CHEMISTRY 202 Hour Exam I. Dr. D. DeCoste T.A.

Transcription

1 CHEMISTRY 202 Hour Exam I September 22, 2016 Dr. D. DeCoste Name Signature T.A. This exam contains 23 questions on 11 numbered pages. Check now to make sure you have a complete exam. You have two hours to complete the exam. Determine the best answer to the first 20 questions and enter these on the special answer sheet. Also, circle your responses in this exam booklet. Show all of your work and provide complete answers to questions 21, 22 and (60 pts.) 21 (20 pts.) 22 (20 pts) 23 (20 pts.) Total (120 pts) Useful Information: Always assume ideal behavior for gases (unless explicitly told otherwise). PV = nrt R = Latm/molK = J/Kmol K = C N A = x υ rms = 3RT M λ = 1 2( N / V )( πd 2 ) Z A = A V N RT 2πM Z = 4 V N d 2 πrt M x = - b ± 2 b - 4ac 2a Solubility Rules: 1. Most nitrate salts are soluble. 2. Most salts of sodium, potassium, and ammonium cations are soluble. 3. Most chloride salts are soluble. Exceptions: silver, lead(ii), and mercury(i) chloride. 4. Most sulfate salts are soluble. Exceptions: calcium, barium, and lead (II) sulfate. 5. Most hydroxide salts can be considered insoluble. Soluble ones: sodium, potassium, and calcium hydroxide. 6. Consider sulfide, carbonate, and phosphate salts to be insoluble.

2 Hour Exam I Page No Caffeine is a stimulant most commonly found in coffee and is 5.19% hydrogen by mass. The molecular formula of caffeine is C x H 10 N 4 O 2. Determine the value of x. a) 2 b) 4 c) 8 d) 10 e) An iron (Fe) bar sits outside and rusts, forming iron(iii) oxide. Once all of the iron has rusted, the mass is 6.28 g. Determine the original mass of the iron bar. a) 2.20 g b) 3.38 g c) 4.01 g d) 4.39 g e) 8.78 g 3. When copper (Cu) metal is dropped into an aqueous AgNO 3 solution, silver (Ag) metal is produced and the solution turns blue from the Cu 2+ ions in solution. Determine the mass of silver that can be produced from g of copper in an excess of silver nitrate solution. a) g b) g c) g d) g e) g 4. Consider a reaction in which two reactants make one product (for example, consider the unbalanced A + B C). You know the following: 2.0 moles of A (with an excess of B) can make a maximum of 2.0 moles C 5.0 moles of B (with an excess of A) can make a maximum of 4.0 moles C If you react 10.0 moles of A and 10.0 moles of B, what is the maximum amount of C that can be produced? a) 8.0 mol b) 10.0 mol c) 12.0 mol d) 18.0 mol e) 20.0 mol 5. Consider mixing an aqueous solution of barium nitrate and an aqueous solution of potassium phosphate. For how many of the following cases is the concentration of the barium ion essentially equal to zero after the reaction is complete? That is, we are assuming the reaction goes to completion and does not reach equilibrium. Reacting ml of M barium nitrate with ml of M potassium phosphate. Reacting ml of M barium nitrate with ml of M potassium phosphate. Reacting ml of M barium nitrate with ml of M potassium phosphate. Reacting ml of M barium nitrate with ml of M potassium phosphate. a) 0 b) 1 c) 2 d) 3 e) 4 6. You have a table sugar solution (sucrose, C 12 H 22 O 11 ) sitting in an open beaker. After several days you test the solution and find that it has a concentration 33.3% higher due to evaporation. What percentage of the water must have evaporated from the sugar solution? a) 25.0% b) 33.3% c) 66.7% d) 75.0% e) The concentration of the solution will not change due to evaporation.

