23. How many unpaired electrons are there in an atom of sulfur in its ground state?
(A) 1
(B) 2
(C) 4
(D)0
(E)3

19. A 100-mL sample of water is placed in a coffee cup calorimeter. When 1.0 g of an ionic solid is added, the temperature decreases from 21.5℃ to 20.8℃ as the solid dissolves. Which of the following is true for the dissolving of the solid? (A) AH ＜O (B) (C) (D) (E) none of these

複選題20. For a spontaneous endothermic process, which conditions must hold? cannot be negative； IV △S is positive. (A) all (B) none (C) I and IIll only (D) I, I, and IV only (E) IIl and IV only

21. At constant pressure, the reaction； 2NO2(g) → N2O4(g) is exothermic. The reaction (as written) (A) spontaneous at low temperatures but not at high temperatures (B) spontaneous at high temperatures but not at low temperatures (C) always spontaneous (D) never spontaneous (E) none of these

22. The reaction below occurs in basic solution. In the balanced equation, what is the sum of the coefficients?(A) 27 (B) 19 (C) 23 (D) 12 (E) 15

24. The valence electron configuration of an element is . To which group does X belong? (A) Group 3A (B) Group 4A (C) Group 6A (D) Group 5A (E) Group 7A

25. The_______ states that in a given atom no two electrons can have the same set of four quantum numbers (n, I, mI, ms). (A) Pauli exclusion principle (B) Huygens-Fresnel principle (C) Cauchy's argument principle (D) Heisenberg uncertainty principle(E) Le Chatelier's principle

26. Of the following, which molecule has the smallest bond angle? (A)CO2(B) H2O (C) SCI2 (D) SO3 (E) NF3

27. What type of structure does the XeOF2 molecule have? (A) trigonal planar (B) octahedral (C) pyramidal (D) T-shaped (E) tetrahedral

28. Which of the following statements is false? (A) The carbon-carbon bond inis shorter than the one in CH3CH3 (B) the carbon-carbon bond in is stronger than the one in CH3CH3 (C) C2 is diamagnetic (D) C2 is paramagnetic (E) two of these statements are false.

29. 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 and II only (B)I, II, and IV (C) I and IV (D) II and III (E) I only