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

Transcription

1 CHEMISTRY 202 Hour Exam I September 28, 2017 Dr. D. DeCoste Name Signature T.A. This exam contains 23 questions on 10 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 NA = x υrms = 3RT M λ = 1 2( N / V )( πd 2 ) ZA = 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 Tetraphenylporphyrin is composed of only C, H, and N atoms. Experiments reveal that tetraphenylporphyrin is 85.96% C and 9.12% N by mass. What is the empirical formula of tetraphenylporphyrin? a) C7HN5 b) C22H15N2 c) C11H8N d) C11H15N e) C7H5N 2. Given equal masses of each of the following, which contains the greatest mass of phosphorus? a) phosphorus pentachloride b) iron(iii) phosphide c) magnesium phosphide d) sodium phosphate e) barium phosphate 3. A g sample of copper (Cu) sits in the air for a period of time. Since copper is a transition metal, the products can be copper(i) oxide and copper(ii) oxide. The mass of the copper oxide that has formed is g. Which of the following best describes the product? a) The product is a mixture of copper(i) oxide and copper(ii) oxide, and there is more copper(i) oxide by mass. b) The product is a mixture of copper(i) oxide and copper(ii) oxide, and there is more copper(ii) oxide by mass. c) The product is pure copper(i) oxide. d) The product is pure copper(ii) oxide. e) The product is a 50-50% by mass mixture of copper(i) oxide and copper(ii) oxide. 4. Consider 2 reactants, A and B, which react according to aa + bb cc, where a, b, and c are the coefficients in the balanced chemical equation. You add equal masses of A and B together and let them react. How many of the following statements are true? I. If a is greater than b in the balanced equation, and the molar mass of A is less than the molar mass of B, then A must be limiting. II. If a is greater than b in the balanced equation, and the molar mass of A is less than the molar mass of B, then B must be limiting. III. If a is less than b in the balanced equation, and the molar mass of A is greater than the molar mass of B, then A must be limiting. IV. If a is less than b in the balanced equation, and the molar mass of A is greater than the molar mass of B, then B must be limiting. a) 0 b) 1 c) 2 d) 3 e) 4 5. A sample of lead(ii) nitrate is dissolved in water and a M aqueous potassium chloride solution is added to the lead(ii) nitrate solution to form 3.14 g of solid. Determine the minimum volume of the potassium chloride solution required to precipitate all of the lead(ii) ions. a) 37.6 ml b) 56.5 ml c) 75.3 ml d) 113 ml e) 226 ml

3 Hour Exam I Page No How many of the following will make a M NaCl solution? I. Take 23.4 g of NaCl and dissolve it in ml solution. II. Take 10.0 ml of 4.00 M NaCl and add ml water. III. Mix ml of M NaCl with ml of M NaCl. IV. Mix 50.0 ml of M NaOH with 50.0 ml of M HCl. a) 0 b) 1 c) 2 d) 3 e) 4 7. Consider mixing ml of M solution of calcium nitrate with ml of a M solution of sodium phosphate to produce a solid. Determine the concentration of the sodium ion after the precipitation is complete. a) M b) M c) M d) M e) M 8. Which of the following solutions contains the greatest number of ions? a) ml of 1.00 M aluminum nitrate b) ml of 1.00 M sodium sulfate c) ml of 1.00 M iron(iii) nitrate d) ml of 1.00 M cobalt(ii) chloride e) ml of 1.00 M barium nitrate 9. Consider heating a g mixture of KCl and KClO3 to completion. When heated, KClO3 decomposes to produce KCl and oxygen gas, while KCl is stable. After heating the g mixture to completion, the mass of KCl is found to be g and the oxygen gas is collected at 1.00 atm and 25 C. Determine the volume of the oxygen gas collected. a) L b) 1.29 L c) 1.60 L d) 2.40 L e) 4.42 L 10. Consider two 1.0-L containers. One container is a cube and the other is a sphere. Recall from geometry that these two shapes have different surface areas, and the surface area of a cube is greater than the surface area of a sphere. You add 1.00 mole of He(g) at 25 C to the cube and 1.00 mole of Ne(g) at 25 C to the sphere. Which of the following correctly describes and explains the relative pressures of the gases in each container? a) The pressure is greater in the cube. Pressure is due to the collisions of the gaseous atoms with the inside of the container. The collision frequency (ZA) is directly proportional to the surface area (A), and since A is greater for the cube, ZA must be greater, so the pressure is greater. b) The pressure is the same in each container. Since the volume, number of moles, and temperature are the same, the total number of collisions between the gaseous atoms and container per second is the same. Because pressure is dependent on total number of collisions, the pressures therefore must be equal. c) The pressure is greater in the sphere. Since the number of moles and temperature are the same, the total number of collisions is the same. The sphere has a smaller surface area and since pressure is a force per unit area; with a smaller area, the pressure is greater. d) None of these answers (a-c) correctly describes and explains the relative pressures of the gases in the two containers.

