Chem 105/107 Exam #3 Fall 2012

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1 November 12 th, 2012 Name: CLID: Score: Chem 105/107 Exam #3 Fall 2012 There are 17 multiple choices that are worth 3 points each. There are 4 problems and 1 bonus problem. Try to answer the questions, which you know first, and then try to solve the problems, which you are not sure about. Here is the summary of the total 100 points plus 6 points bonus. Carefully read the questions. Good luck! Multiple choice (17 total) 3 points each, the total is 51 points Problems #1 13 points #2 10 points #3 12 points #4 14 points #5 (bonus) 6 points Total 100 points + 6 points You could use your calculator. No notes or books of any sort may be used during the exam. I have neither given nor received aid on the exam. Signature and date 1

2 I. Multiple Choices: Choose the best answer for each question. (3 points each, the total is 51 points) 1. H is equal to q (heat) under and E is equal to q (heat) under. a) constant pressure; constant pressure b) constant pressure; constant volume c) constant volume; constant volume d) constant volume; constant pressure 2. Determine Hº rxn for the reaction: 2Al(s) + (3/2)O 2 (g) Al 2 O 3 (s) You are given: 2Al 2 O 3 (s) 4Al(s) + 3O 2 (g), and Hº rxn = 3,340 kj/mol a) 1,670 kj/mol b) -1,670 kj/mol c) 3,340 kj/mol d) -3,340 kj/mol 3. Under which of the following four conditions, the gas moves slowest based on µ rms? a) He gas at 300 K b) Cl 2 gas at 300 K c) N 2 gas at 300 K d) He gas at 500 K 4. Which of the following statements is true about a closed system? a) A closed system can exchange energy, but not matter, with surroundings. b) A closed system can exchange matter, but not energy, with surroundings. c) A closed system can exchange neither energy nor matter, with surroundings. d) Based on the first law of thermodynamics, energy change of a closed system is equal to zero. 5. Calculate the volume occupied by 35.2 g of methane gas (CH 4 ) at 2.50 x 10 C and 1.00 atm. R = L atm/k mol. Molar mass of CH 4 : 16.0 g/mol. (K = C + 273) a) 4.51 L b) 11.3 L c) 49.3 L d) 53.8 L 6. Under what set of conditions, would a gas be expected to behave most likely as an ideal gas? a) High temperature and high pressure b) High temperature and low pressure c) Low temperature and low pressure d) Low temperature and high pressure 2

3 7. State functions are properties that are determined by the state of the system regardless of how that condition was achieved. Which of the following thermodynamic definitions is NOT a state function? a) Enthalpy b) Gibbs free energy c) Temperature d) Work 8. Ethylene (C 2 H 4 ) burns in oxygen to form carbon dioxide (CO 2 ) and water vapor. How many liter(s) of CO 2 are obtained from 1.00 liter (L) of O 2 at the constant temperature and pressure? C 2 H 4 + 3O 2 2CO 2 + 2H 2 O a) L b) 1.00 L c) 1.50 L d) 2.00 L 9. A gas-filled balloon with a volume of 12.5 L at 0.90 atm and 21 C is allowed to rise to the stratosphere where the temperature is -5.0 C and the pressure is 1.0 millibar. What is the final volume of the balloon? atm = bar. (K = C + 273) a) 3.4 x 10 3 L b) 7.9 x 10 3 L c) 1.0 x 10 4 L d) 2.5 x 10 4 L 10. How many liters of chlorine Cl 2 gas at 2.50 x 10 C and atm can be produced by the reaction of 12.0 g of MnO 2 with excess HCl according to the following chemical equation? The molar mass of MnO 2 : g/mol and the molar mass of Cl 2 : 70.9 g/mol, R = L atm/(mol K). (K = C + 273) MnO 2 (s) + 4HCl(aq) MnCl 2 (aq) + 2H 2 O(l) + Cl 2 (g) a) L b) L c) 1.09 L d) 3.55 L 11. The volume of 2 moles of CO gas is under standard temperature and pressure (STP) condition and the volume of 2 moles of CO 2 gas is under standard temperature and pressure (STP) condition. a) 22.4 L; 44.8 L b) 22.4 L; 67.2 L c) 44.8 L; 44.8 L d) 44.8 L; L 3

