CH302 Spring 2009 Practice Exam 1 (a fairly easy exam to test basic concepts) !1) Complete the following statement: We can expect _____ vapor pressure when the molecules of a liquid are held together by _____ intermolecular forces in the liquid and _____ vapor pressure when the intermolecular forces are strong. A. high; weak; low B. low; strong; high C. low; weak; high D. high; strong; low Answer: A 2) A plot of ln(vapor pressure) versus 1/T for benzene gives a straight line with slope –3.70 × 103 K. The enthalpy of vaporization of benzene is A. 2.25 kJmol–1. B. Not enough information is given to permit the calculation. C. 3.70 kJmol–1. D. 30.8 kJmol–1. E. 445 Jmol–1. Answer: D 3) The phase diagram for a pure substance is given below.The substance is stored in a container at 150 atm at 25°C. Describe what happens if the container is opened at 25°C. A. The liquid in the container freezes. B. The solid in the container sublimes. C. The solid in the container melts. D. The vapor in the container escapes. E. The liquid in the container vaporizes. Answer: E 4) Consider the phase diagrams for water and carbon dioxide given in the text on page 315. Explain the following observations: A thin wire with weights attached is draped over a block of “dry ice,” a second wire with weights is draped over a block of ice. The wire cuts through the ice but not through the “dry ice.” Answer Answer: For water, increased pressure reduces the melting point, whereas for carbon dioxide the opposite is true. Remember, ice is less stable at high pressures. 5) For CaCl2, the enthalpies of hydration and solution are –2337 and –81 kJmol–1, respectively, at 25°C. Calculate the lattice enthalpy of calcium chloride.Answer Answer: +2256 kJmol–1 6) Calculate the vapor pressure at 25°C of a mixture of benzene and toluene in which the mole fraction of benzene is 0.650. The vapor pressure at 25°C of benzene is 94.6 Torr and that of toluene is 29.1 Torr. A. 84.4 Torr B. 124 Torr C. 51.3 Torr D. 71.7 Torr E. 61.5 Torr Answer: D 7) Of the following, which would likely dissolve in toluene? A. Na2CO3 B. NaCl C. C6H12O6 D. Ca(HCO3)2 E. Cl2CCCl2 Answer: E 8) True or false: the van't Hoff i of HBr, HCl, and HF should all be the same? Answer Answer: False 9) The addition of 58 g of acetone to 10.0 kg of water lowers the water freezing point by 0.186°C. What is the molar mass of acetone? Give your answer to 2 significant figures.10) An animal cell assumes its normal volume when it is placed in a solution with a total solute molarity of 0.3 M. If the cell is placed in a solution with a total solute molarity of 0.1 M, A. water enters the cell, causing expansion. B. water leaves the cell, causing contraction. C. the escaping tendency of water in the cell increases. D. no movement of water takes place. Answer: A 11) When a certain solid dissolves, the disorder of a system increases (ΔS>0). Which of the following is concluded? A. This solid is more soluble at higher temperatures. B. This solid is less soluble at higher temperatures. C. The change in Gibbs free energy becomes more positive at higher temperatures. D. There is no change in Gibbs free energy at higher temperatures. Answer: A 12) You observe that the solubility of an inert gas increases as the temperature of the solution decreases. Which of the following statements explains this observation? A. The dissolution of the gas in water is an exothermic process. B. The dissolution of the gas in water is driven by an increase in system entropy C. The dissolution of the gas in water is an endothermic process. D. The dissolution of the gas in water is driven by a decrease in system entropy E. More than one of these statements explains the observation. Answer: A 13) According to the given heat curve, which of the following statements is wrong?A. ΔHvap > ΔHfus B. C (s) > C (l) C. C (l) > C (g) D. None of the above Answer: D 14) Which of the following is the expression for the equilibrium constant for the reaction 4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g) ? A. K = [ (PNO)4 × (PH2O)12 ] / [ (PNH3)12 × (PO2)10 ] B. K = [ 6(PH2O) × 4(PNO) ] / [ 5(PO2) × 4(PNH3) ] C. K = [ (PNH3)4 × (PO2)5 ] / [ (PNO)4 × (PH2O)6 ] D. K = [ (PH2O)6 × (PNO)4 ] / [ (PO2)5 × (PNH3)4 ] Answer: D 15) The standard Gibbs free energy of reaction for 2 O3(g) → 3 O2(g) is ΔGr° = −326.4 kJ × mol− 1 at 298 K. Calculate the equilibrium constant for this reaction. A. 1.63 × 1057 B. 11.7 × 106 C. 1.14D. 6.14 × 10− 58 Answer: A 16) For the reaction NH3(g) + H2S(g) NH4HS(s) Kc = 9.7 at 900 K. If the initial concentrations of NH3(g) and H2S(g) are 2.0 M, what is the equilibrium concentration of H2S(g)? A. 1.9 M B. 0.20 M C. 1.7 M D. 0.10 M E. 0.32 M Answer: E 17) The equilibrium constant K for the dissociation of N2O4(g) to NO2(g) is 1700 at 500 K. Predict its value at 300 K. For this reaction, ΔH° is 56.8 kJmol–1. A. 1.32 × 10–6 B. 1.11 × 10–4 C. 15.5 D. 0.188 E. 1.54 × 107 Answer: D 18) Consider the gas-phase reaction, N2(g) + 3 H2(g) <=> 2 NH2 (g), for which Kp = 43 at 400 K. If the mixture is analyzed and found to contain 0.18 bar of N2, 0.36 bar of H2 and 0.62 bar of NH3, describe the situation: A. Q < K and more reactants will be made to reach equilibrium. B. Q < K and more products will be made to reach equilibrium. C. Q > K and more reactants will be made to reach equilibrium. D. Q > K and more products will be made to reach equilibrium.E. Within 1 decimal place, Q = K and the reaction is at equilibrium Answer: C 19) Consider the reaction H2(g) + Cl2(g) <=> 2 HCl(g), which is exothermic as written. What would be the effect on the equilibrium position of decreasing the temperature? A. Reaction would go to the right, making more "products" B. Reaction would go to the left, making more "reactants" C. Reaction would go to the left, making more "products" D. Reaction would go to the right, making more "reactants" E. No change on the equilibrium position Answer: A 20) For each of the following equilibria, state whether reactants or products will be favored by an increase in the total pressure resulting from compression. I. CaC2(s) + 2 H2O(l) Ca(OH)2(s) + C2H2(g) II. Ni(s) + 4 CO(g) Ni(CO)4(g) A. I – Reactants will be favored; II – products will be favored. B. I – products will be favored; II – Reactants will be favored. C. I,
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