3 Hour Exam I Page No Consider the reaction between equal volumes of lead(ii) nitrate and calcium chloride in which both solutions have the same concentration in terms of molarity. How many of the following statements is/are false after the reaction is complete? Note: assume the reaction goes to completion (that is, does not reach equilibrium). The concentration of the nitrate ion is the same as the initial concentration of lead(ii) nitrate. The lead(ii) ion has all precipitated out of solution. The concentration of the chloride ion is equal to that of the nitrate ion. The only ions left in solution are the nitrate ions and the calcium ions a) 0 b) 1 c) 2 d) 3 e) 4 8. A 4.60 g sample of cobalt (Co) metal is reacted with ml of 1.00 M HCl(aq) to produce hydrogen gas and cobalt chloride. There is no excess reactant. Determine the formula of the cobalt chloride produced in the reaction. a) CoCl 2 b) CoCl c) Co 2 Cl 3 d) CoCl 3 e) CoCl 4 9. A classic chemistry lab is to determine the molar mass of a volatile (easily evaporated) liquid by vaporizing the liquid to a known pressure, temperature, and volume. Suppose you vaporize a liquid at 1.00 atm and C in a mL flask. The mass of the vapor is found to be g. Which of the following could be the identity of the liquid? a) CHCl 3 b) CH 3 OH c) C 6 H 14 d) CH 2 Cl 2 e) C 2 H 6 O 10. For metals that form a 2+ charge in an ionic compound, the general equation for such a metal reacting with hydrochloric acid is M(s) + 2HCl(aq) MCl 2 (aq) + H 2 (g) 24.4 g of a metal is reacted completely with an excess of HCl and all of the hydrogen gas produced is collected in a balloon at 25 C and 1.00 atm. The volume of the balloon is measured and found to be 9.12 L. Identify the metal that was added to the HCl. a) Ni b) Mn c) Ca d) Mg e) Zn , 12. Consider two samples He in separate container of equal volume and at equal pressure. Sample 1 has a Kelvin temperature that is 4 times that of Sample 2. Determine the ratio of: 11. [moles (n) in Sample 1] : [moles (n) in Sample 2]. a) 1:1 b) 2:1 c) 1:2 d) 1:4 e) 4:1 12. [collision frequency (Z A ) in Sample 1] : [collision frequency (Z A ) in Sample 2]. a) 1:1 b) 2:1 c) 1:2 d) 1:4 e) 4:1

4 Hour Exam I Page No. 3 13, 14. You have equal masses of neon gas in the left side, and helium gas in the right side, of a twobulb container connected by a valve as shown below. Initially the valve is closed. Ne He 5.0 L 1.0 L 13. After the valve is opened, what is true about the relative partial pressures of helium and neon? Assume constant temperature. a) The partial pressures of helium and neon are equal. b) The partial pressure of neon is 25.0 times as great as the partial pressure of helium. c) The partial pressure of neon is 5.0 times as great as the partial pressure of helium. d) The partial pressure of helium is 25.0 times as great as the partial pressure of neon. e) The partial pressure of helium is 5.0 times as great as the partial pressure of neon. 14. After the valve is opened, how does the final total pressure compare to the initial pressure of helium? Assume constant temperature. a) P (final) = (6/5)P (He initial) b) P (final) = (1/5)P (He initial) c) P (final) = (1/6)P (He initial) d) P (final) = (5/6)P (He initial) e) We cannot determine this without knowing the amounts of neon and helium present , 16. Indicate which of the graphs below best represents each plot described in questions 15 and 16. Note: the graphs may be used once, more than once, or not at all. a) b) c) d) e) 15. Density (y) vs. T (K) (x) for 1 mole of an ideal gas in a rigid steel container. 16. Pressure (y) vs. molar mass (x) for 1 mole of a given volume of a series of ideal gases at the same temperature.

5 Hour Exam I Page No How many of the following changes will always result in shifting the equilibrium position of a gaseous chemical system? I. Changing the pressure of the system. II. Changing the volume of the system. III. Changing the temperature of the system. IV. Changing the total number of moles of gas in the system. a) 1 b) 2 c) 3 d) Consider the system represented by 2H 2 (g) + O 2 (g) 2H 2 O(g) at equilibrium. How many of the following changes would shift the equilibrium position to the left? I. Addition of water vapor at constant volume and temperature. II. Decreasing the volume at constant temperature. III. Decreasing the temperature of the system at constant volume. IV. Addition of helium gas at constant volume and temperature. a) 0 b) 1 c) 2 d) 3 e) , 20. You have a sample of NOBr(g) in an otherwise empty, closed flask at atm where it decomposed at constant temperature according to the following equation: 2NOBr(g) 2NO(g) + Br 2 (g) At equilibrium, 60.0% of the original NOBr(g) has decomposed. 19. Determine the total pressure in the flask at equilibrium. a) 1.00 atm b) 1.15 atm c) 1.30 atm d) 1.50 atm e) 1.75 atm 20. Determine the value of the equilibrium constant, K p. a) b) c) d) e) 0.675

6 Hour Exam I Page No Solid sodium bicarbonate (baking soda) decomposes upon heating to form solid sodium carbonate, water vapor, and carbon dioxide gas. Suppose you are given an unknown mixture of sodium bicarbonate and sodium carbonate and are asked to determine the mass percent of sodium bicarbonate in the mixture. You find the mass of the mixture to be g and begin heating it. After a few minutes your hot plate breaks and you do not know if all of the original baking soda has decomposed. You are running out of time so you cool the sample and measure its mass, finding it to be g. It turns out that, given your data, you can determine at least one of the following (you may be able to determine two, or even all three this is for you to decide). Circle YES or NO after each of the three questions below (remembering that the answer to at least one of these is YES ). Can you determine the mass of sodium bicarbonate that decomposed? YES NO Can you determine the mass of sodium bicarbonate in the original mixture? YES NO Can you determine whether or not the reaction went to completion (that is, whether or not there is any sodium bicarbonate left after heating)? YES NO In the space below, and on the next page, provide verbal and mathematical support for your choices. For example, if you answer YES to either of the first two questions, then determine the mass. If you answer NO, show why you cannot determine the mass. If you answer YES to the third question, then state either the reaction went to completion or the reaction did not go to completion and provide proof. If you answer NO to this, show why you cannot determine it. Explain completely, show all work, and define any variables fully and carefully. [20 points]

