AP Chem 2015

26. The O2(g) produced from the decomposition of the 1.0 L sample of H2O2(aq) is collected in a previously evacuated 10.0 L flask at 300. K. What is the approximate pressure in the flask after 400. s? (For estimation purposes, assume that 1.0 mole of gas in 1.0 L exerts a pressure of 24 atm at 300. K.)

(A) 1.2 atm

39. If equal masses of the following compounds undergo complete combustion, which will yield the greatest mass of CO2 ?

(A) Benzene, C6H6

13. The exothermic dissolution of Mg(OH)2(s) in water is represented by the equation above. The Ksp of Mg(OH)2 is 1.8 ¥ 10-11. Which of the following changes will increase the solubility of Mg(OH)2 in an aqueous solution?

(A) Decreasing the pH

1. A 0.5 mol sample of He(g) and a 0.5 mol sample of Ne(g) are placed separately in two 10.0 L rigid containers at 25°C. Each container has a pinhole opening. Which of the gases, He(g) or Ne(g), will escape faster through the pinhole and why?

(A) He(g) will escape faster because the He(g) atoms are moving at a higher average speed than the Ne(g) atoms.

14. The heating curve for a sample of pure ethanol is provided above. The temperature was recorded as a 50.0 g sample of solid ethanol was heated at a constant rate. Which of the following explains why the slope of segment T is greater than the slope of segment R ?

(A) The specific heat capacity of the gaseous ethanol is less than the specific heat capacity of liquid ethanol.

36. When Reaction 3 occurs, does the hybridization of the carbon atoms change?

(A) Yes; it changes from sp to sp2 .

20. Which of the following is the balanced net-ionic equation for the reaction between Li(s) and water?

(B) 2 Li(s) + 2 H2O(l) → 2 Li+(aq) + 2 OH−(aq) + H2(g)

19. On the basis of the pH curve, the pKa value of the acid is closest to

(B) 5

37. The acid-dissociation constants of HC3H5O3(aq) and CH3NH3+(aq) are given in the table above. Which of the following mixtures is a buffer with a pH of approximately 3 ?

(B) A mixture of 100. mL of 0.1 M HC3H5O3 and 50. mL of 0.1 M NaOH

30. Which of the following accounts for the observation that the pH of pure water at 37°C is6.8?

(B) At 37∞C the autoionization constant for water, Kw , is larger than it is at 25∞C.

12. The first five ionization energies of a second-period element are listed in the table above. Which of the following correctly identifies the element and best explains the data in the table?

(B) B, because it has three valence electrons

44. The proposed mechanism can be written in a more general form, as shown above. Species other than Cl can also decompose O3 through the same mechanism. Which of the following chemical species is most likely to decompose O3 in the upper atmosphere through the above mechanism?

(B) Br

11. The molecular formula and molar mass of two straight-chain hydrocarbons are listed in the table above. Based on the information in the table, which compound has the higher boiling point, and why is that compound's boiling point higher?

(B) C4H10 , because it has more electrons, resulting in greater polarizability and stronger dispersion forces

32. A sample of a compound that contains only the elements C, H, and N is completely burned in O2 to produce 44.0 g of CO2 , 45.0 g of H2O, and some NO2 . A possible empirical formula of the compound is

(B) CH5N

28. The reaction is thermodynamically favorable. The signs of ΔG° and ΔS° for the reaction are which of the following?

(B) G is neg, S is pos

38. The Lewis electron-dot diagrams of the HClO3 molecule and the HClO2 molecule are shown above at the left and right, respectively. Which of the following statements identifies the stronger acid and correctly identifies a factor that contributes to its being the stronger acid?

(B) HClO3(aq) is the stronger acid because the additional electronegative oxygen atom on the chlorine atom stabilizes the conjugate base.

40. The reaction system represented above is at equilibrium. Which of the following will decrease the amount of CaO(s) in the system?

