2018 AP Multiple choice questions

When 70g of Li3N (s) (molar mass 35g/mol) reacts with excess H2 (g), 8.0g of LiH (s) is produced. The percent yield is closest to

25%

Which of the following particulate representations shows a process during which the entropy of the system decreases?

4 (mickey mouse ear) pairs ----> all molecules touching

Which of the following particle diagrams best represents the products when four molecules of H2O2 (l) decompose into water and oxygen gas at room temperature?

4 pairs of 2 hydrogen/1 oxygen, 2 pairs of hydrogen

The forward reaction is thermodynamically favored at which of the following temperatures?

Low temperatures only

A mixture of CO(g) and H2(g) is pumped into a previously evacuated 2.0L reaction vessel. The total pressure of the system is 1.2at, at equilibrium. What will be the total pressure of the system if the volume of the reaction vessel is reduced to 1.0L at constant temperature?

Less than 1.2atm

The temperature of the CH3OH (l) is increased from 300K to 400K to vaporize all the liquid, which increases the pressure in the vessel to 0.30atm. The experiment is repeated under identical conditions but this time using half the mass of CH3OH that was used originally. What will be the pressure in the vessel at 400K?

0.15atm

The value of Kp for the evaporation of CH3OH (l) at 300K is closest to

0.2

If 3.80g of Cu3(PO4)2 was recovered from step 1, what was the approximate {Cu2+} in the original solution? The molar mass of Cu3(PO4)2 is 381g/mol

0.30M

The pH of a solution made by combining 150.mL of 0.10M KOH (aq) with 50.0 mL of 0.20 M HBr is closest to which of the following?

12

A 0.0300 mol sample of NO2 (g) is placed in a rigid 1.00 L reaction vessel and allowed to reach equilibrium at a certain temperature. What is the value of Kc at this temperature if 0.00500 mol of N2O4 (g) is present at equilibrium?

12.5

Based on the values of ^G for the three reactions represented above, what is the value of ^G for the reaction represented below?

-1320kJ/molrxn

In an experiment a student mixes a 50.0mL sample of 0.100M AgNO3 with a 50.0mL sample of 0.100M NaCl at 20.0 C in a coffee cup calorimeter. Which of the following is the enthalpy change of the precipitation reaction represented above if the final temperature of the mixture is 21.0C

-84.0kJ/molrxn

Which of the following best describes the change that takes place immediately after the CH3OH (l) is introduced into the previously evacuated vessel?

A physical change takes place because intermolecular attractions are overcome

A student performs an acid-base titration and plots the experimental results in the graph above. Which of the following statements best explains the experimental findings?

A weak acid was titrated with a strong base, as evidenced by the equivalence point at pH>7

NH3 (g) was synthesized at 200C in the presence of a powdered Os(s) catalyst, leading to the equilibrium system represented above. Which of the following changes would result in more NH3 in the mixture after equilibrium is reestablished?

Adding some N2 (g)

Dissolution of the slightly soluble salt, CaF2, is shown above. Which of the following changes will decrease [Ca2+] in a saturated solution of CaF2 and why?

Adding some NaF because the reaction will proceed toward the reactants

Which of the following best help explain why an increase in temperature increases the rate of a chemical reaction?

At higher temperatures, high energy collisions happen more frequently

Which of the following Lewis electron-dot diagrams represents the molecule that is the most polar?

Br-----F

When a small amount of 12M HNO3 is added to a buffer solution made by mixing CH3NH2 and CH3NH3Cl, the pH of the buffer solution changes from 10.64 to 10.62. Which of the following equations represents the reaction that accounts for the fact that the pH does not change significantly when the HNO3 is added?

CH3NH2(aq) + H+ ----> CH3NH3+ (aq)

Equal volumes of solutions in two different vessels are represented above. If the solution represented in vessel 1 is KCL (aq), then the solution represented in vessel 2 could be an aqueous solution of

CaCl2 with twice the molarity of the solution in vessel 1

The standard reduction potentials for the half reactions related to the galvanic cell represented above are listed in the table below

E cell = +0.20V

Which of the following explains the effect on the equilibrium constant, Kc, when the temperature of the reaction system is increased to 650K?

Kc will decrease because the reaction is endothermic

The Maxwell-Boltzmann distributions of molecular speeds in samples of two different gases at the same temperature shown to the left. Which gas has the greater molar mass?

Gas A

Which of the following happens to H atoms in the forward reaction?

H atoms are both oxidized and reduced

A 20mL sample of 0.50 M HC2H3O2 (aq) is titrated with 0.50M NaOH. Which of the following best represents the species that react and the species produced in the reaction?

HC2H3O2(aq) + OH- ---> H2O (l) + C2H3O2- (aq)

Equimolar samples of each of three gases, H2, I2, and HI are introduced into a 3.0L container that is heated to 600C. According to the information above, which of the following will occur as the system approaches equilibrium?

More HI will be produced because Q<Kc

Three substances were studied in the laboratory and the data in the table above were collected. Based on the data, which of the following shows the type of bonding in each substance?

Network Covalent (X), Metallic (Y), Ionic (Z)

The reaction between NO (g) and NO3 (g) is represented by the equation above. Which of the following orientations of collision between NO (g) and NO3 (g) is most likely to be effective?

