19. Which of the following substances does not show increased solubility as the pH of the solution becomes more acidic?
(A) Hg₂Cl₂, K_sp = 1.1 × 10^-18 (Hg₂²⁺ is the cation in solution.)
(B) Ag₃PO₄, K_sp = 1.8 × 10^-18
(C) CaCO₃, K_sp = 8.7 × 10^-9
(D) CdCO₃, K_sp = 5.2 × 10^-12
(E) Sr₃(PO₄)₂, K_sp = 1.0 × 10^-31

15. Suppose the reaction system 2HI(g)⇌H2(g) + I2(g) has already reached equilibrium. Which of the following statements correctly predict the effect of each change on the equilibrium position? (A) Additional H2(g) added to the system will shift to the right to obtain the equilibrium. (B) As an exothermic reaction, the system will shift to the right to attain the equilibrium when increasing the system temperature. (C) When iodine gas is removed, the system will shift to the left to achieve the equilibrium. (D) Addition of argon gas into the reaction system will shift to the right to achieve the equilibrium. (E) When volume of the reaction container is doubled, the change will have no effects on its equilibrium.

16. In a 0.10-M aqueous solution of pyridine (Kь = 1.7 × 10-9), the percentage of pyridine (C5H5N) that forms pyridinium ion, C5H5NH+, is _____%. (A) 0.013 (B) 6.4 (C) 4.2 (D) 1.3 (E) 5.5

17. Consider the titration of 40.0 mL of 0.200 M HCIO4 by 0.100 M KOH. After adding 10.0 mL of the KOH solution, the pH of the resulting solution is _____. (A) 1.15 (B) 7.00 (C) 2.72 (D) 0.85 (E) 4.26

18. Which of the following mixtures would result in buffered solutions when 1.0 L of each of the two solutions are mixed? (A) 0.1 M KOH and 0.1 M CH3NH3Cl (B) 0.1 M KOH and 0.2 M CH3NH3Cl (C) 0.1 M KOH and 0.2 M CH3NH2 (D) 0.2 M KOH and 0.1 M CH3NH2 (E) 0.2 M KOH and 0.1 M CH3NH3CI

20. For the reaction at 800 K: N2(g) + 3F2(g)→2NF3(g), an equilibrium mixture contains the partial pressures of P(nitrogen) = 0.021 atm, P(fluorine) = 0.063 atm, and P(nitrogen trifluoride) = 0.48 atm. The △G° for the reaction at 527°C is thus _____kJ/mol. (A) 17 (B)-50 (C) 50 (D)-17 (E)-71

21. Which one of the following statements is correct? (A) The value of S° for N2O at 0 K is greater than that for He at 10 K. (B) At 1 atm and 25°C, the value of S° for He(g) is greater than that for N2O(g). (C) The sign of S° for the reaction of 2H2S(g) + SO2(g) → 3Srhombic(s) + 2H2O(g) is negative. (D) The sign of S° for the reaction of 2SO3(g) → 2SO2(g) + O2(g) is negative. (E) The sign of S° for the reaction of Fe2O3(s) + 3H2(g) → 2Fe(s) + 3H2O(g) is only positive.

22. When electrolyzing a sodium chloride solution at 50°C and 2.50 atm, the volume of _____ L H2(g) is acquired in addition of the production of 257 L Cl2(g) with both volumes measured in the same time. (A) 257(B) 128.5(C) 85.7(D) 514(E) 771

23. A rock contains 0.688 mg of 206 Pb for every 1.000 mg of 238U present. Assuming that no lead was originally present, which all the 206Pb formed over the years has remained in the rock, and that the number of nuclides in intermediate stages of decay between 238U and 206Pb is negligible, the age of the rock is calculated to be _____ years. (For 238U, t1/2 = 4.5 ×109 years.) (A) 3.8 × 106(B) 3.8 × 107(C) 3.8 × 108(D) 3.8 × 109(E) 3.8 × 1010

24. Which of the following statements is correct? (A) The diamagnetic Ni(CN)42- ion has a tetrahedral d8 structure. (B) The V(en)33+ ion has 0 unpaired electrons. (C) The systematic name for K2(PtCl4) is potassium tetrachloroplatinate(II). (D) The formulas for sodium dicyanobis(oxalate)ferrate(III) is Na4(Fe(CN)2(C2O4)2). (E) The transition metal ion in (Co(NH3)2(NH2CH2CH2NH2)2)l2 has an electron configuration of [Ar]3d6.

25. The average molar mass of one base pair of nucleotides in DNA is approximately 600 g/mol. The spacing between successive base pairs is about 0.34 nm, and a complete turn in the helical structure of DNA occurs about every 3.4 nm. If a DNA molecule has a molar mass of 4.5 × 109 g/mol, there are approximately _____ complete turns existing in the DNA a-helix structure. (A) 5.2 × 1012 (B) 4.6 × 109 (C) 8.0 × 105 (D) 2.6 × 106 (E) 9.4 × 103