4 Hour Exam I Page No Recall during the first lecture when I inhaled SF6(g) and it lowered my voice. You also have probably seen someone inhale He(g) which makes the voice higher. This is in part a function of the density differences of the gases compared to the density of air. Suppose you wish to make a mixture of SF6(g) and He(g) so that this mixture has the same density as air (1.179 g/l) at room conditions (1.00 atm and 25 C). Determine the partial pressure of the SF6(g) in such a mixture. a) atm b) atm c) 0.50 atm d) atm e) atm 12. Speaking of SF6(g) and He(g): determine the temperature at which the effusion rate of SF6(g) equals that of He(g) at 25 C. a) 639 C b) 912 C c) 1527 C d) 10,601 C e) 10,874 C 13. Suppose a sealed 1.00 L flask contains pure O2(g) and this is completely converted to ozone [O3(g)] at constant temperature. Determine the ratio of the frequency of molecular collisions with the walls of the container before and after the conversion. That is: [ZA of O2] : [ZA of O3]. a) : 1 b) : 1 c) 1.22 : 1 d) 1.50 : 1 e) 1.84 : Indicate which of the graphs below best represents each plot described in questions 14, 15, and 16. Note: the graphs may be used once, more than once, or not at all. a) b) c) d) e) 14. Change in momentum (mass x velocity) per impact (y) vs. molar mass (x) for an ideal gas at constant T. 15. Mean free path (λ) (y) vs. T (K) (x) for a 1.0 mole of an ideal gas in a container fitted with a frictionless, massless piston. 16. n (y) vs. T (x) for an ideal gas at constant P and V.

5 Hour Exam I Page No How many of the following statements is/are true considering chemical equilibrium? I. A system that is disturbed from an equilibrium condition responds in a manner to restore equilibrium. II. Because K is an equilibrium constant, its value for a given equation can never change. III. Adding an inert gas to a system at equilibrium cannot change the equilibrium position. IV. For a given reaction at a given temperature, there is only one set of equilibrium concentrations for the reactants and products. V. The value of the equilibrium constant at a given temperature for a given reaction mixture is the same regardless of the direction from which equilibrium is attained. a) 1 b) 2 c) 3 d) 4 e) Consider the decomposition of ammonia as shown below: 2NH3(g) N2(g) + 3H2(g) At a certain temperature, Kp = When pure ammonia is placed in an otherwise empty vessel at this temperature, equilibrium is reached after 25.00% of the ammonia has decomposed. Calculate the partial pressure of hydrogen gas at equilibrium. a) atm b) atm c) atm d) atm e) atm 19. Consider an initial mixture N2 and H2 in which the pressure of N2 is 2.00 atm and the pressure of H2 is 4.00 atm. Determine the pressure of NH3 at equilibrium (assume constant volume and temperature). These gases react according to the following equation: N2(g) + 3H2(g) 2NH3(g) Kp = 3.14 x 10-5 a) 1.26 x 10-4 atm b) 2.51 x 10-4 atm c) 4.31 x 10-3 atm d) 3.17 x 10-2 atm e) 6.33 x 10-2 atm 20. You add an excess of NaCl(s) to a beaker with ml of water and let this sit until the system comes to equilibrium at 25 C. You carefully remove 20.0 ml of the solution without disturbing the solid NaCl on the bottom of the beaker and transfer it to a new beaker. You then add g of solid NaCl to the solution in this new beaker at 25 C. What will happen? a) All g of NaCl(s) will dissolve in the solution and the system will not be able to reach equilibrium. b) Some of the g of NaCl(s) will dissolve in the solution. When the system re-reaches equilibrium, there will be less than g of NaCl(s) remaining. c) Some of the NaCl(aq) will precipitate out of the solution. When the system re-reaches equilibrium, there will be more than g of NaCl(s) remaining. d) g of NaCl(s) will remain and the system will remain at equilibrium. e) g of NaCl(s) will remain and the system will not be able to re-achieve equilibrium.

