CHEMISTRY 0 Hour Exam I September 5, 014 Dr. D. DeCoste Name Signature T.A. This exam contains 33 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 30 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 31, 3 and 33. 1-30 (60 pts. 31 (0 pts. 3 (0 pts 33 (0 pts. Total (10 pts Useful Information: Always assume ideal behavior for gases (unless explicitly told otherwise. PV = nrt R = 0.0806 Latm/molK = 8.3145 J/Kmol STP = standard temperature and pressure = 0 C and 1.00 atm K = C + 73 N A = 6.0 x 10 3 υ rms = 3RT M λ = 1 ( N / V ( πd Z A = A V N RT πm Z = 4 V N d πrt M Solubility Rules: 1. Most nitrate salts are soluble.. 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.
Hour Exam I Page No. 1 1. What is the molar mass of calcium phosphate? a 135.05 g/mol b 175.13 g/mol c 310.18 g/mol d 365.07 g/mol e 405.15 g/mol. Substance AB is 4.86% A by mass. What is the percent B by mass for substance A B? a 14.8% b 5.00% c 8.57% d 57.14% e 78.57% 3. Consider a compound that is 3.0% Co by mass, 15.30% nitrogen by mass, and the rest oxygen. Is it possible to determine the name of this compound? a Yes, and it is cobalt(i nitrate. b Yes, and it is cobalt(ii nitrate. c Yes, and it is cobalt(iii nitrate. d Yes, and it is cobalt nitrate. e No because we need to know the molar mass to determine the molecular formula. 4. Ethane (C H 6 reacts with oxygen in the air to produce carbon dioxide and water. What mass of oxygen is required to react completely with 10.0 g ethane? a 10.6 g b 6.6 g c 37. g d 53. g e 74.5 g 5. When iron(iii oxide is heated with carbon, the products are iron metal and carbon monoxide. Balance the equation in standard form and determine the sum of the coefficients. a 4 b 5 c 6 d 8 e 9 6. Consider the balanced chemical equation: 4A + 3B C + D. When equal masses of A and B are reacted, which is limiting? a If the molar mass of A is greater than the molar mass of B, then A must be limiting. b If the molar mass of A is less than the molar mass of B, then A must be limiting. c If the molar mass of A is greater than the molar mass of B, then B must be limiting. d If the molar mass of A is less than the molar mass of B, then B must be limiting. e None of the above. 7. A 5.000 g sample of copper reacts with oxygen in the air to form 5.69 g of an oxide of copper. Determine the formula for this compound. a Cu O b CuO c Cu O 3 d Cu 3 O 4 e Cu 4 O 3
Hour Exam I Page No. 8. Earlier in the semester we estimated the number of atoms in an average adult human. To try to get an accurate estimation, we looked at the percent by mass composition of the major elements in a human body. The partial data are: Major Elements Mass Percent Oxygen 65.0 Carbon 18.0 Hydrogen 10.0 Nitrogen 3.0 Calcium 1.4 Phosphorus 1.0 From the above data, determine the percent by moles of oxygen in the human body to the nearest integer. a 4% b 1 % c 18 % d 6% e 74% 9. How much water must be added to 50.0 ml of a 0.1000 M NaCl solution to make a 0.0800 M solution of NaCl? a 4.1 ml b 6.5 ml c 180.0 ml d 31.5 ml e 41.5 ml ---------------------------------------------------------------------------------------------------------------- For questions 10 and 11, consider mixing 15.0 ml of a 1.00 M aqueous solution of AgNO 3 and 10.0 ml of a 1.00 M aqueous solution of K CrO 4. When these are mixed, a solid is observed. 10. What is present in solution after the reaction is complete? Note: the solid is not considered to be in solution. a Ag +, NO - 3, K +, CrO - 4, water b Ag +, NO - 3, K +, water c K +, CrO - 4, water d NO - 3, K +, CrO - 4, water e water 11. Determine the mass of solid produced. a 1.01 g b.49 g c 3.3 g d 4.98 g e 5.5 g ----------------------------------------------------------------------------------------------------------------
Hour Exam I Page No. 3 1. In lab next week you will dissolve 7.000 g of copper(ii sulfate in water and react this solution with.000 g of iron powder. Because iron can form more than one type of ion, there are two potential reactions that could be occuring: Reaction 1: copper(ii sulfate (aq + iron (s copper (s + iron(ii sulfate (aq Reaction : copper(ii sulfate (aq + iron (s copper (s + iron(iii sulfate (aq Your goal in lab will be to determine which reaction occurs. After the reaction you dry the copper and note a mass of.76 g. Which of the following can you conclude? a Reaction 1 occurs and the iron metal is limiting. b Reaction occurs and the iron metal is limiting. c Reaction 1 occurs and the copper(ii sulfate is limiting. d Reaction occurs and the copper(ii sulfate is limiting. e We cannot tell which reaction occurs from the above data, but we can tell which reactant is limiting. 