Name I.D. # Chem 102H Exam 2 - Spring 2005 PHYSICAL CNSTANTS/CNVERSIN FACTRS Speed of light = 3.00! 10 8 m/s Planck!s const. = 6.63! 10-34 J s Avagadro!s Number = 6.02! 10 23 Electron charge = 1.602! 10-19 C Faraday!s const. = 96487 C mol -1 (e ) Gas const.(r) = 0.0821 L atm mol -1 K -1 = 62.4 L torr mol-1 K-1 = 8.314 J mol -1 K -1 = 1.987 cal mol -1 K -1 Mass of e = 0.00055 amu = 9.1094! 10-31 kg 0 C = 273 K 1.00 atm = 760 torr 1 inch = 2.54 cm 1.00 lb = 454 g 1 Å = 1.0! 10-8 cm 1 ev = 1.602! 10-19 J 1.0 cal = 4.184 J (exact) Mass of p = 1.0073 amu = 1.6726! 10-27 kg Mass of p = 1.0087 amu = 1.6749! 10-27 kg Scores 1 /10 2 /8 3 /6 4 /6 5 /8 6 /16 7 /14 8 /10 9 /15 10 /7 Tot. /100 2.1 1.0 1.0 H Li Na 0.9 0.9 K Rb 0.8 Cs 0.8 1.008 6.941 22.99 39.10 85.47 132.9 Fr (223) 1 3 1.5 11 1.2 19 1.0 37 1.0 55 1.0 87 1.0 Pauling Electronegativity Be 9.012 Mg 24.30 Ca 40.08 Sr 87.62 Ba 137.3 Ra (226) 4 12 20 1.3 38 1.2 56 1.1 88 1.1 Sc 44.96 Y 88.91 Ti Zr La Hf 138.9 V Nb Ta Cr Mo W Mn Tc Re AcUnqUnpUnhUns (227) 21 1.4 39 1.3 57 1.3 89 47.88 91.22 178.5 22 1.5 40 1.5 72 1.4 104 50.94 2.1 92.91 180.9 23 1.6 41 1.6 73 1.5 105 52.00 95.94 183.8 24 1.6 42 1.7 74 1.7 106 54.94 (98) 186.2 25 1.7 43 1.8 75 1.9 107 (261) (262) (263) (262) 2.1 Fe 55.85 Ru 101.1 s 190.2 H 1.008 1 26 1.7 44 1.8 76 1.9 Co 58.93 Rh 102.9 Ir 192.2 27 45 77 1.8 1.8 Ni Pd 1.8 58.69 106.4 Pt 195.1 Atomic molar mass (Atomic weight) 28 1.8 46 1.6 78 1.9 Cu 63.55 Ag 107.9 Au 197.0 29 1.6 47 1.6 79 1.7 Zn 65.39 Cd 112.4 Hg 200.6 2.0 1.5 30 1.7 48 1.6 80 1.6 B 10.81 Al 26.98 Ga 69.72 In 114.8 Tl 204.4 5 2.5 13 1.8 31 1.9 49 1.8 81 1.7 C 12.01 Si 28.09 Ge 72.61 Sn 118.7 Pb 207.2 6 3.0 14 2.1 32 2.1 50 1.9 82 1.8 N 14.01 P 30.97 As 74.92 Sb 121.8 Bi 209.0 7 3.5 15 2.5 33 2.4 51 2.1 83 1.9 16.00 S 32.07 Se 78.96 Te 127.6 Po (209) 8 4.0 16 3.0 34 2.8 52 2.5 84 2.1 F 19.00 Cl 35.45 Br 79.90 I 126.9 At (210) 9 17 35 53 85 He 4.003 Ne 20.18 Ar 39.95 Kr 83.80 Xe 131.3 Rn (222) 2 10 18 36 54 86 Lanthanide series Actinide series 1.1 Ce 1.2 140.1 58 1.1 59 1.1 60 1.1 61 1.1 62 1.1 63 1.1 64 1.1 65 1.1 66 1.1 67 1.1 68 1.1 69 1.0 70 1.2 Pr 140.9 Nd 144.2 Pm (145) Sm 150.4 Eu 152.0 Gd 157.2 Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 232.0 90 1.3 231.0 91 1.5 238.0 92 1.3 (237) 158.9 162.5 93 1.3 94 1.3 95 1.3 96 1.3 97 1.3 98 1.3 99 1.3 100 1.3 101 1.3 102 1.5 (244) (243) (247) (247) (251) 164.9 (252) 167.3 (257) 168.9 (258) 173.0 (259) 175.0 (260) 71 103 1
Read questions carefully - some questions may require data in tables on the last two pages of this exam (1) (10 pts) Consider the following equilibrium: SrC 3 (s)! C 2 (g) + Sr(s) Suppose we have closed, 1 liter container at 700 C. There is solid at the bottom of the container consisting of 2 g of Sr and 8 g of SrC 3 and there is a mixture of gases above the solid with the following gas pressures: [P 2 ] = 0.120 atm, [P N2 ] = 0.670 atm, and [P C2 ] = 0.230 atm. Which of the following statements is true: (a) The equilibrium pressure of C 2 can be increased by either adding 2 g of SrC 3 or by removing 1 g of Sr while maintaining the temperature at 700 C. (b) If the container instead contained 8 g of Sr and 2 g of SrC 3, the C 2 pressure would be the same at 700 C. (c) The equilibrium pressure of C 2 can be changed by rapidly pumping out all the gases, reclosing the container, and allowing equilibrium to be reestablished at 700 C. (d) If the 2 and N 2 could be removed at the same temperature (700 C), then the C 2 pressure would increase to 1.12 atm. (e) If the temperature were raised to 900 C the C 2 pressure would be approximately (1173/973) (0.230) = 0.277 atm. (2) (8 pts) Put a number (1 5) by each letter (a-e) to rank the following solutions in order of increasing ph (i.e., from 1 for the lowest ph up to 5 for the highest ph): (a) a 0.1 M NaCN solution (b) a 0.1 M HF solution (c) a 0.1 M pyridine solution (d) a 0.1 M NaN 3 solution (e) a 0.1 M Na(CH 3 C) solution ph rank 2
