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Chapters 13 and 14 1. 1980 #3 A solution containing 3.23 grams of an unknown compound dissolved in 100.0 grams of water freezes at -0.97 °C. The solution does not conduct electricity. a. Calculate the molecular weight of the compound. (The molal freezing point depression constant for H2O is 1.86 °C kg mole¯1.) b. Elemental analysis of this unknown compound yields the following percentages by weight: H = 9.74%; C = 38.70%; O = 51.56%. Determine the molecular formula of the compound. c. Complete combustion of a 1.05-gram sample of the compound with the stoichiometric amount of oxygen gas produces a mixture of H2O(g) and CO2(g). What is the pressure of this gas when it is contained in a 3.00-liter flask at 127 °C? 2. 1982 #8 Describe a laboratory procedure needed to carry out each of the following. a. Separate a mixture of powdered solid CaCl2 and CaCO3. b. Determine the concentration of solute in an aqueous sodium chloride solution and give the concentration units that your method provides. c. Separate a mixture of two volatile liquids. 3. 1984 #4 a d Give a scientific explanation for the following observations. Use equations of diagrams if they are relevant. a. It takes longer to cook an egg until it is hard-boiled in Denver (altitude 1 mile above sea level) than it does in New York City (near sea level). d. The addition of antifreeze to water in a radiator decreases the likelihood that the liquid in the radiator will either freeze or boil. 4. 1984 #8 Discuss some differences in physical and chemical properties of metals and nonmetals. What characteristic of the electronic configurations of atoms distinguishes metals from nonmetals? On the basis of this characteristic, explain why there are many more metals than nonmetals. 5. 1985 #3 The formula and the molecular weight of an unknown hydrocarbon compound are to be determined by elemental analysis and the freezing-point depression method. a. The hydrocarbon is found to contain 93.46 percent carbon and 6.54 percent hydrogen. Calculate the empirical formula of the unknown hydrocarbon. b. A solution is prepared by dissolving 2.53 grams of p-dichlorobenzene (molecular weight 147.0) in 25.86 grams of naphthalene (molecular weight 128.2). Calculate the molality of the p-dichlorobenzene solution. c. The freezing point of pure naphthalene is determined to be 80.2 °C. The solution prepared in (b) is found to have an initial freezing point of 75.7 °C. Calculate the molal freezing-point depression constant of naphthalene. d. A solution of 2.43 grams of the unknown hydrocarbon dissolved in 26.7 grams of naphthalene is found to freeze initially at 76.2 °C. Calculate the apparent molecular weight of the unknown hydrocarbon on the basis of the freezing-point depression experiment above. e. What is the molecular formula of the unknown hydrocarbon? 6. 1985 #9 Substance Melting Point, °C H2 -129 C3H8 -190 HF -92 CsI 621 LiF 870 SiC >2,000 a. Discuss how the trend in the melting points of the substances tabulated above can be explained in terms of the types of attractive forces and/or bonds in these substances. b. For any pairs of substances that have the same kind(s) of attractive forces and/or bonds, discuss the factors that cause variation in the strengths of the forces and/or bonds. 7. 1986 #5 The first ionization energy of sodium is +496 kilojoules per mole, yet the standard heat of formation of sodium chloride from its elements in their standard state is -411 kilojoules per mole. a. Name the factors that determine the magnitude of the standard heat of formation of solid sodium chloride. Indicate whether each factor makes the reaction for the formation of sodium chloride from its elements more or less exothermic. b. Name the factors that determine whether the reaction that occurs when such a salt dissolves in water is exothermic or endothermic and discuss the effect of each factor on the solubility. 8. 1986 #8 Give a scientific explanation for each of the following observations. Use equations or diagrams if they seem relevant. a. Graphite is used to make electrodes, while diamond, another allotrope of carbon, is a very poor conductor of electricity. b. Putting rock salt on an icy driveway melts the ice even when the air temperature is -10 °C. c. A hot-air balloon must be much larger than a helium-filled balloon in order to lift the same weight. d. Carbon dioxide, rather than a stream of water, should be used to extinguish an oil fire. 9. 