6 Hour Exam I Page No In the second Lon-Capa homework assignment ( Quiz 1 ) you were given a problem in which you were told that ammonia and oxygen gases react to form nitrogen dioxide gas and water. It turns out that a better way to think of this overall process is as two separate reactions. That is, the ammonia gas reacts with oxygen gas to produce nitrogen monoxide gas and water vapor, and the oxygen gas will also react with the nitrogen monoxide that is produced in the first reaction to produce nitrogen dioxide. These reactions happen simultaneously once nitrogen monoxide is produced. We will consider these reactions to go to completion (that is, not to reach equilibrium). Suppose, like in the Lon-Capa problem, you react equal masses of ammonia and oxygen gases to completion. In this case, you do so in a container fitted with a frictionless, massless piston at constant temperature. You make three observations after the reactions are complete: The mass of water vapor in the reaction mixture is g. Both nitrogen monoxide and nitrogen dioxide gases are in the reaction mixture. The final volume of the container after the reactions are complete is the same as the initial volume of the container before the first reaction started (when equal masses of ammonia and oxygen gases were added to the container). Use this information to determine the mass percent of the original oxygen gas that reacted with the ammonia (as opposed to the nitrogen monoxide). Note: full credit is reserved for explaining and showing all work in a systematic fashion. An additional sheet, if needed, is provided on the next page. [20 points]

7 Hour Exam I Page No (con t). Additional space is provided below if needed.

8 Hour Exam I Page No Consider three binary gaseous compounds (A, B, and C) at room conditions (1.00 atm and 25 C) made from the same two elements (X and Z). You have the following data on two of the compounds: Compound Density Mass % of Z A g/l 63.2 B g/l 74.1 You also have a mixture of equal masses of gas B and gas C, and this gaseous mixture has a density of g/l (at 1.00 atm and 25 C). Your goal is to determine the molecular formulas and names for the three compounds. If you believe that there are multiple possible answers for a given compound, list the choices and support them. Note: full credit is reserved for explaining and showing all work in a systematic fashion, and for deriving any formulas that you use but are not given on the first page of the exam. An additional sheet, if needed, is provided on the next page. [20 points]

9 Hour Exam I Page No (con t). Additional space is provided below if needed.

10 Hour Exam I Page No At high temperatures, diatomic molecules can be made to dissociate into their constituent atoms. For example, consider a container fitted with a frictionless, massless piston as shown below: P external The container is filled with H2(g) at constant temperature and a constant external pressure. Over time, a certain percentage of hydrogen molecules in a sample dissociate to atomic hydrogen to reach equilibrium: H2(g) 2H(g). a. In order to decrease the percent dissociation for a given sample of hydrogen molecules at this temperature does the external pressure need to be raised or lowered? Explain your answer with a discussion of partial pressures. [8 points]

11 Hour Exam I Page No b. Suppose the container is filled with H2(g) at 4500.K and a constant external pressure of atm. Over time, 87.00% of hydrogen molecules in a sample dissociate to atomic hydrogen to reach equilibrium. Determine the external pressure such that 66.00% of the hydrogen molecules in the sample dissociate at 4500.K. Note: full credit is reserved for explaining and showing all work in a systematic fashion Also, explain how your answer is consistent with your explanation in part a on page 9. [12 points]