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solutions for practice exercises MT1_2006

# solutions for practice exercises MT1_2006 - Solutions for...

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- Solutions for practice exercises of midterm1 2006 - 1. n mixture = RT PV K KPa 300 mol K kPa L 8.31 L 107 80 1 - 1 - × × = = 3.433mol n 2 N = mol gmol g 696 . 2 2 01 . 14 53 . 75 1 = × - n 2 O = 3.433mol – 2.696mol = 0.737mol x 2 O = 2 2 2 N O O n n n + = 3.433mol 0.737mol = 0.215 P 2 O = P x 2 O = 80kPa × 0.215 = 17.2 kPa 2. A is false due to the Avogadro principle 3. . 2 2 1 1 T V T V = 2 5 300 10 T L K L = , so T 2 = 150K = -123.15 o C 4. A state function is independent of the path by which a state was reached. So q is not a state function, but q+w = U, H and PV are all state functions. 5. V 1 = P nRT = Pa K gmol g 5 1 - 1 - 3 1 10 00 . 1 300 mol K L Pa 10 x 8.31 00 . 32 20 . 3 × × × - = 2.49L V 2 = 1.49L For an isothermal and reversible process w = -nRT ln 1 2 V V = - K gmol g 300 mol K L Pa 10 x 8.31 00 . 32 20 . 3 1 - 1 - 3 1 × × - × ln L L 49 . 2 49 . 1 = 1.28 × 10 5 PaL = 128 J 6. A: The maximum expansion work is obtained when the expansion occurs reversibly. B: A closed system still exchanges energy with the surroundings. C: Correct. The universe is an isolated system. D: Perpetual motion does not exist. All the motions need a sustaining energy source.

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7. A. Is true B: Formation reactions are defined as the reactions that form one mole of a given compound in a specified state from stoichiometric amounts of the elements in their reference states (which is not necessarily the gas state!). D: The equation: C P = H/ T is valid for all gases. (not only ideal gases!) 8. The balanced combustion reaction for sucrose is: C 12 H 22 O 11 (s) + 12O 2 (g) 11H 2 O(l) + 12CO 2 (g) Δ c H o = 12 Δ f H o (CO 2 (g)) + 11Δ f H o (H 2 O (l)) – 12Δ f H o (O 2 (g)) – Δ f H o (C 12 H 22 O 11 (s)) = 12 × (-393.51 kJ/mol) + 11 × (-285.83 kJ/mol) – 0 – (-2222 kJ/mol) = -5644 kJ/mol = -1349 Cal/mol n sugar = mol 0117 . 0 g/mol 342.30 g 4 = so the calories we gain when we eat one cube of sugar are 0.0117mol × 1349 Cal/mol = 15.8 Cal 9. Only D is false because there are some formation reactions that are endothermic ( e.g. liquid benzene)
10. 11. 12. 13. 14.

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15. 16. 17.
18. 19. (20) Which of the following statements is true? a. The work associated with the expansion of an ideal gas depends on the chemical nature of the gas b. The work associated with the expansion of an ideal gas depends only on the initial and final pressures and volumes c. The internal energy of an isolated system is always constant. d. All the previous statements are false. Solutions: (a) false because for ideal gases the chemical nature does not matter; (b) false because work is not a state function; (c) true as it is the 1 st law correct answ. Is (c)

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