant forms of fluoride and EDTA are F⁻ and Y⁴⁻. (Given : (i) log K₁ = 6.11, log K₂ = 5.01, log K₃ = 3.88, log K₄ = 3.00, log K₅ = 1.4, log K₆ = 0.4 for the complexes between Al³⁺ and F⁻ and (ii) log K for the complex between EDTA and Al³⁺ = 16.3) 10

(c) (i) Write the following ligand-exchange reactions :

1. [Co(H₂O)₆]³⁺ + 6NH₃ →

2. [Co(H₂O)₆]³⁺ + 3en →

(ii) Which of the reactions should have a larger ΔS°?

(iii) Given that Co—N bond length is approximately same in both complexes. Which reaction will have a larger equilibrium constant? Explain your choice. 10

11. (a) Calcium in powdered milk is determined by ashing a 1.50 g sample and then titrating calcium with EDTA solution, 12.1 mL being required. EDTA was standardized by titrating 10.0 mL of a zinc solution prepared by dissolving 0.632 g zinc metal in acid and diluting to 1 L (10.8 mL EDTA is required for titration). What is the concentration of calcium in the powdered milk in parts per million? 10

(b) Give the trends in the stability of higher oxidation states in the first row d-block metal compounds. 10

(c) (i) Explain why isostructural HfO₂ and ZrO₂ have densities of 9.68 g cm⁻³ and 5.73 g cm⁻³ respectively.

(ii) Why are high coordination numbers not usual for first row d-block elements? 10

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