13. When solid potassium chlorate (KClO 3 is heated, the products are potassium chloride and oxygen gas. A sample of potassium chlorate is heated to completion and.83 L of pure oxygen gas is collected at 1.00 atm and 5 C. Determine the mass of solid product. a 5.75 g b 8.63 g c 9.45 g d 1.94 g e 17. g 14. The pop bottles that begin our lectures are filled with a mixture of hydrogen and oxygen gases that produce water. Suppose we ignite an equimolar (equal number of moles mixture of hydrogen and oxygen gases to produce water vapor in a rigid closed container at constant temperature. Determine the final pressure (P f in the container in terms of the original pressure (P o. a P f = P o b P f = (1/P o c P f = (/3P o d P f = (3/4P o e None of these 15. You are holding two balloons of equal volume, one filled with SF 6 (g and the other with He(g. How do the collision rates of gas particles with the inside of the balloon compare? a. The collision rate of He atoms is greater than the collision rate of SF 6 molecules. Thus, the pressure is greater in the balloon filled with He. b. The collision rate of SF 6 molecules is greater than the collision rate of He atoms. Thus, the pressure is greater in the balloon filled with He. c. The pressures of the gases are equal so the collision rates must be the same. d. The collision rate of SF 6 molecules is greater than the collision rate of He atoms but the pressures are the same in the balloons. e. The collision rate of He atoms is greater than the collision rate of SF 6 molecules but the pressures are the same in the balloons.
Hour Exam I Page No. 4 16. You are standing in a line with 9 other friends, evenly spaced apart. You are at one end of the line (we will call you person #1. You release laughing gas (N O at the exact same time the person at the other end of the line (person #10 releases the tear gas commonly referred to as CS gas. CS gas has the formula C 10 H 5 ClN. Assuming ideal behavior, which person is the first person to simultaneously laugh and cry? a Person 3 b Person 4 c Person 5 d Person 6 e Person 7 17. What mass of carbon dioxide will exert twice the pressure of 150.0 g of helium gas at the same conditions of volume and temperature? a 13.64 g b 150.0 g c 1.649 kg d 3.98 kg e 4.185 kg 18. Consider a gas mixture consisting of equal masses of helium and oxygen at 1.00 atm and 5.0 C. Determine the density of this gas mixture. a 0.91 g/l b 0.736 g/l c 1.15 g/l d 1.47 g/l e More information is needed to answer this question. 19. For which of the following constant volume cases is the number of collisions of gas particles with the walls of the container the greatest? Volume is the same in all cases. a 1.0 mol of oxygen gas at 5 C. b 1.0 mol of oxygen gas at 15 C. c 1.0 mol of helium gas at 5 C. d 1.0 mol of helium gas at 15 C. e Choices a and b have equal collision rates, and these are higher than those in c and d. 0. Consider the two flasks in the diagram below. Assuming the connecting tubes have negligible volume, what is the total pressure after the stopcock is opened? He Ar.00 atm 3.00 atm 4.00 L.00 L a.11 atm b.33 atm c.50 atm d.7 atm e 5.00 atm
Hour Exam I Page No. 5 --------------------------------------------------------------------------------------------------------------------- 1-4. Indicate which of the graphs below represents each plot described in questions 1-4. Note: the graphs may be used once, more than once, or not at all. a b c d e 1. Collision frequency (Z A (y vs. P (x for an ideal gas at constant V and T. a. Gas density (y vs. T(x for 1.0 mol of an ideal gas at constant P. e 3. Force per impact (y vs. T (x for an ideal gas at constant V. c 4. Collision frequency (Z A (y vs. T (x for 1.0 mol of an ideal gas at constant V. c -------------------------------------------------------------------------------------------------------------------- 5. Consider an initial mixture SO and O in which the partial pressure of SO is 3.00 atm and the partial pressure of O is.00 atm. These gases react according to the following equation: SO (g + O (g SO 3 (g K p = 1.80 x 10-6 Determine the partial pressure of SO 3 at equilibrium (assume constant volume and temperature. a 1.64 x 10-3 atm b.85 x 10-3 atm c 5.69 x 10-3 atm d 5.40 x 10-6 atm e 8.10 x 10-6 atm 6. Consider a chemical system at equilibrium. The reaction is endothermic (takes in energy as heat as written and the temperature of the system is raised. Which of the following is true? a Equilibrium shifts to the left and the value of K increases. b Equilibrium shifts to the right and the value of K decreases. c Equilibrium shifts to the left and the value of K decreases. d Equilibrium shifts to the right and the value of K increases. e Equilibrium shifts, but the value of K stays constant.
