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

Transcription

1 CHEMISTRY 202 Practice Hour Exam I Fall 2016 Dr. D. DeCoste Name Signature T.A. This exam contains 23 questions on 9 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 Practice Hour Exam I Page No A metal ion has 23 electrons in an ionic compound that is 30.06% oxygen by mass. Which metal is in the compound? a) Vanadium b) Chromium c) Manganese d) Iron e) Cobalt 2. A metal reacts with water to form the hydroxide of the metal ion along with hydrogen gas. If 4.61 g of water is required to react with exactly 10.0g of a metal, what is the identity of the metal? a) Li b) Na c) K d) Ca e) Ba 3. Ethane (C 2 H 6 ) reacts with oxygen gas to produce carbon dioxide gas and water. Suppose you react g of ethane with 100.0g of oxygen gas. Determine the mass of left-over reactant. a) g b) g c) g d) g e) No limiting reactant 4. A 6 mole sample of copper metal is reacted with excess sulfur to produce a 5 mole mixture of copper(i) sulfide and copper(ii) sulfide. What is the mole ratio of copper(i) sulfide to copper(ii) sulfide in the product mixture? a) 1:4 b) 4:1 c) 2:1 d) 1:2 e) 1:1 5. Determine the volume of water required to add to 15.0 ml of 3.25 M NaOH to make a solution with a concentration of M NaOH. a) 26.0 ml b) 187 ml c) 254 ml d) 375 ml e) 390. ml , 7. Consider mixing ml of 0.125M solution of calcium nitrate with ml of a 0.125M solution of sodium phosphate to produce a solid. Answer the following questions. 6. Determine the concentration of the phosphate ion after the precipitation reaction is complete. a) M b) M c) M d) M e) 0 M 7. Determine the mass of solid formed. a) g b) 1.94 g c) 2.53 g d) 3.88 g e) 5.82 g A 5.00-g sample of silver nitrate is heated in an evacuated container with a volume of 1.75 L. The sample decomposed to Ag 2 O(s), nitrogen dioxide gas, and oxygen gas. Assuming complete decomposition, determine the pressure in the container after cooling to room temperature (25 C). Assume the Ag 2 O(s) takes up negligible volume. a) atm b) atm c) atm d) 1.03 atm e) 2.06 atm 9. Consider two 1.00 mol samples of He(g) each in identical 1.00 L containers. The temperature of sample 1 = C and that of sample 2 = C. Determine the ratio of Z A2 /Z A1. a) b) c) 1.00 d) 1.73 e) 3.00

3 Practice Hour Exam I Page No A gaseous compound known to contain only carbon, hydrogen, and nitrogen is mixed with exactly the volume of oxygen gas required for complete combustion to CO 2, H 2 O, and N 2. It is found that reacting 9 total volumes of reactant mixture (compound plus O 2 ) produces 4 volumes of CO 2, 6 volumes of H 2 O, and 2 volumes of N 2, all at the same temperature and pressure. What is the molecular formula of the compound? a) C 2 H 6 N 2 b) C 4 H 6 N 2 c) CH 3 N d) C 4 H 12 N 4 e) Only an empirical formula can be determined with the information given. 11. Consider a gas mixture consisting of xenon and neon at 3.14 atm and 25.0 C. The density of this mixture is g/l. Determine the partial pressure of the xenon. a) 1.14 atm b) 1.26 atm c) 1.57 atm d) 1.88 atm e) More information is needed to answer this question. 12. Baking soda (sodium bicarbonate) reacts with acetic acid (HC 2 H 3 O 2 ) in vinegar to produce sodium acetate, carbon dioxide gas, and water. In lecture we reacted 10.5 g of baking soda with and excess of acetic acid and collected the carbon dioxide in a balloon. Disregarding water vapor pressure, determine the size of the balloon at room conditions (1.00 atm and 25 C). a) L b) 1.53 L c) 3.06 L d) 6.12 L e) 12.2 L Indicate which of the graphs below best represents each plot described in questions Note: the graphs may be used once, more than once, or not at all. a) b) c) d) e) 13. Collision frequency (Z A ) (y) vs. T (K) (x) for 1 mole of an ideal gas at constant P. 14. Average kinetic energy (y) vs. T ( C) (x) for a an ideal gas.