1987 #7a-b In 1884 the Swedish chemist Svante Arrhenius proposed that salts dissociate into two or more separate, independent, ionic fragments when they dissolve in water. a. Give one piece of experimental evidence that more than 1 mole of particles is formed when 1 mole of a salt dissolves in water. b. Give one piece of experimental evidence that the particles formed when a salt dissolves in water are charged. 10. 1987 #8b When crystals of barium hydroxide, Ba(OH)2 . 8H2O, are mixed with crystals of ammonium thiocyanate, NH4SCN, at room temperature in an open beaker, the mixtures liquefies, the temperature drops dramatically, and the odor of ammonia is detected. The reaction that occurs is the following: Ba(OH)2 . 8H2O(s)+2 NH2SCN(s) Ba2++2SCN¯ +2 NH3(g)+10H2O(l) If the beaker in which the reaction is taking place is put on a block of wet wood, the water on the wood immediately freezes and the beaker adheres to the wood. Yet the water inside the beaker, formed as the reaction proceeds, does not freeze even though the temperature of the reaction mixture drops to -15 °C. Explain these observations. 11. 1988 #5a-c Using principles of chemical bonding and/or intermolecular forces, explain each of the following. a. Xenon has a higher boiling point than neon has. b. Solid copper is an excellent conductor of electricity, but solid copper chloride is not. c. SiO2 melts at a very high temperature, while CO2 is a gas at room temperature, event though Si and C are in the same chemical family. 12. 1988 #8 The normal boiling and freezing points of argon are 87.3 K and 84.0 K, respectively. The triple point is at 82.7 K and 0.68 atmosphere. a. Use the data above to draw a phase diagram for argon. Label the axes and label the regions in which the solid, liquid, and gas phases are stable. On the phase diagram, show the position of the normal boiling point. b. Describe any changes that can be observed in a sample of solid argon when the temperature is increases from 40 K to 160 K at a constant pressure of 0.50 atmosphere. c. Describe any changes that can be observed in a sample of liquid argon the pressure is reduced from 10 atmospheres to 1 atmosphere at a constant temperature of 100 K, which is well below the critical temperature. d. Does the liquid phase of argon have a density greater than, equal to, or less than the density of the solid phase? Explain your answer, using information given in the introduction to this question. 13. 1989 #6 The melting points of the alkali metals decrease from Li to Cs. In contrast, the melting of the halogens increase from F2 to I2. a. Using bonding principles, account for the decrease in the melting point of the alkali metals. b. Using bonding principles, account for the increase in the melting points of the halogens. c. What is the expected trend in the melting points of the compounds LiF, NaCl, KBr, and CsI? Explain this trend using bonding principles. 14. 1991 #2 The molecular formula of a hydrocarbon is to be determined by analyzing its combustion products and investigating its colligative properties. a. The hydrocarbon burns completely, producing 7.2 grams of water and 7.2 liters of CO 2 at standard conditions. b. Calculate the mass in grams of O2 required for the complete combustion of the sample of the hydrocarbon described in (a). c. The hydrocarbon dissolves readily in CHCl3. The freezing point of a solution prepared by mixing 100. grams of CHCl3 and 0.600 gram of the hydrocarbon is -64.0 °C. The molal freezing-point depression constant of CHCl3 is 4.68 °C / molal and its normal freezing point is -63.5 °C. Calculate the molecular weight of the hydrocarbon. d. What is the molecular formula of the hydrocarbon? 15. 1991 #8 Experimental data provide the basis for interpreting differences in properties of substances. Table 1 Compound Melting Point Electrical conductivity of (°C) molten state BeCl2 405 0.086 MgCl2 714 >20 SiCl4 -70 0 MgF2 1261 >20 Substance F2 Br2 N2 Table 2 Bond Length (angstroms) 1.42 2.28 1.09 Account for the differences in properties given in Tables 1 and 2 above in terms of the differences in structure and bonding in each of the following pairs. a. MgCl2 and SiCl4 b. MgCl2 and MgF2 c. F2 and Br2 d. F2 and N2 16. 