(3) (6 pts) What is the ph of a 0.1 M Ba(H) 2 solution in otherwise pure water? (4) (6 pts) What is the ph of a 0.5 M aqueous solution of the salt sodium cyanide, NaCN? (5) (8 pts) 50 ml of solution is 0.4 M in NH 3 and 0.2 M in NH 4 Br is combined with 50 ml of a 0.1 M NaH solution. What is the ph of the final solution? 3
(6) (16 pts) Consider the reaction, C(s, graphite) + C 2 (g)! 2 C(g). When 0.075 mol of C 2 and an excess of solid carbon are heated in a 1.0 L container at 1100 K, the equilibrium concentration of C is 7.0 10-2 M. (a) (4 pts) What is the equilibrium concentration of C 2? (b) (4 pts) What is the value of the equilibrium constant K C at 1100 K? (c) (8 pts) H = +172.45 kj for this reaction. Compute an estimate for the value of the thermodynamic equilibrium constant, K P, at 800 K. 4
(7) (14 pts) (a) (7 pts) The most efficient buffer solution for a given ph is one for which the ph will remain closest to that given ph when a either strong acid or base is added to it. Examine the table of pk a and pk b values given at the end of this exam, then choose the conjugate acid/base pair you would use to prepare a buffer that would be best for use near ph = 10.5. (b) (7 pts) Compute the exact ratio of the conjugate acid and base that would yield ph = 10.5. 5
(8) (10 pts) The presence of a nonvolatile solute in a dilute solution always leads to raising the solution s boiling point and a lowering of the solution s freezing point when compared to the solvent as a pure substance. If the solution is sufficiently dilute, the effect depends only on the concentration of solute particles and not on the specific identity of the solute. Give a clear thermodynamic explanation of these facts. You should probably use mathematical expressions to explain this, but it isn t necessary to use much math if your explanations are physically clear. (9) (15 pts) In the presence of ammonia in aqueous solution, Ag + forms a stable two-coordinate complex, [Ag(NH 3 ) 2 ] + : + + Ag (aq) + 2 NH 3! [Ag(NH 3 ) 2 ] K f =1.6! 10 7 Suppose that 0.10 mol of solid silver nitrate is dissolved into one liter of pure water and then 1.20 mol of gaseous ammonia (NH 3 ) is bubbled into the solution (notes: (i) NH 3 is highly soluble in water; (ii) you can ignore the volume change). (a) (7 pts) What percentage of the silver ions remain uncomplexed (i.e., are not present as Ag(NH 3 ) 2 + ) in this solution? (b) (8 pts) What mass of NaCl can be dissolved into this solution before AgCl begins to precipitate? 6
(10) (7 pts) The data plotted below show how the equilibrium constant varies for a reaction involving only gaseous reactants and gaseous products. T (K) 1000 800 600 500 400 20 10 0-10 -20 ln K -30-40 -50-60 -70 2.5!10-3 2.0!10-3 1.5!10-3 1.0!10-3 0.5!10-4 0 1/T (K -1 ) Suppose this data described the reaction between molecules of N 2 to make N 2 4. For the data plotted above, write the reaction so that the forward reaction is appropriate (i.e., Should the forward reaction describe the making or breaking of N 2 4? Briefly explain your reasoning.) N N N 7
Acidity and Basicity Constants Acids pk a Bases pk b chloroacetic acid, CH 2 ClCH 2.85 aniline, C 6 H 5 NH 2 9.37 hydrofluoric acid, HF 3.46 pyridine, C 5 H 5 N 8.75 acetic acid, CH 3 CH 4.75 hydroxylamine, NH 2 H 7.97 hypochlorous acid, HCl 7.53 ammonia, NH 3 4.75 hydrocyanic acid, HCN 9.31 methylamine, CH 3 NH 2 3.44 hydrosulfuric acid, H 2 S 6.89 (pk a1 ) 14.15 (pk a2 ) Solubility Products K sp magnesium hydroxide, Mg(H) 2 1.1 10-11 magnesium fluoride, MgF 2 6.4 10-9 barium carbonate, BaC 3 8.1 10-9 mercury(ii) sulfide, HgS 1.6 10-52 copper(i) bromide, CuBr 4.2 10-8 nickel hydroxide, Ni(H) 2 6.5 10-18 copper(i) iodide, CuI 5.1 10-12 tin(ii) iodide, SnI 2 1.0 10 4 iron(iii) hydroxide, Fe(H) 3 2.0 10-39 silver bromide, AgBr 7.7 10-13 lithium fluoride, LiF 1.7 10-3 silver chloride, AgCl 1.6 10-10 K sp 8