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； 10^2.75 = 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%

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

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

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

87. Biphenyl (C12H10) is an organic molecule formed by connecting two benzene rings via single bond. Which of the following statements about biphenyl molecules is incorrect? (A) Benzene is an aromatic compound. (B) The molecular structure has twelve carbon atoms in a coplanar fashion. (C) The molecular structure has five hydrogen atoms in a coplanar fashion. (D) The molecule has a total of six double bonds. (E) The molecular hybridization in this molecule are the same as graphite.