1992 #8 Explain each of the following in terms of atomic and molecular structures and/or intermolecular forces. a. Solid K conducts an electric current, whereas solid KNO3 does not. b. SbCl3 has a measurable dipole moment, whereas SbCl5 does not. c. The normal boiling point of CCl4 is 77 °C, whereas that of CBr4 is 190 °C. d. NaI(s) is very soluble in water whereas I2(s) has a solubility of only 0.03 gram per 100 grams of water. 17. 1993 #2 Elemental analysis of an unknown pure substance indicates that the percent composition by mass is as follows: Carbon - 49.02% Hydrogen - 2.743% Chlorine - 48.23% A solution that is prepared by dissolving 3.150 grams of the substance in 25.00 grams of benzene, C6H6, has a freezing point of 1.12°C. (The normal freezing point of benzene is 5.50°C and the molal freezing-point depression constant, Kf, for benzene is 5.12 C°/molal.) a. Determine the empirical formula of the unknown substance. b. Using the data gathered from the freezing point depression method, calculate the molar mass of the unknown substance. c. Calculate the mole fraction of benzene in the solution described above. d. The vapor pressure of pure benzene at 35°C is 150. millimeters of Hg. Calculate the vapor pressure of benzene over the solution described above at 35°C. 18. 1994 #5 b and d Discuss the following phenomena in terms of the chemical and physical properties of the substances involved and general principles of chemical and physical change. b. A bell jar is connected to a vacuum pump containing an open beaker of water. As the air pressure under the bell jar decreases, what behavior of the water in the beaker will be observed? Explain why this occurs. d. A water solution of I2 is shaken with an equal volume of a nonpolar solvent such as TTE (trichlorotrifluoroethane). Describe the appearance of the system after shaking. Account for the observation. 19. 1994 #8 a-c For each of the following, use appropriate chemical principles to explain the observation. a. Sodium chloride may be spread on an icy sidewalk in order to melt the ice; equimolar amounts of calcium chloride are even more effective. b. At room temperatue, NH3 is a gas and H2O is a liquid, even though NH3 has a molar mass of 17 grams and H2O has a molar mass of 18 grams. c. C (graphite) is used as a lubricant, whereas C (diamond) is used as an abrasive. 20. 1995 #5 The conductivity of several substances was tested. The results of the tests are summarized in the following data table. AgNO3 Sucrose Na H2SO4 Melting point (°C) 212 185 99 Liquid at room temperature Liquid Good conductor Nonconductor Good conductor Poor conductor Water solution Good conductor Nonconductor Good conductor Good conductor (dissolves, (conduction accompanied by increases as the the evolution of a acid is added to gas) water) Solid nonconductor nonconductor conductor Not tested Using models of chemical bonding and atomic or molecular structure, account for the differences in conductivity between the two samples in each of the following pairs. a. Sucrose solution and silver nitrate solution b. Solid silver nitrate and solid sodium metal c. Liquid (fused) sucrose and liquid (fused) silver nitrate d. Liquid (concentrated) sulfuric acid and sulfuric acid solution 21. 1995 #6 Pressure (atm) W V 1.0 X Y Z Temperature The phase diagram for a pure substance is shown above. Use this diagram and your knowledge about changes of phase to answer the following questions. a. What does point V represent? What characteristics are specific to the system only at point V? b. What does each point on the curve between V and W represent? c. Describe the changes that the system undergoes as the temperature slowly increases from X to Y to Z at 1.0 atmosphere. d. In a solid-liquid mixture of this substance, will the solid float or sink? Explain. 22. 1996 #4 Concentrated sulfuric acid (18.4-molar H2SO4) has a density of 1.84 grams per milliliter. After dilution with water to 5.20-molar, the solution has a density of 1.38 grams per milliliter and can be used as an electrolyte in lead storage batteries for automobiles. a. Calculate the volume of concentrated acid required to prepare 1.00 liter of 5.20-molar H2SO4. b. Determine the mass percent of H2SO4 in the original concentrated solution. c. Calculate the volume of 5.20-molar H2SO4 that can be completely neutralized with 10.5 grams of sodium bicarbonate NaHCO3. d. What is the molality of the 5.20-molar H2SO4? 23. 