19. Consider the following molecules.

Ⅰ.BF₃ 

Ⅱ.CHBr₃ (C is the central atom.) 

III. Br₂

IV. XeCl₂ 

V. CO

VI. SF₄ Select the molecule(s) that fit the given statement. These molecules have a zero net dipole moment.
(A) III, V
(B) III, IV
(C) I, III, IV
(D) III, IV, V
(E) I, IIIL, IV, VI Il.

15. The wavelength of light associated with the n = 2 to n = 1 electron transition in the hydrogen spectrum is 1.216 x 10-7 m. By what coefficient should this wavelength be multiplied to obtain the wavelength associated with the same electron transition in the Li2+ ion? (A) 1/9 (B) 1/7 (C) 1/4 (D) 1/3 (E) 1

16. A strip of copper is placed in a 1 M solution of copper nitrate, and a strip of silver is placed in a 1 M solution of silver nitrate. The two metal strips are connected to a voltmeter by wires, and a salt bridge connects the solutions. The following standard reduction potentials apply: When the voltmeter is removed and the two electrodes are connected by a wire, which of the following does not take place? (A) Electrons flow in the external circuit from the copper electrode to the silver electrode. (B) The silver electrode increases in mass as the cell operates. (C) There is a net general movement of silver ions through the salt bridge to the copper half-cell. (D) Negative ions pass through the salt bridge from the silver half-cell to the copper half-cell. (E) Some positive copper ions pass through the salt bridge from the copper half-cell to the silver half-cell.

17. The standard potential for the reaction is 0.51 V. What is the equilibrium constant K for this reaction at 250C? (A) 6.3 x 1025 (B) 1.6 x 10-26 (C) 17 (D) -17 (E) 0.015

18. Which of the following is not a valid resonance structure for N3-?(A)(B)(C)(D)(E) All are valid.

20. Which of the following is diamagnetic? (A) H2+ (B) C2+ (C) N2+ (D) N2 (E) F2+

21. Below is a phase diagram for compound X. You wish to purify a sample of X that was collected at P = 1.0 atm and T = 100 by subliming it. In order to sublime the sample, you should (A) increase P to 1.5 atm and then increase T'to 300 K. (B) increase T to 300 K, keeping P = 1.0 atm. (C) lower P to 0.5 atm and then increase T to 200 K. (D) increase T to 300 K and then lower P to 0.5 atm. (E) abandon the attempt to sublime X.

22. Alkali halides commonly have either the sodium chloride structure or the cesium chloride structure. The molar mass of CsCl is 2.88 times the molar mass of NaCl, and the edge length of the unit cell for NaCl is 1.37 times the edge length of the CsCl unit cell. Determine the ratio of the density of CsCl to the density of NaCl. (A) 0.541 (B) 0.984 (C) 1.02 (D) 1.85 (E) 2.10

23. Liquid A has vapor pressure x. Liquid B has vapor pressure y, and x>y.What is the mole fraction of A in the liquid mixture if the vapor above the solution is 30% A? (A)0.3y/(0.7x+0.3y) (B) 0.7y/(0.3x+0.7y) (C) 0.3x/(0.3.xc+0.7y) (D)0.7x/(0.7x+0.3y) (E) none of these

24. The oxidation states of P in KPF6 is: (A)+1 (B)+3 (C)+5 (D)-3 (E) -5

25. Which of the following compounds is the strongest acid? (A) Acetic acid (B) Chloric acid (C) Hypochlorous acid (D) Boric acid (E) Chlorous acid