81. A sample of helium gas has been contaminated with argon gas. At 1 atm and 25°C, the density of the mixture is 0.200 g/L. What is the volume percent helium in the sample?
(A) 90.0%
(B) 97.5%
(C) 80.3%
(D) 2.5%
(E) 99%

77. It is desired to determine the concentration of arsenic in a lake sediment sample by means of neutron activation analysis. The nuclide As captures a neutron to form As , which in turn undergoes β decay. The daughter nuclide produces the characteristic γ rays used for the analysis. What is the daughter nuclide? (A) (B) (C) (D) (E)

複選題78. As shown below, compound IV can be prepared from reagents I, II, and III. Which of the following reaction conditions is the most suitable for the preparation of IV? (A) I is slowly added to II, and III is then added to the above mixture. (B) I is slowly added to II, and the resultant mixture is slowly added to III. (C) II is slowly added to I, and III is then added to the above mixture. (D) II is slowly added to I, and the resultant mixture is slowly added to III. (E) I is slowly added to III, and II is then added to the above mixture.

79. The equilibrium constant of a certain reaction was measured at various temperatures to give the plot shown below. What is Δ S o (J/mol∙K ) for the reaction? (R = 8.314 J/mol∙K ) (A) 0.20 (B) 3.0 (C) −50 (D) −8.3 × 103 (E) 33

80. For the reaction N2(g) + 2H2(g) → N2H4(l), if the percent yield for this reaction is 41.0%, what is the actual mass of hydrazine (N2H4) produced when 30.57 g of nitrogen reacts with 4.45 g of hydrogen? (A) 24.00 g (B) 28.60 g (C) 15.00 g (D) 12.00 g (E) 14.60 g

82. Which of the following is NOT a state function? (A) entropy (B) enthalpy (C) internal energy (D) heat (E) temperature

83. A metal ion in a high–spin octahedral complex has four more unpaired electrons than the same ion does in a low–spin octahedral complex. The metal ion could be: (A) V2+ (B) Cu2+ (C) Mn2+ (D) Cr3+ (E) Co2+

84. Which of the following coordination compounds will form a precipitate when treated with an aqueous solution of AgNO3? (A) [Cr(NH3)3Br3] (B) [Mo(Cl)2(NH3)4](NO3) (C) Na3[WCl6] (D) [Pt(NOlinear)2(OAc)2](Br)2 (E) None of the above.

85. Which of the following species is diamagnetic? (A) CN (B) B2 (C) O2 (D) All of the above. (E) None of the above.

86. Which of the lines in the figure below is the best representation of the relationship between the volume of a gas and its pressure, other factors remaining constant? (A) a (B) b (C) c (D) d (E) e

87. Sulfur tetrafluoride adopts a see-saw geometry with two axial F atoms with a F–S–F angle of about 180º and two equatorial F atoms at about 90º from the axial fluorines. Which statement most accurately describes the axial and equatorial S–F bonds? (A) The axial S–F bonds are longer because the two fluorine atoms must share bonding to the same orbital on sulfur. (B) The axial S–F bonds are longer because they experience greater repulsion from the other fluorine atoms in the molecule. (C) The equatorial S–F bonds are longer because the equatorial F–S–F bond angle is the smallest in the molecule. (D) The equatorial S–F bonds are longer because they experience greater repulsion from the lone pair on sulfur. (E) The equatorial S–F bonds are longer because they experience greater repulsion from two axial fluorine atoms.