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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.

i)
$straight T space equals space 1508 space years straight N subscript 0 space equals space 100 straight N space equals space 100 minus 20 space equals space 80 percent sign straight T space equals space fraction numerator 0.693 over denominator straight lambda end fraction straight lambda space equals space fraction numerator 0.693 over denominator straight T end fraction straight lambda space equals space fraction numerator 0.693 over denominator 1580 end fraction per space year straight N space equals space straight N subscript 0 straight e to the power of negative λt end exponent straight N over straight N subscript 0 space equals space straight e to the power of negative λt end exponent λt space equals space log subscript straight e space 100 over 80 straight t space equals space fraction numerator 2.3026 space left parenthesis log subscript straight e space 100 over 80 right parenthesis space straight x space 1580 over denominator 0.693 end fraction space space equals space 509 space years space left parenthesis approx. space right parenthesis$

ii) Both the samples will take same time to disintegrate 20%