Exam 1-solutions

# With 760 torr 1 atm this amounts to 292 atm from the

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2,220 mm Hg=2,220 Torr. With 760 Torr = 1 atm, this amounts to 2.92 atm from the

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Version 237 – Exam 1 – holcombe – (51395) 4 water column PLUS 1 atm from barometric pressure gives a total pressure on the diver of about 3.9. At the surface the diver experi- ences about 1 atm. This leads to a ratio of about 4. 010 2.9points Consider the following pressure diagram for a binary liquid containing A and B. 0 1 x A 40 torr 70 torr A solution is mixed such that x A = 0 . 5. What is the vapor pressure of B? 1. 40 torr 2. 35 torr 3. 70 torr 4. 55 torr 5. 20 torr correct Explanation: 0 1 0 . 5 70 torr 20 torr 011 2.9points Which of the following would have the largest absolute entropy? 1. N 2 ( ) 2. N 2 (aq) (nitrogen dissolved in water) 3. N 2 (s) 4. N 2 (g) correct Explanation: S is high for systems with high degrees of freedom, disorder or randomness and low for systems with low degrees of freedom, disorder or randomness. Gases have more degrees of freedom, disorder and randomness than aque- ous solutions, liquids, or solids. 012 2.9points The vapor pressure of water above 40 mL of water in a 100 mL container is 23.8 torr at 25 C. What is the vapor pressure of the water if the volume of the container is changed to 50 mL? 1. about 20 torr 2. 47.6 torr 3. about 25 torr 4. 11.9 torr 5. 23.8 torr correct Explanation: Vapor pressure is the pressure exerted by a vapor that exists in equilibrium with the liquid of the substance in question. As long as there is some liquid, the actual amount of liquid is irrelevant; the vapor pressure does not change. (Temperature affects vapor pres- sure.) 013 2.9points For the combustion reaction of ethylene (C 2 H 4 ) C 2 H 4 + 3 O 2 2 CO 2 + 2 H 2 O assume all reactants and products are gases, and calculate the Δ H 0 rxn using the following Δ H 0 f values (in kJ/mol): Δ H 0 f C 2 H 4 = 52 Δ H 0 f CO 2 = - 393
Version 237 – Exam 1 – holcombe – (51395) 5 Δ H 0 f O 2 = 0 Δ H 0 f H 2 O = - 242 1. 1322 kJ/mol 2. - 687 kJ/mol 3. 0 kJ/mol 4. - 1322 kJ/mol correct 5. 687 kJ/mol Explanation: Reactants: Δ H f C 2 H 4 (g) = 52 kJ/mol Δ H f O 2 (g) = 0 kJ/mol Products: Δ H f CO 2 (g) = - 393 kJ/mol Δ H f H 2 O(g) = - 242 kJ/mol Applying Hess’s Law: Δ H 0 rxn = summationdisplay n Δ H 0 f products - summationdisplay n Δ H 0 freactants = bracketleftbigg 2 parenleftbigg - 393 kJ mol parenrightbigg + 2 parenleftbigg - 242 kJ mol parenrightbiggbracketrightbigg - parenleftbigg 52 kJ mol parenrightbigg = - 1322 kJ mol 014 2.9points What is the molar mass of a compound if 25.0 g of the compound dissolved in 750. mL gives a molarity of 0.290 M? 1. 44.4 g/mol 2. 246 g/mol 3. 9.67 g/mol 4. 115 g/mol correct 5. 0.103 g/mol Explanation: m = 25 . 0 g V = 750 . mL M = 0 . 290 M ? g mol = 25 . 0 g 0 . 750 L × 1 L soln 0 . 290 mol = 115 g / mol 015 2.9points Calculate the vapor pressure at 25 C of a mixture of benzene and toluene in which the mole fraction of benzene is 0.650. The vapor pressure at 25 C of benzene is 94.6 torr and that of toluene is 29.1 torr. 1. 51.3 torr 2. 84.4 torr 3. 71.7 torr correct 4. 124 torr 5. 61.5 torr Explanation: 016 2.9points If you had 40 molecules of CO gas in 250000 molecules of air, what is the concentration of CO in parts per million?

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• Fall '07
• Holcombe
• Chemistry, Vapor pressure, Torr, Law of Partial Pressure

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