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## ISC Chemistry 2015 Class-12 Previous Year Question Paper Solved

-: Select Options :-

Part – I

Part – II Section – A

Part – II Section – B

Part – II Section – C

Maximum Marks: 70
Time allowed: 3 hours

• Answer all questions in Part I and six questions from Part II, choosing two questions from Section A, two from Section B and two from Section C.
• All working, including rough work, should be done on the same sheet as, and adjacent to, the rest of the answer.
• The intended marks for questions or parts of questions are given in brackets [ ].
• Balanced equations must be given wherever possible and diagrams where they are helpful.
• When solving numerical problems, all essential working must be shown.
• In working out problems use the following data:
Gas constant R = 1.987 cal deg-1 mol-1 = 8.314 JK-1 mol-1 = 0.0821 dm3 atm K-1 mol-1. 1L atm = 1 dm3 atm = 101.3 J.
1 Faraday = 96500 Coulombs.
Avogadro’s number = 6.023 × 1023

Part – I (20 Marks)

### ISC Chemistry 2015 Class-12 Previous Year Question Paper Solved

Question 1.
(a) Fill in the blanks by choosing the appropriate word/words from those given in the brackets: [5] (increases, decreases, positive, efficient, 68, non-efficient, no α-hydrogen, α-hydrogen, negative, Rosenmund’s, greater, Cannizzaro, 74, common-ion effect, lesser, buffer action, diamagnetic, paramagnetic)
(i) The more ……………. the standard reduction potential of metal, the …………. is its ability to displace hydrogen from acids.
(ii) Both ccp and hep are …………. packings and occupy about ………….. % of the available space.
(iii) The solubility of silver chloride ………….. in the presence of sodium chloride because of …………
(iv) Benzaldehyde undergoes …………… reaction on treatment with concentrated sodium hydroxide because it has ………. atom.
(v) The transition metals show ………….. character because of the presence of impaired electrons and Cu+ is ……….. because its electronic configuration is [Ar] 3d10.

(b) Complete the following statements by selecting the correct alternative from the choices given: [5] (i) The molal freezing point constant of water is 1.86 K kg mol-1. Therefore, the freezing point of 0.1 M NaCl solution in water is expected to be:
(1) -1.86°C
(2) -0.372°C
(3) -0.186°C
(4) +0.372°C

(ii) For a first-order reaction, the rate constant for the decomposition of N2O5 is 6 × 10-4 sec-1. The half-life period for the decomposition in seconds is:
(1) 11.55
(2) 115.5
(3) 1155
(4) 1.155

(iii) When acetaldehyde is treated with Grignard reagent, followed by hydrolysis the product formed is:
(1) Primary alcohol
(2) Secondary alcohol
(3) Carboxylic acid
(4) Tertiary alcohol

(iv) The geometry of XeF6 molecule and the hybridization of the Xe atom in the molecule is:
(1) Distorted octahedral and sp3d3
(2) Square planar and sp3d2
(3) Pyramidal and sp3
(4) Octahedral and sp3d3

(v) In the complexes [Fe(CN)6]3- and [Pt(en) (H2O)2 (NO2) (Cl)]2+ the respective oxidation numbers of central metal atoms are:
(1) +3 and+4
(2) +6 and+4
(3) +6 and +3
(4) +3 and +3

(c) Answer the following questions: [5]
(i) What is the effect of temperature on the ionic product of water? How will it change the pH value of a neutral solution?
(ii) How many hours does it take to reduce 3 moles of Fe3+ to Fe2+ with 2.0 A current intensity?
(iii) How is urea prepared by Wohler synthesis?
(iv) Two liquids A and B form type II non-ideal solution which shows a minimum in its temperature mole fraction plot (T- χ diagram). Can the two liquids be completely separated by fractional distillation?
(v) The aqueous solution of sodium acetate is basic. Explain.