Hour Exam I Page No. 6 7. Which of the following is true about a system at equilibrium? a The concentration(s of the reactant(s is equal to the concentration(s of the product(s. b The concentration of reactant(s is constant over time. c No new product molecules are formed. d The rate of the reverse reaction is equal to the rate of the forward reaction and both rates are equal to zero. e None of the above (a-d is true. 8. You mix 500.0 ml of 4.00 M Fe(NO 3 3 with 500.0 ml 6.00 M KSCN. They react according to the following equation: Fe 3+ (aq + SCN - (aq FeSCN + (aq Since FeSCN + in solution appears red (and the reactants are colorless you use a spectrophotometer and determine the concentration of FeSCN + at equilibrium to be 1.00 M. Determine the value of the equilibrium constant, K, for the reaction above. a 0.0417 b 0.0667 c 0.100 d 0.167 e 0.500 9. How many of the following statements 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. When two opposing processes are proceeding at identical rates, the system is at equilibrium. IV. For a given reaction at a given temperature, there is only one set of equilibrium concentrations for the reactants and products. a 0 b 1 c d 3 e 4 30. For which of the following reactions is the value of K p greater than the value of K at a temperature of 300K? a N (g + 3H (g NH 3 (g b CO (g + H (g CO(g + H O(l c 3Fe(s + 4H O(g Fe 3 O 4 (s + 4H (g d H (g + O (g H O(g e CaCO 3 (s CaO(s + CO (g
Hour Exam I Page No. 7 31. Uranyl fluoride is a solid compound consisting of uranium, oxygen, and fluorine. It is produced by reacting gaseous uranium hexafluoride (UF 6 with water. There is another product of the reaction as discussed below. 4.67 g of UF 6 is reacted with excess water to produce solid uranyl fluoride along with 1.186 L of a binary gas at 1.00 atm and 5 C. The binary gas is 95% fluorine by mass. a. Use the data above to determine the empirical formula for uranyl fluoride. [10 pts.] UF6(g + HO(l UxOyFz(s + A(g 4.67 g x 1mol 35.089 g = 0.011 moles UF6 gas PV na = = RT (1.00 atm(1.186 L (0.0806 Latm/molK (98K = 0.0485 mol A [4X as much as UF6] Gas A is HF (must contain hydrogen, 95% F: 4.96 mol H, 5 mol F 1UF6(g + HO(l?UxOyFz(s + 4HF(g [Note 1:4 ratio, to balance] Empirical formula = UOF(s b. With the above data why are you able to find only the empirical formula? Provide two potential chemical equations (balanced that show you cannot determine the molecular formula from the given data. [ pts.] Could be, for example, UO4F4(s UF6(g + HO(l UOF(s + 4HF(g UF6(g + 4HO(l UO4F4 (s + 8HF(g Both have the 1:4 mol ratio of UF6(g:HF(g c. With the empirical formula of uranyl fluoride (instead of the molecular formula, is it possible to determine the mass of uranyl fluoride produced from 4.67 g of UF 6 with excess water? Whether you answer yes or no, explain your answer and support it mathematically. If you believe it is possible, determine the mass produced. If you believe it is not possible, show why it is not possible. [8 pts] Yes, the answer is the same regardless. 0.011 mol UF6 gas = 0.011 mol UOF(s = (0.011 x308.089 = 3.733 g 0.011 mol UF6 gas = 0.00605 mol UO4F4(s = (0.00606 x616.0578 = 3.733 g
Hour Exam I Page No. 8 3. The formula for mean free path is λ = 1 ( N / V ( πd. a. Discuss mean free path by answering the following questions. Use the postulates of the kinetic molecular theory when appropriate. [8 pts] What is meant by mean free path? Notice that according to the formula above, the mean free path is independent of temperature. Why is this? Explain the conditions such that this is true. Is there a scenario in which mean free path can change with temperature? If so, explain this along with describing the relationship (inverse or direct. If not, explain why not. The mean free path is the average distance between collisions between gas particles in a sample. In the formula above, we are dealing with a constant volume situation, like a rigid steel tank, for example. We can see this because V is in the equation, so it must be a constant number. Changing the temperature will increase the speed of the gas particles, but because the volume is constant, the distance the particles travel in unchanged. [They may collide more often with each other but the distance they travel is the same.]