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ISC Class 12 Chemistry Solved Question Paper 2013

Short Answer Type

11. If the half-life period for a first order reaction is 69.3 seconds, what is the value of its rate constant ?

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12. Define cryoscopic constant.

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13. Ethylene glycol is used as an antifreeze agent. Calculate the amount of ethylene glycol to be added to 4 kg of water of prevent it from freezing at -6°C. (K_ffor H₂0 = 1.85 K mole^-1 kg)

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14. The freezing point of a solution containing 0.3 gms of acetic acid in 30 gms of benzene is lowered by 0.45K. Calculate the Van’t Hoff factor. (at. wt. of C = 12, H = 1, 0 =16, K_f for benzene = 5.12K kg mole^-1)

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15.

(b) Name the law or principle confirmed by the following observations :

(i) When water is added to 0.01M aqueous solution of acetic acid the number of hydrogen ions increase.

(ii) When 96500 coulombs of electricity is passed through acidulated water, 5.6 litres of oxygen at s.t.p. is liberated at the anode.

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16.

Arrange Ag, Cr and Hg metals in the increasing order of reducing power. Given :

E° _Ag+/Ag = +0.80V E° cr⁺³/cr = -0.74V E° Hg²/Hg = +0.79V

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17.

In a first order reaction, 10% of the reactant is consumed in 25 minutes. Calculate :

(i) The half-life of the reaction.

(ii) The time required for completing 17% of the reaction.

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18.
(a) Explain giving reasons why (Give equations in support of your answer):

(i) A solution of NH₄Cl and NH₄OH acts as a buffer.

(ii) Cu is precipitated as CuS while Zn is not precipitated when H₂S is passed through an acidic solution of Cu(NO₃)₂ and Zn(NO₃)₂ respectively.

(i) NH₄Cl (strong electrolyte) ⇌ NH₄⁺ (high) + Cl^‒ (high)
NH₄OH (weak electrolyte) ⇌ NH4 + (low) + OH‒ (low)

(1) When an acid is added, it gives H⁺ ions which combine with OH^‒ ions (from NH₄OH) to produce H₂O (weak electrolyte) in absence of OH^‒ ions NH₄OH dissociates more and maintains the pH.
(2) When an alkali is added, it gives OH^‒ ions which combine with NH₄⁺ ions (from NH₄Cl) to produce NH₄OH (weak electrolyte). NH₄OH dissociates partially and OH^‒ ions produced are not sufficient to increase the pH.

(ii) HCl ⇌ H ⁺ + Cl^‒ , H₂S ⇌ 2H+ + S^2‒
In the presence of HCl (strong electrolyte), the dissociation of H₂S (weak electrolyte) is suppressed due to the presence of common ion H+ [common ion effect] and less S^2‒ ions are produced. In analytical chemistry groups are formed in increasing order of Ksp, Cu²⁺ belongs to 2nd group while Zn²⁺ belongs to 4th group. The Ksp of CuS is low and hence [Cu²⁺][S^2‒ ] > K_sp, CuS gets precipitated. But Ksp of ZnS is high and hence [Zn²⁺][S^2‒] < Ksp, ZnS doesn’t get precipitated.