80. A weak acid HA is dissolved in enough water to prepare an acidic solution. The pH value and osmotic pressure of resulting solution at 27o C are 6 and 0.246 atm, respectively. What is the Ka of HA? (R = 0.082 L· atm/K·mol； assuming Van’t Hoff factor = 1)
(A) 1.00 × 10^–10
(B) 2.30 × 10^–10
(C) 1.10 × 10^–9
(D) 6.10 × 10^–9
(E) 5.80 × 10–8

76. An unknown compound is cooled at 1 atm, and it freezes at 50.0 K to form Solid I. At a lower temperature, Solid I rearranges to Solid II, which has a different crystal structure. Thermal measurements show that ΔH an ΔS for the I → II phase transition are –700.0 J/mol and –20.0 J/K mol, respectively. What is the temperature of Solids I and II in equilibrium? (A) 13.6 K (B) 19.8 K (C) 35.0 K (D) 59.8 K (E) 98.2 K

77. What are the signs (+, –, or 0) of ΔH, ΔS, and ΔG for the spontaneous vaporization of a liquid to a vapor? (A) (ΔH, ΔS, ΔG) = (+, +, +) (B) (ΔH, ΔS, ΔG) = (+, –, +) (C) (ΔH, ΔS, ΔG) = (0, 0, 0) (D) (ΔH, ΔS, ΔG) = (+, +, –) (E) (ΔH, ΔS, ΔG) = (–, – , –)

78. The reduction potentials for Au3+ and Cr3+ are as follows: (A) –7.37 × 102 kJ (B) –5.79 ×102 kJ (C) –1.6 × 102 kJ (D) 7.37 × 102 kJ (E) 10 × 102 kJ

79. How many different structures do SeCl4, CBr4, KrF4, CH4, and TeF4 have? (A) 1 (B) 2 (C) 3 (D) 4 (E) 5

81. The pH of a bottle of vinegar is 2.75 at 25℃. What is the mass percentage concentration (w/w) of acetic acid in this vinegar, assuming no other acid is presented? (Ka of acetic acid: 1.8x10-5； pKa of acetic acid: 4.74； 10-2.75 = 0.0018； 102.75 = 562.3； assuming the density of vinegar = 1 g/cm3 ； CH3COOH=60 g/mol) (A) 5.0% (B) 4.0% (C) 3.0% (D) 2.0% (E) 1.0%

82. Which of the following complexes can exhibit optical isomerism? (en = H2N–CH2–CH2–NH2) (A) cis–Co(NH3)4Cl2 (B) trans–Co(en)2Br2 (C) cis–Co(en)2Cl2 (D) Co(NH3)3Cl3 (E) none of these

83. A compound has a formula of [Pt(PPh3)2(NCS)2]. How many isomers does this compound have? (A) 2 (B) 3 (C) 4 (D) 5 (E) 6

84. Aluminum (Al) metal crystallizes in a face-centered cubic structure. The relationship between the radius r of an Al atom and the length of an edge E of the unit cell is________. (A) r = 0.354E (B) r = 0.433E (C) r = 0.5E (D) r = 2E (E) r = 4E

85. Which of the following statements correctly describes the signs of q (heat) and w (work) for the following exothermic process at P = 1 atm and T = 370 K? H2O(g) ―→ H2O(l) (A) The q and w are both negative. (B) The q is positive, but the w is negative. (C) The q and w are both positive. (D) The q is negative, but the w is positive. (E) The q and w are both zero.

86. Carminic acid, a naturally occurring red pigment extracted from the cochineal insect, contains 53.66% C, 4.09% H and 42.25% O by mass. A titration required 10.00 mL of 0.5 M NaOH to neutralize 2.46 g carminic acid. Assuming there is only one acidic hydrogen per molecule, what is the empirical formula and molecular formula of carminic acid? (Na = 23.0 g/mole) (A) empirical formula: C22H20O13； molecular formula: C44H40O26 (B) empirical formula: C22H20O13； molecular formula: C22H20O13 (C) empirical formula: C44H40O26； molecular formula: C22H20O13 (D) empirical formula: C26H20O10； molecular formula: C26H20O10 (E) empirical formula: C26H20O10； molecular formula: C52H40O20