7 Hour Exam I Page No (con t) Explain completely, show all work, and define any variables fully and carefully.

8 Hour Exam I Page No Recall from lecture and your textbook homework that the lift (or buoyancy) of a hot air balloon is due to the difference between the mass of the cooler air that was displaced and the mass of the hot air in the balloon. Remember that the bottom of the balloon (where the heater is) is open. a. For both of the following scenarios you are to consider two different balloons of the same volume and you are to determine which balloon, if either, has the greater lift. That is, your answer for each may be Balloon #1, Balloon #2, or They both have equal lift. Only one variable will be different between the balloons, as described below. Support your answer by using the gas laws and premises of the kinetic molecular theory. i. Balloon #1 is flown on a day in which atmospheric pressure is greater than the day in which balloon #2 is flown. All other conditions [including the make-up of the air, the outside air temperature (cool), and temperature of air inside the balloons (higher than outside)] are the same for the two balloons. Explain completely. [6 points] ii. Balloon #1 is flown on a planet which has an atmosphere of helium (that is, the gas inside and outside the balloon is helium) and balloon #2 is flown on Earth (atmosphere of air). All other conditions [including atmospheric pressure, outside (cool) temperature, and temperature of gas inside the balloon (higher than outside)] are the same for the two balloons. Explain completely. [6 points]

9 Hour Exam I Page No (con t) b. In the space below, derive the relationship that allows you to determine the lift or buoyancy of a hot air balloon. Your final equation should only include the following variables: atmospheric pressure (P), volume of the balloon (V), temperature outside the balloon (T cool ), temperature inside the balloon (T hot ), and molar mass of the gas inside and outside the balloon (MM; note the same gas is inside and outside the balloon). If you decide the lift is independent of one or more of these variables, do not include it in your equation. In addition, explain (using the premises of the kinetic molecular theory) the direct or inverse relationship of all variables and explain how your equation is consistent with your explanations to part a. [8 points]

10 Hour Exam I Page No In a 1.00-L rigid steel container at constant temperature you place 7.00 mol of PCl 3 (g) and 6.00 mol of Cl 2 (g). The system reaches equilibrium according to the following chemical equation: PCl 3 (g) + Cl 2 (g) PCl 5 (g) After equilibrium is reached, you note that the partial pressure of Cl 2 (g) is two times as large as the partial pressure of PCl 5 (g). a. Determine the equilibrium concentrations for all species, and the value of K, the equilibrium constant, for the chemical equation as given above. Show all work. [4 points] b. You add 6.00 mol of PCl 3 (g) to the equilibrium mixture as determined in part a above. In Row 1 of the table on page 11, fill in the new initial number of moles for each of the three species before the system returns to equilibrium (that is, simply add 6 to the number of moles of PCl 3 (g) and keep the number of moles of Cl 2 (g) and PCl 5 (g) the same as you have determined in part a above (you ll see why you are doing this shortly) c. Next, determine the new equilibrium concentrations for all species after adding 6.00 mol of PCl 3 (g). Also, fill in the number of moles for each of the three species in Row 2 of the table on page 11. Show all work. [3 points]

11 Hour Exam I Page No d. Suppose that you have the same equilibrium mixture as determined in part a above (still in a 1.0-L container) and you add 6.00 mol of PCl 3 (g) to this as before, but in this case the container is fitted with a massless, frictionless piston. Determine the equilibrium concentrations for all species. Also, fill in the number of moles for each of the three species in Row 3 of the table on page 11. Explain how the approach to solving this problem is different from part c, and show all work. [7 points]

12 Hour Exam I Page No (con t) e. Fill in the table with the number of moles of reactants and product [1 point] Addition of PCl 3 (g) (initial conditions) Equilibrium (part c steel tank) Equilibrium (part d cont. w/ piston) moles PCl 3 (g) moles Cl 2 (g) moles PCl 5 (g) f. By adding PCl 3 (g), the equilibrium position should have been pushed to the right in both parts c and d. You can tell this by comparing the number of moles of all species in Rows 2 and 3 to the initial moles in Row 1 in the table above. Was there a difference in how far the system went to the right to reach equilibrium, depending on whether it was done in rigid tank or container with a piston? If so, explain in which case it went further to the right and why this makes sense. If there was no difference, explain why this makes sense. [5 points] Note: if you cannot determine any of the moles for the table, or if you believe your relative numbers do not make sense, explain what the relative numbers should have been and why. Otherwise, use your numbers