(B) Lowering the temperature of the system

42. Which of the following rate laws for the overall reaction corresponds to the proposed mechanism?

(B) Rate = k[Cl][O3]

22. Acid-dissociation constants of two acids are listed in the table above. A 20. mL sample of a 0.10 M solution of each acid is titrated to the equivalence point with 20. mL of 0.10 M NaOH. Which of the following is a true statement about the pH of the solutions at the equivalence point?

(B) Solution 1 has a higher pH at the equivalence point because CH3CO2H has the stronger conjugate base.

18. What will be the effect on the amount of gas produced if the experiment is repeated using 0.35 g of K(s) instead of 0.35 g of Li(s) ?

(B) Some gas will be produced but less than the amount of gas produced with Li(s).

8. At 127°C, the entire sample of I2 is observed to have vaporized. How does the mass of vessel 5 at 127°C compare to its mass at 27°C?

(B) The mass is the same, since the number of each type of atom in the vessel is constant.

24. The reaction between NO(g) and O2(g) to produce NO2(g) in a rigid reaction vessel is represented in the diagram above. The pressure inside the container is recorded using a pressure gauge. Which of the following statements correctly predicts the change in pressure as the reaction goes to completion at constant temperature, and provides the correct explanation?

(B) The pressure will decrease because there are fewer molecules of product than of reactants.

27. Which of the following statements is a correct interpretation of the data regarding how the order of the reaction can be determined?

(B) The reaction is first order if the plot of ln [H2O2] versus time is a straight line.

The temperature of the reaction mixture is held constant. In which direction will the reaction proceed?

(B) The reaction will form more reactant.

45. A solution of a salt of a weak acid HY is added to a solution of another weak acid HX. Based on the information given above, which of the following species is the strongest base?

(B) Y−(aq)

35. Reaction 2 occurs when an excess of 6 M HCl(aq) solution is added to 100. mL of NaOCl(aq) of unknown concentration. If the reaction goes to completion and 0.010 mol of Cl2(g) is produced, then what was the molarity of the NaOCl(aq) solution?

(C) 0.10 M

7. At 127°C, the pressure in vessel 1 is found to be higher than that in vessel 2. Which of the following reactions best accounts for the observation?

(C) 2 NaHCO3(s) Æ Na2CO3(s) + CO2(g) + H2O(g)

6. A hot iron ball is dropped into a 200.g sample of water initially at 50.° C. If 8.4 kJ of heat is transferred from the ball to the water, what is the final temperature of the water? (The specific heat of water is 4.2 J/(g·°C).)

(C) 60.°C

5. Which of the following is the strongest type of interaction that occurs between the atoms within the circled areas of the two molecules represented above?

(C) Hydrogen bond

2. The lattice energy of a salt is related to the energy required to separate the ions. For which of the following pairs of ions is the energy that is required to separate the ions largest? (Assume that the distance between the ions in each pair is equal to the sum of the ionic radii.)

(C) Mg2+(g) and O2−(g)

25. Which of the following identifies the element(s) being oxidized and reduced in the reaction?

(C) Oxygen is both oxidized and reduced.

41. To maximize the yield in a certain manufacturing process, a solution of a weak monoprotic acid that has a concentration between 0.20 M and 0.30 M is required. Four 100. mL samples of the acid at different concentrations are each titrated with a 0.20 M NaOH solution. The volume of NaOH needed to reach the end point for each sample is given in the table above. Which solution is the most suitable to maximize the yield?

(C) Solution C

23. A 1.0 g sample of a cashew was burned in a calorimeter containing 1000. g of water, and the temperature of the water changed from 20.0°C to 25.0°C. In another experiment, a 3.0 g sample of a marshmallow was burned in a calorimeter containing 2000. g of water, and the temperature of the water changed from 25.0°C to 30.0°C. Based on the data, which of the following can be concluded about the energy content for 1.0 g of each of the two substances? (The specific heat of water is 4.2 J/(g⋅°C).)

(C) The combustion of 1.0 g of cashew releases more energy than the combustion of 1.0 g of marshmallow.

