5. Which of the following statements is (are) true?
I. An excited atom can return to its ground state by absorbing electromagnetic radiation.
II. The energy of an atom is increased when electromagnetic radiation is emitted from it.
III. The energy of electromagnetic radiation increases as its frequency increases.
IV. An electron in the n = 4 state in the hydrogen atom can go to the n = 2 state by emitting electromagnetic
radiation at the appropriate frequency.
V. The frequency and wavelength of electromagnetic radiation are inversely proportional to each other.
(A) II, III, IV；
(B) III, V；
(C) I, II, III；
(D) III, IV, V；
(E) I, II, IV

1. A 0.4987 g sample of a compound known to contain only carbon, hydrogen, and oxygen was burned in oxygen to yield 0.9267 g of CO2 and 0.1897 g of H2O. What is the empirical formula of the compound? (A) CHO； (B) C2H2O； (C) C3H3O2； (D) C6H3O2； (E) C3H6O2

2. A 0.307 g sample of an unknown triprotic acid is titrated to the third equivalence point using 35.2 mL of 0.106 M NaOH. Calculate the molar mass of the acid. (Na: 22.99 g/mol) (A) 247 g/mol； (B) 171 g/mol； (C) 165 g/mol； (D) 151 g/mol； (E) 82.7 g/mol

3. The partial pressures of CH4, N2, and O2 in a sample of gas were found to be 135 mm-Hg, 508 mm-Hg, and 571 mm-Hg, respectively. Calculate the mole fraction of nitrogen. (A) 20.4； (B) 0.470； (C) 0.418； (D) 0.751； (E) 0.359

4. A 32.5 g piece of aluminum (which has a molar heat capacity of 24.03 J/°C·mol) is heated to 82.4°C and dropped into a calorimeter containing water (specific heat capacity of water is 4.18 J/g°C) initially at 22.3°C. The final temperature of the water is 24.2°C. Ignoring significant figures, calculate the mass of water in the calorimeter. (Al: 26.98 g/mol) (A) 212 g； (B) 5.72 kg； (C) 6.42 g； (D) 1.68 kg； (E) none of these

6. Consider the following reaction: A2 + B2  2AB ∆H = -365 kJ The bond energy for A2 is half the amount of AB. The bond energy of B2 = 447 kJ/mol. What is the bond energy of A2? (A) 812 kJ/mol； (B) 630 kJ/mol； (C) 271 kJ/mol； (D) –183 kJ/mol； (E) none of these

7. Consider the molecular orbital energy level diagrams for O2 and NO. Which of the following is true? I. Both molecules are paramagnetic. II. The bond strength of O2 is greater than the bond strength of NO. III. NO is an example of a homonuclear diatomic molecule. IV. The ionization energy of NO is smaller than the ionization energy of NO+. (A) I only； (B) I and II； (C) I and IV； (D) II and III； (E) I, II, and IV

8 If equal, rigid spheres are arranged in a simple cubic lattice in the usual way (i.e., in such a way that they touch each other), what fraction of the corresponding solid will be empty space? (A) 0.52； (B) 0.32； (C) 0.68； (D) 0.48； (E) none of these

9 At a given temperature, you have a mixture of benzene (vapor pressure of pure benzene = 745 torr) and toluene (vapor pressure of pure toluene = 290 torr). The mole fraction of benzene in the vapor above the solution is 0.590. Assuming ideal behavior, calculate the mole fraction of toluene in the solution. (A) 0.213； (B) 0.778； (C) 0.641； (D) 0.359； (E) 0.590

10. A 1 L container originally holds 0.4 mol of N2, 0.1 mol of O2, and 0.08 mole of NO. If the volume of the container holding the equilibrium mixture of N2, O2, and NO is decreased to 0.5 L without changing the quantities of the gases present, how will their concentrations change? (A) The concentration of NO will increase； the concentrations of N2 and O2 will decrease. (B) The concentrations of N2 and O2 will increase； and the concentration of NO will decrease. (C) The concentrations of N2, O2, and NO will increase. (D) The concentrations of N2, O2, and NO will decrease. (E) There will be no change in the concentrations of N2, O2, and NO.

11. In titrating 0.20 M hydrochloric acid, HCl, with 0.20 M NaOH at 25°C, the solution at the equivalence point is (Cl: 35.45 g/mol): (A) 0.20 M NaCl； (B) very acidic； (C) slightly acidic； (D) 0.10 M HCl and 0.20 M NaOH； (E) 0.10 M NaCl