1996 #9 a Explain each of the following in terms of the electronic structure and/or bonding of the compounds involved. a. At ordinary conditions, HF (normal boiling point = 20°C) is a liquid, whereas HCl (normal boiling point = -114°C) is a gas. 24. 1998 #2 An unknown compound contains only the three elements C, H, and O. A pure sample of the compound is analyzed and found to be 65.60 percent C and 9.44 percent H by mass. a. Determine the empirical formula of the compound. b. A solution of 1.570 grams of the compound in 16.08 grams of camphor is observed to freeze at a temperature 15.2 °C below the normal freezing point of pure camphor. Determine the molar mass and apparent molecular formula of the compound. (The molal freezing-point depression constant, Kf, for camphor is 40.0 kg-K-mol¯1.) c. When 1.570 grams of the compound is vaporized at 300 °C and 1.00 atmosphere, the gas occupies a volume of 577 milliliters. What is the molar mass of the compound based on this result? d. Briefly describe what occurs in solution that accounts for the difference between the results obtained in parts (b) and (c). 25. 1998 #9 Answer each of the following using appropriate chemical principles. a. Why does it take longer to cook an egg in boiling water at high altitude than it does at sea level? b. When NH3 gas is bubbled into an aqueous solution of CuCl2, a precipitate forms initially. On further bubbling, the precipitate disappears. Explain these two observations. c. Dimethyl ether, H3C-O-CH3, is not very soluble in water. Draw a structural isomer of dimethyl ether that is much more soluble in water and explain the basis of its increased water solubility. d. Identify a chemical species that is i. capable of oxidizing Cl¯(aq) under standard conditions ii. capable of reducing Cl2(aq) under standard conditions. In each case, justify your choice. 26. 2000 #5. The molar mass of an unknown solid, which is nonvolatile and a nonelectrolyte, is to be determined by the freezing-point depression method. The pure solvent used in the experiment freezes at 10˚C and has a known molal freezing-point depression constant, Kf,. Assume that the following materials are also available. Test tubes Stirrer Pipet Thermometer Balance Beaker Stopwatch Graph paper Hot water bath ice a. Sketch cooling curves for (i) the pure solvent and for (ii) the solution as each is cooled from 20˚C to 0.0˚C on two separate temperature versus time graphs. b. Information from these graphs may be used to determine the molar mass of the unknown solid. i. Describe the measurements that must be made to determine the molar mass of the unknown solid by this method. ii. Show the setup(s) for the calculation(s) that must be performed to determine the molar mass of the unknown solid from the experimental data. iii. Explain how the difference(s) between the two graphs in part (a) can be used to obtain information needed to calculate the molar mass of the unknown solid. c. Suppose that during the experiment a significant but unknown amount of solvent evaporates from the test tube. What effect would this have on the calculated value of the molar mass of the solid (i.e., too large, too small, or no effect?) Justify your answer. d. Show the setup for the calculation of the percentage error in a student’s result if the student obtains a value of 126 g mol-1 for the molar mass of the solid when the actual value is 120. g mol-1. 27. 2001 #5a Answer the questions below that relate to the following five 0.10 M aqueous solutions at 25˚C: Pb(NO3)2, NaCl, KMnO4, C2H5OH, KC2H3O2 a. Which solution has the highest boiling point? Explain 28. 2001 #8 Account for each of the following observations about pairs of substances. In your answers, use appropriate principles of chemical bonding and/or intermolecular forces. In each part, your answer must include references to both substances. a. Even though NH3 and CH4 have similar molecular masses, NH3 has a much higher normal boiling point (-33˚C) than CH1 (-164˚C). b. At 25˚C and 1.0 atm, ethane (C2H6) is a gas and hexane (C6H14) is a liquid. c. Si melts at a much higher temperature (1410˚C) than Cl2 (-101˚C) d. MgO melts at a much higher temperature (2852 ˚C) than NaF (993˚C) 29. 2002 #6d Use the principles of atomic structure and/or chemical bonding to explain each of the following. In each part, your answer must include references to both substances. d. The boiling point of Cl2 is lower than the boiling point of Br2. 30. 