17. Which of the following changes will most likely increase the rate of reaction between Li(s) and water?

(C) Using a 0.35 g sample of Li(s) cut into small pieces

46. A 1.0 mol sample of X(g) and a 1.0 mol sample of Q(g) are introduced into an evacuated, rigid 10.0 L container and allowed to reach equilibrium at 50°C according to the equation above. At equilibrium, which of the following is true about the concentrations of the gases?

(C) [R] = [Z] > [Q]

31. To gravimetrically analyze the silver content of a piece of jewelry made from an alloy of Ag and Cu, a student dissolves a small preweighed sample in HNO3(aq). Ag+(aq) and Cu2+(aq) ions form in the solution. Which of the following should be the next step in the analytical process?

(D) Adding a solution containing an anion that forms an insoluble salt with only one of the metal ions

10. An acetate buffer solution is prepared by combining 50. mL of 0.20 M acetic acid, HC2H3O2(aq), and 50. mL of 0.20 M sodium acetate, NaC2H3O2(aq). A 5.0 mL sample of 0.10 M NaOH(aq) is added to the buffer solution. Which of the following is a correct pairing of the acetate species present in greater concentration and of the pH of the solution after the NaOH(aq) is added? (The pKa of acetic acid is 4.7.)

(D) C H O - > 4.7

34. Ca(OH)2 (s) precipitates when a 1.0 g sample of CaC2 (s) is added to 1.0 L of distilled water at room temperature. If a 0.064 g sample of CaC2(s) (molarmass64g/mol)isusedinstead and all of it reacts, which of the following will occur and why? (The value of Ksp for Ca(OH)2 is 8.0 108.)

(D) Ca(OH)2 will not precipitate because Q < Ksp .

The dissociation of the weak acid HF in water is represented by the equation above. Adding a 1.0 mL sample of which of the following would increase the percent ionization of HF(aq) in 10mLofasolutionof1.0M HF?

(D) Distilled water

15. A solution is prepared by mixing 50 mL of 1 M NaH2PO4 with 50 mL of 1 M Na2HPO4. On the basis of the information above, which of the following species is present in the solution at the lowest concentration?

(D) PO43−

3. The mass spectrum of element X is presented in the diagram above. Based on the spectrum, which of the following can be concluded about element X?

(D) The atomic mass of X is between 90 and 92.

21. Benzene, C6H6 , has the structure shown above. Considering the observation that benzene is only sparingly soluble in water, which of the following best describes the intermolecular forces of attraction between water and benzene?

(D) There are dipole-induced dipole and London dispersion interactions between water and benzene.

49. In a paper chromatography experiment, a sample of a pigment is separated into two components, X and Y, as shown in the figure above. The surface of the paper is moderately polar. What can be concluded about X and Y based on the experimental results?

(D) Y is more polar than X.

48. In trial 2 which of the reactants would be consumed more rapidly, and why?

(D) Y, because the rate of disappearance will be double that of X.

4. Which of the following diagrams best depicts an alloy of Ni and B ?

A

9. The gas particles in vessel 3 at 27°C are represented in the diagram above. The lengths of the arrows represent the speeds of the particles. Which of the following diagrams best represents the particles when vessel 3 is heated to 127°C?

A

47. The potential energy of a system of two atoms as a function of their internuclear distance is shown in the diagram above. Which of the following is true regarding the forces between the atoms when their internuclear distance is x ?

A) The attractive and repulsive forces are balanced, so the atoms will maintain an average internuclear distance x.

29. Assume that the bond enthalpies of the oxygen- hydrogen bonds in H2O are not significantly different from those in H2O2 . Based on the value of ΔH° of the reaction, which of the following could be the bond enthalpies (in kJ/mol) for the bonds broken and formed in the reaction?

B

43. Which of the following reaction energy profiles best corresponds to the proposed mechanism?

B

50. The diagram above represents the absorption spectrum for a pure molecular substance. Which of the following correctly indicates the type of transition observed for the substance in each of the regions of the absorption spectrum?

D