4 Practice Hour Exam I Page No. 3 15, 16. Consider an equimolar (equal number of moles) mixture of N 2 and O 2 gases in a rigid steel container. At equilibrium 25.0% of the nitrogen gas has reacted to produce NO(g) as follows: N 2 (g) + O 2 (g) 2NO(g) 15. Assuming the molecular diameters of N 2, O 2, and NO are essentially the same, determine the intermolecular collision frequency ratio initially and at equilibrium (Z initial : Z equilibrium ) a) 1:4 b) 4:1 c) 1:2 d) 2:1 e) 1:1 16. Determine the value of the equilibrium constant, K, at this temperature. a) b) c) d) e) Consider a sample of pure UF 6 (g) initially at a pressure of 5.00 atm at 357 C. The UF 6 (g) decomposes as shown: UF 6 (g) UF 2 (g) + 2F 2 (g) After the system has reached equilibrium the pressure is atm at 357 C. Determine the value of K p for this decomposition at 357 C. a) 3.60 b) 5.00 c) 13.3 d) 25.0 e) , 19. Ammonium carbamate decomposes as follows: NH 4 OCONH 2 (s) 2NH 3 (g) + CO 2 (g) In an experiment carried out in a 10.0-L container at 25 C a certain amount of ammonium carbamate is placed in an evacuated container and allowed to come to equilibrium. When the system has reached equilibrium the total pressure in the flask was observed to be atm. 18. Determine the value of K p for this reaction at 25 C. a) 1.85 x 10-4 b) 2.19 x 10-4 c) 1.48 x 10-3 d) 2.89 x 10-3 e) 3.25 x Determine the minimum mass of ammonium carbamate required for the system to reach equilibrium. a) 1.21 g b) 1.82 g c) 2.97 g d) 3.63 g e) Need more info Consider the pop bottles at the beginning of lecture in which we react hydrogen gas and oxygen gas to produce water. Which of the following correctly describes what would happen if we carried out the reaction at a higher temperature? a) Equilibrium would be shifted to the right and the value of K would increase. b) Equilibrium would be shifted to the right and the value of K would decrease. c) Equilibrium would be shifted to the left and the value of K would decrease. d) Equilibrium would be shifted to the left and the value of K would increase. e) Equilibrium would be shifted but the value of K would remain constant.

5 Practice Hour Exam I Page No Ammonia gas [NH 3 (g)] reacts with oxygen gas to produce H 2 O(l) and either NO(g) or NO 2 (g), depending on the supply of oxygen. If oxygen gas is limiting, NO(g) is one of the products. If oxygen gas is in excess, NO 2 (g) is a product. Suppose you react ammonia and oxygen gases in a container fitted with a piston in such a ratio that the reactant mixture at 25.0 C and 1.00 atm has the same density as air at these conditions (consider air to be 21.0% oxygen gas and 79.0% nitrogen gas by volume). a. Determine the balanced equation for the reaction that occurs and defend your answer with numbers. b. Determine the ratio of the volume of the container before and after the reaction. Assume the liquid water that is produced takes up negligible volume. Show all work.

6 Practice Hour Exam I Page No (con t) c. Specify the temperature so that the gaseous mixture after the reaction is complete has the same density as the initial reactant gaseous mixture at 25.0 C and 1.00 atm. If there is a range of temperatures that work, explain. Show all work.

7 Practice Hour Exam I Page No In this problem you will differentiate between a law and a theory, and apply your results. a. Starting with the ideal gas law equation, derive an equation (law) relating the moles of gas to the Kelvin temperature at a given pressure and volume. Show all steps. b. Sketch a plot of n (y) vs. T (K) (x) at constant P and V. Explain how your graph supports the law you derived in part a. c. Use the tenets of the kinetic molecular theory to explain the law you derived in part a. Use complete sentences.

8 Practice Hour Exam I Page No (con t) d. A hot-air balloon is open at the bottom and top. Even after a hot-air balloon reaches maximum volume it will ascend when the air inside is heated. Use the law (part a) and theory (part c) to explain why this is true. Use complete sentences. e. You heat the air in a hot-air balloon to 65 C. Determine the volume (in liters) required for this balloon to be able to lift 25.0 kg if the surrounding conditions are 25 C and 1.00 atm. Assume air is 21.0% oxygen gas and 79.0% nitrogen gas by volume. Show all work.

9 Practice Hour Exam I Page No Consider a gas-phase reaction (that is, all species are gases) that has reached equilibrium. To this system we add an inert gas (such as helium) at constant temperature. a. Which of the following describes what happens? Fill in the circles next to any statement that applies. o There is no shift because adding an inert gas never shifts the equilibrium position. o If the system is at constant volume and K = K p, then the equilibrium position will shift. o If the system is at constant volume and K K p, then the equilibrium position will shift. o If the system is at constant pressure and K = K p, then the equilibrium position will shift. o If the system is at constant pressure and K K p, then the equilibrium position will shift. b. Explain your answer to part a. As part of your explanation provide an example of a chemical equation in which K = K p, and an example of a chemical equation in which K K p and justify your choices. Full credit is reserved for explanations that address all of the conditions given in part a, that use the actual (real) chemical equations, and that correctly incorporate partial pressures (and/or partial concentrations) of reactants and products. You may continue your answer on the next page if needed.

10 Practice Hour Exam I Page No (con t). Continue your answer on this page if needed.