2003 #6. For each of the following, use appropriate chemical principles to explain the observation. Include chemical equations as appropriate. a. In areas affected by acid rain, statues and structures made of lime stone (calcium carbonate) often show signs of considerable deterioration. b. When table salt (NaCl) and sugar (C12H22O11) are dissolved in water, it is observed that i. Both solutions have a higher boiling point than pure water, and ii. The boiling point of 0.10 M NaCl (aq) is higher than that of 0.10 M C12H22O11 (aq). c. Methane gas does not behave as an ideal gas at low temperatures and high pressure. d. Water droplets form on the outside of a beaker containing an ice bath. 31. 2004 #7 a-b; d Use appropriate chemical principles to account for each of the following observations. In each part, your response must include specific information about both substances. a. At 25°C and 1 atm, F2 is a gas, whereas I2 is a solid. b. The melting point of NaF is 993 °C, whereas the melting point of CsCl is 645°C. d. Ammonia, NH3, is a very soluble in water, whereas phosphine, PH3, is only moderately soluble in water. 32. 2005 #2 Answer the following question about a pure compound that contains only carbon, hydrogen, and oxygen. a. A 0.7549 g sample of the compound burns in O2 (g) to produce 1.9061 g of CO2 (g) and 0.3370 g of H2O (g). i. Calculate the individual masses of C, H, and O in the 0.7549 g sample. ii. Determine the empirical formula for the compound. b. A 0.5246 g sample of the compound was dissolved in 10.0012 g of lauric acid, and it was determined that the freezing point of the lauric acid was lowered by 1.68 ˚C. The value of Kf of lauric acid is 3.90˚Cm-1. Assume the compound does not dissociate in lauric acid. i. Calculate the molality of the compound dissolved in the lauric acid. ii. Calculate the molar mass of the compound from the information provided. c. Without doing any calculations, explain how to determine the molecular formula of the compound based on the answers to parts (a)(ii) and (b)(ii). d. Further tests indicate that a 0.10 M aqueous solution of the compound has a pH of 2.6. Identify the organic functional group that accounts for this pH. 33. 2005 #7 a-b Use principles of atomic structure, bonding, and/or intermolecular forces to respond to each of the following. Your responses must include specific information about all substances referred to in each question. a. At a pressure of 1 atm, the boiling point of NH3 (l) is 240 K, whereas the boiling point of NF3 (l) is 144 K. i. Identify the intermolecular force(s) in each substance. ii. Account for the difference in the points of the substances. b. The melting point of KCl (s) is 776 ˚C, whereas the melting point of NaCl (s) is 801 ˚C. i. Identify the type of bonding in each substance. ii. Account for the difference in the melting points of the substances. 34. 2005 #8 d-e Use principles of atomic structure, bonding, and intermolecular forces to answer the following questions. Your responses must include specific information about all substances referred to in each part. H H H O | | | || H - C - C - C - H | | | H H H C H H O d. The complete structural formulas of propane, C3H8, and methanoic acid, HCOOH, are shown above. In the table below, write the type(s) of intermolecular attractive force(s) that occur in each substance. Substance Boiling Point Intermolecular Attractive Force(s) Propane 229 Methanoic acid 374 e. Use principles of intermolecular attractive forces to explain why methanoic acid has a higher boiling point than propane. 35. 2006 #6 a-c a. The structures for glucose, C6H12O6, and cyclohexane, C6H12, are shown below. OH H H OH H H H H H H OH H H H HO OH H H H OH H H Identify the type(s) of intermolecular attractive forces in i. Pure glucose ii. Pure cyclohexane b. Glucose is soluble I water by cyclohexane is not soluble in water. Explain. c. Consider the two processes represented below. Process 1: H2O(l) H2O(g) ΔH°= +44.0 kJ/mol Process 2: H2O(l) H2(g) + 1/2 O2(g) ΔH°= +286 kJ/mol i. ii. For each of the two processes, identify the type(s) of intermolecular or intramolecular attractive forces that must be overcome for the process to occur. Indicate whether you agree or disagree with the statement in the box below. Support your answer with a short explanation. When water boils, H2O molecules break apart to form hydrogen molecules and oxygen molecules.