chem

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. (B) There is a net repulsive force pushing the atoms apart, so the atoms will move further apart. (C) There is a net attractive force pulling the atoms together, so the atoms will move closer together. (D) It cannot be determined whether the forces between atoms are balanced, attractive, or repulsive, because the diagram shows only the potential energy.

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

44. The forward reaction is thermodynamically favored at which of the following temperatures? (A) All temperatures (B) Low temperatures only (C) High temperatures only (D) No temperature

(B) Low temperatures only

The reaction system represented above is at equilibrium. Which of the following will decrease the amount of CaO(s) in the system? (A) Increasing the volume of the reaction vessel at constant temperature (B) Lowering the temperature of the system (C) Removing some CO2(g) at constant temperature (D) Removing some CaCO3(s) at constant temperature

(B) Lowering the temperature of the system

46. A mixture of NO 2(g) and N2O4(g) is at equilibrium in a rigid reaction vessel. If the temperature of the mixture is decreased, then (A) [NO2] will increase and the mixture will turn a darker brown (B) [N2O4] will increase and the mixture will turn a lighter brown (C) [N2O4] will decrease and the mixture will turn a lighter brown (D) no change will be observed

(B) [N2O4] will increase and the mixture will turn a lighter brown

36. The amount of H2(g) present in a reaction mixture at equilibrium can be maximized by (A) increasing the temperature and increasing the pressure by decreasing the volume (B) increasing the temperature and decreasing the pressure by increasing the volume (C) decreasing the temperature and increasing the pressure by decreasing the volume (D) decreasing the temperature and decreasing the pressure by increasing the volume

(B) increasing the temperature and decreasing the pressure by increasing the volume

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 (B) 2.4 atm (C) 12 atm (D) 24 atm

(A) 1.2 atm

20. For which of the equilibrium systems represented below will the amount of product(s) at equilibrium increase if the volume of the reaction vessel is increased at a constant temperature? (A) PCl5(g) ← PCl 3(g) + Cl2 → (g) (B) 2 NO(g) + O2(g) ← 2 NO2 → (g) (C) N2(g) + O2(g) ←→ 2 NO(g) (D) 2 CO(g) ← C(s) + CO2 → (g)

(A) PCl5(g) ← PCl 3(g) + Cl2 → (g)

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).) (A) The combustion of 1.0 g of cashew releases less energy than the combustion of 1.0 g of marshmallow. (B) The combustion of 1.0 g of cashew releases the same amount of energy as the combustion of 1.0 g of marshmallow. (C) The combustion of 1.0 g of cashew releases more energy than the combustion of 1.0 g of marshmallow. (D) No comparison can be made because the two systems started with different masses of food, different masses of water, and different initial temperatures.

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

17. Compared to the equilibrium vapor pressure of CH3OH(l) at 300 K, the equilibrium vapor pressure of C2H5OH(l) at 300 K is (A) the same, because both compounds have hydrogen bonding among their molecules (B) higher, because London dispersion forces among C2H5OH molecules are greater than those among CH3OH molecules (C) lower, because London dispersion forces among C2H5OH molecules are greater than those among CH3OH molecules (D) lower, because of the larger number of hydrogen bonds among C2H5OH molecules

(C) lower, because London dispersion forces among C2H5OH molecules are greater than those among CH3OH molecules

43. Which of the following experimental techniques will allow the most accurate determination of the concentration of NO2(g) at equilibrium? (A) Paper chromatography (B) Gravimetric analysis (C) Titration (D) Spectrophotometry

(D) Spectrophotometry

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? (A) X has a larger molar mass than Y does. (B) Y has a larger molar mass than X does. (C) X is more polar than Y. (D) Y is more polar than X.

(D) Y is more polar than X.

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? (A) The pressure will increase because the product molecules have a greater mass than either of the reactant molecules. ( B) The pressure will decrease because there are fewer molecules of product than of reactants. (C) The pressure will decrease because the product molecules have a lower average speed than the reactant molecules. (D) The pressure will not change because the total mass of the product molecules is the same as the total mass of the reactant molecules

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

7. A 20. mL sample of 0.50 M HC2H3O2(aq) is titrated with 0.50 M NaOH(aq). Which of the following best represents the species that react and the species produced in the reaction? (A) H+(aq) + OH−(aq) → H2O(l) (B) H+(aq) + C2H3O2 −(aq) + Na+(aq) + OH−(aq) → H2O(l) + NaC2H3O2(aq) (C) HC2H3O2(aq) + OH−(aq) → C2H3O2 −(aq) + H2O(l) (D) HC2H3O2(aq) + NaOH(aq) → H2O(l) + Na+(aq) + C2H3O2 −(aq)

C NaOH dissociates completely because it is a strong base while HC2H3O2 does not because it is a weak acid