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ISC Class 12 Chemistry Solved Question Paper 2005

Short Answer Type

11.

Correct the following statements :

Graphite has a two-dimensional sheet-like structure in which each carbon atom is sp³ hybridized.

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12.

Correct the following statements :

For water-system, the number of phases that can exist in equilibrium is two.

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13. Calculate the mole fraction of water in a sodium hydroxide solution which has 80 g of NaOH and 54 g of H₂O. [Relative atomic masses : Na = 23, O = 16, H = 1].

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14. The bond angles in CH₄ (109.5°), NH₃ (107°) and H₂O (104.5°) are different although these molecules have the same hybridization, sp³. Give a suitable reason for this.

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15.

Radium has a half-life of 1580 years. How long would it take for each of the samples of radium given below to disintegrate ?

(i) 20% of a 5 g sample.

(ii) 20% of a 50 g sample.

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16. State the main characteristics of an ionic and a network type of crystal citing a suitable example for each.

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17.

For the cell

Zn | Zn²⁺ (a = 1) | | Cu²⁺ (a = 1) | Cu

Given that E_Zn . Zn²⁺ = +0.761 volt; E_cu²⁺, Cu = +0.339 volt

(i) Write the cell reaction

(ii) Calculate the emf and free energy change at 298 K involved in the cell. [Faraday’s constant F = 96500 coulomb equivalent^{– 1}].

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18.

(i) What is the physical significance of entropy ?

(ii) How is the entropy of the universe changing ?

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19.
For the extraction of aluminium, write

(i) The most important ore.

(ii) One method for the purification of the mentioned ore.

(iii) The role of cryolite and calcium fluoride in electrolysis.

i) Bauxite (A1₂O₃ . 2H₂O) is the most important ore of aluminium.

(ii) Hall’s Process of purification of Bauxite : Bauxite ore is first fused with sodium carbonate. The Al₂O₃ forms sodium meta aluminate with sodium carbonate. The fused mass is now extracted with water where Fe₂O₃ and SiO₂remain insoluble in the residue.

Al₂O₃ +Na₂CO₃ --> 2NaAlO₃ +CO₂

This solution is warmed to 50 - 60°C and then CO₂ is passed through it. Al(OH)₃ is separated out as precipitate.

2NaAlO₂ +CO₂ +3H₂O --> 2Al(OH)₃ +Na₂CO₃
Now the precipitate of Al(OH)₃ is filtered, washed and dried up.

(iii) Role of cryolite and calcium fluoride : The pure alumina is a bad conductor of electricity and has a fusion temperature of about 2000°C which is more than the boiling point of aluminium (1800°C). So, to overcome these difficulties a mixture of alumina, cryolite and calcium fluoride in the ratio of 1 : 2 : 1 is used for electrolysis. This mixture is a good conductor of electricity and has a fusion temperature of 900°C.

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