(d) Match the following: [5]

 (i) Disaccharide (a) Lucas reagent (ii) Carbylamine (b) Condensation polymer (iii) Dacron (c) Obnoxious smell (iv) Low spin complex, d2sp3 (d) Sucrose (v) Anhydrous ZnCl2 + conc. HCl (e) Hexaamminecobalt (III) ion

(a)

(i) common ion effect, lesser
(ii) non-efficient, 68
(ii) decreases, buffer action
(iv) Cannizzaro, α-hydrogen
(v) paramagnetic, diamagnetic.

(b)

(i) -3
(ii) -4
(iii) -3
(iv) -3
(v) -2

(c)

(i) $\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}^{+}(a q)+\mathrm{OH}^{-}(a q)$ …(i)
At 25°C, [H+] = [OH] = 1.01 × 10-7 M.
Kw = 10-14, K = 1.8 × 10-6 (at 25°C)
The value of Kw increases with the increase of temperature i.e., the concentration of H+ and OH ions increases with increase in temperature. Neutral solution pH will increase.
(ii) Fe3+ + e → Fe2+
3 mole Fe3+ will require 3-mole electrons.
Required charge = 3 × 96500 coulomb = 289500 coulomb
Charge = Current × Time
$\text { Time }=\frac{\text { Charge }}{\text { Current }}=\frac{289500}{2}=1.44750 \times 10^{5} \mathrm{sec} .=40 \mathrm{hrs}$
(iii) By Vital force theory
NH2CONH2 → CO(NH2)2

(v) CH3COONa → CH+3 + COONa
Aqueous solution of CH3COONa is basic in nature.

(d)

(i) (d)
(ii) (a)
(iii) (b)
(iv) (e)
(v) (c)

### Part – II (50 Marks)

A nswer six questions choosing two from Section A, two from Section B and two from Section C.

Section – A
(Answer any two questions)

ISC Chemistry 2015 Class-12 Previous Year Question Paper Solved

Question 2.
(a)

(i) A solution containing 0.5 g of KCl dissolves in 100 g of water and freezes at -0.24°C. Calculate the degree of dissociation of the salt. (Kf for water = 1.86°C), Atomic weights [K = 39, Cl = 35.5] [3] (ii) If 1.71 g of sugar (molar mass = 342) are dissolved in 500 mL of an aqueous solution at 300 K, what will be its osmotic pressure? [1] (iii) 0.70 g of an organic compound, when dissolved in 32 g of acetone produces an elevation of 0.25°C in the boiling point. Calculate the molecular mass of organic compound (Kb for acetone = 1.72 K kg mol-1) [1] (b)

(i) What is the difference between the order of a reaction and the molecularity of a reaction? [2] (ii) A substance decomposes by following first-order kinetics. If 50% of the compound is decomposed in 120 minutes, how long will it take for 90% of the compound to decompose? [2] (c) Name the crystal structure of the copper metal. [1] Answer 2:
(a)

(i) w = 0.5 g of KCl
Kf = 1.86°C
W = 100 g of water
m of KCl = K + Cl = 39 + 35.5 = 74.5 g

Question 3.
(a)

(i) Chromium metal crystallises with a body-centred cubic lattice. The edge length of the unit cell is found to be 287 pm. Calculate the atomic radius. What would be the density of chromium in g/cm3? (atomic mass of Cr = 52.99) [2] (ii) Why does sodium chloride on heating with sodium vapours acquire yellow colour? [1] (iii) The equilibrium constant for the reaction:
$\mathrm{N}_{2}(g)+3 \mathrm{H}_{2}(g) \rightleftharpoons 2 \mathrm{NH}_{3}(g) \text { at } 715 \mathrm{K} \text { , is } 6.0 \times 10^{-2}$
If, in a particular reaction, there are 0.25 mol L-1 of H2 and 0.06 mol L-1 of NH3 present, calculate the concentration of N2 at equilibrium.
(iv) Calculate the concentration of OH ions in solution when [H+] = 6.2 × 10-2 mol L-1. [1] (v) State Le Chatelier’s principle. [1]

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