. If we have a container fitted with a piston or a balloon, in which pressure is constant and volume changes, the mean free path will be directly related to temperature. That is, as the temperature is increased, the mean free path increases. This is because an increase in temperature will increase the volume of the container, thus allowing for a greater distance between particles. 1 Knowing n/v is related to P/T, we can derive: λ = ( P / T ( πd and multiplying, we see temperature is in the numerator.. By inverting b. One assumption of the kinetic molecular theory is that gas particles do not interact with each other at all. That is, the gas particles collide with the walls of the container but not with each other. What must be true about the relative sizes of the mean free path and the distance between the walls of the container for this assumption to be valid? Explain your answer. [4 pts] For the particles not to collide with each other, the mean free path must be much greater than the distance between the walls of the container. Some particles collide in less than this distance, some in more; the mean free path is the average distance between collisions, so if the walls are much closer than this distance, no molecules will collide with each other.
Hour Exam I Page No. 9 3. (continued c. Suppose you have helium gas at 5 C in a 1000.-L cubic container (such that each side of the cube is 1.000 m, since 1m 3 = 1000 L. Determine the range of pressures that would make the assumption valid as you discussed in part b. Explain the significance of your answer. [8 pts] Note: use 3. x 10-11 m as the diameter of a helium atom ========================================================== λ = 1 ( N / V ( πd Suppose λ = 1.000 m = 1 ( N / V ( π (3.x10 11 N/V =.198 x 10 0 molecules/m 3 N =.198 x 10 0 molecules; n = 3.650 x 10-4 mol V = 1 m = 1000 L PV=nRT P = nrt/v = 8.96 x 10-6 atm We know that λ must be greater than the distance, P must be less than 8.96 x 10-6 atm. (even less than this since the as particles do not necessarily travel perpendicularly into a wall we could use 1.73 m, for example, as the diagonal of the cube In either case, the pressure is extremely small, and this makes sense because a gas behaves more ideally at low pressure since we assume no interactions between gas particles.
Hour Exam I Page No. 10 33. Gaseous hydrazine (N H 4 decomposes to nitrogen gas and hydrogen gas. At a certain temperature, K p = 3.50 x 10 4 for this reaction balanced in standard form. Pure gaseous hydrazine is placed in an otherwise empty rigid vessel and the pressure is noted. The reaction is allowed to reach equilibrium at this temperature and it is observed that the equilibrium pressure is double its initial value. a. Determine the initial pressure of the hydrazine (in atm. Define any variables. [11 pts] Pressure directly related to number of moles (constant volume and temperature, so we can use pressures in the ICE chart. NH4(g N(g + H(g I X 0 0 C -Y +Y +Y E X-Y Y Y X = initial pressure (pressure of hydrazine only Y = change in pressure of hydrazine (X-Y + Y + Y = X+Y = final pressure (X+Y/X = or X =Y; 3.5 x 10 4 = (Y (Y = (Y (Y ( Y ; Y = 93.5 Y (Y Initial pressure of hydrazine = X = Y = 187 atm b. What percent (in terms of moles of hydrazine decomposed to reach equilibrium? Explain your answer. [7 pts] Initial pressure = Y Change in pressure = Y Thus Y/Y or 50% decomposed (pressure related to moles (constant volume and temperature, so 50% moles decomposed. c. Determine the equilibrium pressure of hydrogen (in atm. [ pts] Equilibrium pressure of hydrogen gas = Y = 187 atm