19. Which of the following resonance structures is the most stable?
(A)
(B)
(C)
(D)
(E) All of them are the same.

15. A circuit contains a resistance and a charged capacitance. The resistance is 20 ohms, and the capacitance is 5 F. If the circuit is switched off, how long will it take when the current decreases to a half of initial value? (ln2 = 0.69) (A) 34 s (B) 50 s (C) 69 s (D) 100 s (E) 127 s

16. Using the rules of significant figures, calculate the following: 0.102 × 0.0821 × 273/1.01 (A) 2.2635 (B) 2.264 (C) 2.26 (D) 2.3 (E) 2.66351

17. Which of the following best describes an orbital? (A) Space where electrons are likely to be found in an atom. (B) Space which may contain electrons, protons, and/or neutrons. (C) The space in an atom where an electron is most unlikely to be found. (D) Small, walled spheres that contain electrons. (E) A single space within an atom that contains all electrons of that atom.

18. Which of the following is the correct electron configuration for OF− ? (A) (B) (C) (D) (E) None of the above.

20. Which of the following does NOT contain at least one pi bond? (A) H2CO (B) CO2 (C) C2H2 (D) NO (E) All of the above (A-D) contain at least one pi bond.

21. Order the intermolecular forces (dipole-dipole, London dispersion, ionic, and hydrogen-bonding) from weakest to strongest. (A) dipole-dipole, London dispersion, ionic, and hydrogen-bonding (B) London dispersion, dipole-dipole, hydrogen-bonding, and ionic (C) hydrogen-bonding, dipole-dipole, London dispersion, and ionic (D) dipole-dipole, ionic, London dispersion, and hydrogen-bonding (E) London dispersion, ionic, dipole-dipole, and hydrogen-bonding

22. Which separation technique is based on differences in the affinity of the substances to be separated? (A) filtration (B) distillation (C) solvent extraction (D) paper chromatography (E) None of the above.

23. Which of the following gives correct rank of the ionization energies for Cs, Na, O, F, and S? (A) Cs ＜ Na ＜ S ＜ O ＜ F (B) Cs ＜ S ＜ Na ＜ O ＜ F (C) F ＜ O ＜ Na ＜S ＜ Cs (D) F ＜ O ＜ S ＜ Na ＜ Cs (E) Na ＜ S ＜ Cs ＜ F ＜ O

24. Naturally occurring copper exists in two isotopic forms: 63Cu and 65Cu. The atomic mass of copper is 63.55 amu. What is the approximate natural abundance of 63Cu? (A) 63% (B) 90% (C) 80% (D) 85% (E) 72%

25. The correct mathematical expression for finding the molar solubility(s) of Al(OH)3 is . (A) 9 s2 = Ksp (B) 3 s3 = Ksp (C) 27 s4 = Ksp (D) s4 = Ksp (E) 9 s3 = Ksp