4 12. Methane (CH 4 ) has the molar mass of 16.0 g/mol. A gas X effuses 2.5 times slower than CH 4. What is the molar mass of the gas X? a) 25.0 g/mol b) 50.0 g/mol c) 1.00 x 10 2 g/mol d) 2.00 x 10 2 g/mol 13. Calculate the density in g/l, of CO 2 gas at 2.70 x 10 C and atm pressure. R = L atm/(mol K). The molar mass of CO 2 : 44.0 g/mol. (K = C + 273) a) g/l b) 1.12 g/l c) 2.93 g/l d) 4.02 g/l 14. A sample of natural gas contains 5.0 moles of CH 4, 4.0 moles of C 2 H 6 and 2.0 moles of C 3 H 8. If the total pressure of the natural gas mixture is 22.0 atm, calculate the partial pressure of C 3 H 8. a) 2.5 atm b) 4.0 atm c) 6.0 atm d) 8.0 atm 15. Copper metal has a specific heat of J/g C. Calculate the amount of heat required to raise the temperature of 22.8 g of Cu from 20.0 C to 875 C. a) 1.97 x 10-5 J b) 329 J c) 7.51 kj d) 10.5 kj 16. Suppose a 50.0 g block of silver (specific heat = J/g C) at 1.00 x 10 2 C is placed in contact with a 50.0 g block of iron (specific heat = J/g C) at 0.00 C, and the two blocks are insulated from the rest of the universe. Assume that the two blocks are considered as an isolated system. The final temperature of the two blocks is a) 34.3 C b) 50.0 C c) 65.7 C d) 69.5 C 4

5 17. A gaseous compound is 30.4% nitrogen and 69.6% oxygen by mass. A 5.25-g sample of the gas occupies a volume of 1.00 L and exerts a pressure of 1.26 atm at C. Which of these choices is its molecular formula? Hint: calculate its molar mass of this gaseous compound. R = L atm/(mol K). (K = C + 273) a) NO b) NO 2 c) N 2 O 4 d) N 3 O 6 III. Problem Solving 1. Glycine, C 2 H 5 O 2 N, is important for biological energy. The combustion reaction of glycine is given by the following balanced equation: 4C 2 H 5 O 2 N(s) + 9O 2 (g) 8CO 2 (g) + 10H 2 O(l) + 2N 2 (g) H rxn = kj/mol You are given that H f [CO 2, g] = kj/mol H f [H 2 O, l] = kj/mol Hº f [O 2, g] = 0 kj/mol Hº f [N 2, g] = 0 kj/mol (a) Calculate the enthalpy of formation of glycine, C 2 H 5 O 2 N, Hº f [C 2 H 5 O 2 N, s]. Show your calculations. (8 points) (b) Calculate the heat change in kj when one mole of glycine, C 2 H 5 O 2 N reacted with excess O 2. Show your calculations or justify your answer. Assume constant pressure. (3 points) (c) Is this reaction an endothermic or exothermic reaction? Justify your answer. (2 points) 5

6 2. Calculate the standard enthalpy change for the following reaction. Show your calculations. (10 points) 2C 8 H O 2 16CO + 18H 2 O given two following reactions: 2C 8 H O 2 16CO H 2 O H = -11,020 kj/mol 2CO + O 2 2CO 2 H = kj/mol 3. Chloroform formula is CHCl x. Determine the molar mass of chloroform gas if a sample weighing g is collected in a flask with a volume of 102 cm 3 at 97 C. The pressure of the chloroform is 728 mmhg. (a) What is the molar mass of chloroform gas? Show your calculations. (10 points) You are given: 1 L = 1,000 cm 3 R = L atm/(mol K) K = C (b) What is the value of X in the formula of CHCl x? Show your calculation. (2 points) 6

7 4. (a) Determine the amount of heat changed when 39.7 g of H 2 O gas is converted to H 2 gas and O 2 gas under constant pressure condition. Show your calculations. For the convenience of calculation: the molar mass of H 2 O is 18.0 g/mol; the molar mass of O 2 is 32.0 g/mol; the molar mass of H 2 is 2.00 g/mol (8 points) 2H 2 O (g) 2H 2 (g) +O 2 (g) H = 484 kj/mol (b) Is the reaction above is exothermic or endothermic reaction? Justify your answer. (2 points) (c) If the pressure is 1.00 atm and temperature is 3.00 x 10 2 K, what is E for this reaction? Hint: E = H RT n and R = J/K mol (4 points) 5. Bonus question (total 6 points) Gasoline (which can be considered to be octane liquid, C 8 H 18 ) burns in oxygen to produce carbon dioxide and water. What volume of carbon dioxide at STP is generated as a result of the combustion of 1.0 gal of gasoline? Show your calculations. (The density of gasoline is 0.81 g/ml. 1 gal = 3.78 L) 7

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Chem 105 Final Exam. Here is the summary of the total 225 points plus 10 bonus points. Carefully read the questions. Good luck!

Chem 105 Final Exam. Here is the summary of the total 225 points plus 10 bonus points. Carefully read the questions. Good luck! May 3 rd, 2012 Name: CLID: Score: Chem 105 Final Exam There are 50 multiple choices that are worth 3 points each. There are 4 problems and 1 bonus problem. Try to answer the questions, which you know first,