19. Rubidium–87 decays by beta decay with a half-life of 4.9×10¹⁰ years. How many 87Rb atoms are in a moon rock sample that has a rubidium decay rate of 3500 disintegrations per hour?
(A)9.0×10¹⁶ atoms
(B)4.3×10⁻⁴ atoms
(C) 2.2×10¹⁸ atoms
(D)2.5×10¹⁴ atoms
(E)2.6×10¹⁵ atoms

15. Consider the titration of 100.0 mL of 0.250 M aniline (Kb = 3.8x10-10) with 0.500 M HCl. Calculate the pH of the solution at the stoichiometric point. (A)-0.85(B)8.70(C)2.68 (D)11.62(E)7.00

16. The Kf for the complex ion Ag(NH₃) 2+ is 1.7x107 . The Ksp for AgCl is 1.6x10-10. Calculate the molar solubility of AgCl in 1.0 M NH₃. (A)5.2x10-2 M(B)4.7x10-2 M(C)2.9x10-3 M (D)1.3x10-2 M(E)3.5x10-2 M 

17. The equilibrium constant K for the dissociation reaction of a molecule X2 X2(g) ←→ 2X(g) was measured as a function of temperature (in Kelvin). A graph of ln K versus 1/T for this reaction gives a straight line with a slope of –1.352x10⁴ and an intercept of 15.59 K. The value of ∆S for this dissociation reaction is: (A)1.875 J/K mol(B)259.2 J/K mol(C)32.4 J/K mol (D)64.81 J/K mol(E)129.6 J/K mol

18. An excess of finely divided iron is stirred up with a solution that contains Cu²⁺ ion, and the system is allowed to come to equilibrium. The solid materials are then filtered off and electrodes of solid copper and solid iron are inserted into the remaining solution. What is the value of the ratio [Fe²⁺]/[Cu²⁺] at 25°C? The following standard reduction potentials apply: (A)1(B)1.3x1026(C)2.5x1026 (D)4.4x10-27(E)8.8x10-27

20. What two oxidation states are more frequently observed in the first transition series than in the third? (A)+3 and +7 (B)+2 and +3 (C) +2 and +7 (D)+5 and +6 (E)+2 and +5

21. Give the number of geometrical isomers for the octahedral compound [MA₂B₂C₂], where A, B, and C represent ligands. (A)1(B)2(C)3 (D)5(E)6

22. A particular first order reaction has a rate constant of 1.35×10² s⁻¹ at 25.0 °C. What is the magnitude of k at 75.0 °C if Ea = 55.3 kJ/mol? (A)2.24 × 10⁴(B)3.34 × 10³(C)433 (D)1.05 × 10³(E)2.49 × 10⁶

23. For a particular reaction in a galvanic cell ∆S° is negative. Which of the following statements is true? (A)E⁰will increase with an increase in temperature. (B)E⁰will decrease with an increase in temperature. (C) E⁰will not change when the temperature increases. (D)∆G° > 0 for all temperatures. (E)None of these

24. Consider the reaction: NH₃(g) + HCl(g) → NH₄Cl(s) Given the following table of thermodynamic data at 298 K, the value of equilibrium constant for the reaction at 25 °C is ________. Substance ∆Hf o (kJ/mol)) So (J/K・mol) NH3(g) -46.19 192.5 HCl(g) -92.30 186.69 NH4Cl(s) -314.4 94.6 (A)150(B)9.3×1015(C)8.4×10⁴ (D)1.1×10-16(E)7.2×1012

 25. The molar enthalpy of vaporization of hexane (C₆H₁₄) is 28.9 kJ/mol, and its normal boiling point is 68.73°C. What is the vapor pressure of hexane at 25°C? (A)4.44 torr(B)117 torr(C)241 torr (D)759 torr(E)171 torr