105b_1999_e3_blank reisler

105b_1999_e3_blank reisler - 1 1. 16 points (a) How many...

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Unformatted text preview: 1 1. 16 points (a) How many moles of NaCN must be added to 1.00 L of 0.50 M HCN solution to give a buffer of pH = 9.00? Ka(HCN) = 7.20x10-10 (b) Write reactions that show how a buffer made of HCN and NaCN reacts when: (i) a strong acid is added (ii) a strong base is added 2 (2) (14 points) A 1.00 L buffer solution was prepared from 0.31 M NH3 and 0.26 M NH4F. 0.10 mol of H+ ions were added to the solution (without changing the volume). Calculate the pH of the final solution. Kb(NH3)= 1.8x10-5 3 3. (14 points) Ag2SO4 dissolves in water to the extent of 8.7x10-5 mol/L. a). Calculate Ksp for this salt. b). What is the molar solubility of this salt in a 0.1 M solution of AgNO3? 4 4. (10 points) When ignited, solid ammonium dichromate decomposes in a fiery display (the "volcano reaction"), according to the reaction: (NH4)2Cr2O7(s) Cr2O3(s) + 4H2O(l) + N2(g) The thermodynamic values for this reaction are: Substance Cr2O3(s) H2O(l) 242 N2(g) (NH4)2Cr2O7(s) DHf0 (kJ/mol) 1140 118.7 0 22.5 191.5 113.7 S0 (J/mol.K) 81.2 Calculate DG0 for this reaction 5 (5) (18 points) (a) Assume that the reaction CO(g) + H2O(g) CO2(g) + H2(g) occurs in the gas phase at 700 K. If the measured equilibrium constant is Kp = 5.10 at this temperature, what is the value of DG0? (b) consider the reaction: 2SO2(g) + O2(g) 2SO3(g) for which DH0 = 200 kJ and DS0 = 187 J/K. Assuming that DH0 and DS0 are independent of temperature, calculate the temperature for which Kp = 1. 6 6. (12 points) (i) Which of the following has the highest buffer capacity: (a) 0.01 M HCN and 0.01 NaCN (b) 0.1 M HCN and 0.1 NaCN (c) 1.0 M HCN and 1.0 NaCN Answer: ______ (ii) NaCl is added slowly to a solution that is 0.010 M in Cu+, Ag+ and Au+. The Ksp values for the chloride salts are 1.9x10-7, 1.6x10-10, and 2.0x10-13, respectively. Which compound will precipitate first? (a) CuCl(s) Ksp = 1.9x10-7 (b) AgCl(s) Ksp = 1.6x10-10 (c) AuCl(s) Ksp = 2.0x10-13 Answer: ______ (iii) Which of the following substances will be more soluble in 1.0 M H+ than in pure water: (a) KNO3 (b) FePO4 (c) NaCl (d) AgCl (e) KCl Answer: ______ (iv) Cu2+(aq) ions react with OH(aq) ions to form an insoluble copper hydroxide. Cu2+(aq) is also known to react with ammonia to form the soluble Cu(NH3)4 2+ ion. Therefore, the addition of NH3 to copper hydroxide would: (a) (b) (c) decrease its solubility increase its solubility not affect its solubility Answer: ______ 7 7. (16 points) (i) For which one of the following processes you expect DS0 to be the most positive? (a) 2SO2(g) + O2(g) 2SO3(g) (b) H2O(l) H2O(s) (c) NH3(g) + HF(g) NH4F(g) (d) 2NH4NO3(s) O2(g) + 4H2O(g) + 2N2(g) (e) N2O4(g) 2NO2(g) Answer: ______ (ii) Which of the following processes shows decreased entropy? (a) freezing of water (b)burning of methane (c) melting of ice (d)coal gasification (e) boiling of alcohol Answer: ______ A chemical reaction is most likely to be spontaneous if it is accompanied by: (a) increasing energy and increasing entropy (b) lowering energy and increasing entropy (c) increasing energy and decreasing entropy (d) lowering energy and decreasing entropy Answer: ______ (iv) There are two known crystalline forms of white phosphorous known as form I and form II. The reaction: P4(s, I) P4(s, II) is exothermic, but it is spontaneous only at T < 550 K. What are the signs of DH and DS for this reaction? (iii) DH: ______ DS: ______ ...
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