O in top left corner on N ----> colliding into O

The rate of the reaction represented above increases significantly in the presence of P (s). Which of the following best explains this observation?

One of the reactants binds on the surface of P(s), which introduces an alternative reaction pathway with a lower activation energy

Which of the following procedures will allow the student to determine the rate constant, k , for the reaction?

Plot ln[X] versus time and determine the magnitude of the slope

A balloon filled with 0.25 moles He (g) at 273K and 1.0 atm is allowed to rise through the atmosphere. Which of the following explains what happens to the volume of the balloon as it rises from ground level to an altitude where the air temperature is 220K and the air pressure is 0.10atm?

The volume will increase because the decrease in air pressure will have a greater effect than the decrease in temperature

The ionization energies of an unknown element, X, are listed in the table to the left. Which of the following is the most likely empirical formula of a compound formed from element X and phosphorus, P?

X3P2

Compared to the equilibrium vapor pressure of CH3OH at 300K, the equilibrium vapor pressure of C2H5OH at 300 K is

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

The equation above represents an elementary step in a chemical reaction. Which of the following is the correct expression for the rate law of the elementary step?

r = k (NOBr)2

HCHO2 (aq), a weak acid (Ka=2 * 10^-4), dissociates in water according to the equation above. Which of the following provides the best estimate of the pH of 0.5M HCHO2 and identifies the species at the highest concentration (excluding H2O) in the solution?

2

On the basis of the information above, what is the appropriate percent ionization of HNO2 in a 1.0M HNO2 solution?

2.0%

In which of the following liquids do the intermolecular forces include dipole-dipole forces?

CH2F2 (l)

Which of the following most likely describes the solid represented in the diagram to the left?

It is brittle, water soluble electrolyte that is a poor thermal and electrical conductor as a solid.

What can be inferred about S for the reaction at 600K?

It must be positive since G is negative and H is positive

Which of the following shows the relationship between K1 and K2 in the reactions represented above?

K2 = 1/(K1)^2

For which of the equilibrium systems represented below will the amount of products at equilibrium increase if the volume of the reaction vessel is increased at a constant temperature?

PCl5 <----> PCl3 + Cl2

The values of dissociation constants for H3PO4 are given in the table to the left. Given that the solution in the beaker at the end of step 2 had a pH of approximately 1, which of the following chemical species had the lowest concentration among the products of step 2?

PO4 ^3-

Equimolar samples of Pb(OH)2, PbI2, and PbF2 are placed in three separate beakers, each containing 250mL of water at 25C. After the solutions are stirred, solid remains in the bottom of each beaker. Based on the Ksp values for the compounds listed in the table below, a solution of which of the compounds will have the lowest [Pb2+] ?

Pb(OH)2 (s)

Based on the information in the table to the left, which of the compounds has the highest boiling point, and why?

Propanoic acid, because it can form intermolecular hydrogen bonds

The step-wise dissociation of selenous acid, H2SeO3 (aq), is represented by the equations above. Which of the following best helps explain why the value of Ka2 is so much smaller than the value of Ka1?

Removing the first H+ from H2SeO3 requires less energy than removing the second H+. because the second H+ is removed from a negatively charged species.

Based on coulomb's law and the information in the table above, which of the following anions is most likely to have the strongest interaction with nearby water molecule in an aqueous solution?

S2-

The masses of carbon and hydrogen in samples of four pure hydrocarbons are given to the left. The hydrocarbon in which sample has the same empirical formula as propene, C3H6?

Sample B (72g carbon, 12g hydrogen)

The mass of the Cu (s) produced in step 3 was slightly more than the mass predicted from the 3.80 g Cu3(PO4)2 recovered from step 1. Which of the following could account for the discrepancy in the yield of Cu from step 3?

Some unreacted Zn (s) was mixed in with the Cu(s)

Which of the following experimental techniques will allow the most accurate determination of the concentration of NO2(g) at equilibrium ?

Spectrophotometry

Which of the following best helps to explain why CCl4 is a liquid whereas CI4 is a solid when both are at 25C

The London dispersion forces are stronger in CI4 than in CCL4 because CI4 has a more polarizable electron cloud than CCl4

Based on the data above, what can be concluded regarding the strength of the C-H, N-H, and O-H bonds in the molecules shown?

The O-H bond is the strongest

Which of the following correctly compares periodic properties of two elements and provides an accurate explanation for that difference?

The atomic radius of Cl is smaller than that of S because Cl has a larger nuclear charge than S does

Which of the following statements is true about bond energies in this reaction?

The energy absorbed as the bonds in the reactants are broken is less than the energy released as the bonds in the product are formed.

A 50.0g of Fe (s) at 100C is added to 500mL of water at 35C in a perfectly insulated container. Which of the following statements is true?

The heat energy lost by the Fe will be equal to the heat energy gained by the water

Which of the following is the most likely reason that the reaction occurs at a significant rate only if the temperature of the reaction mixture is greater than 200C?

The reaction has a high activation energy

What happens to the temperature of the contents of the vessel as the reaction occurs?

The temperature must decrease, because the reaction is endothermic

A 1.0 mol sample of CO (g) and a 1.0 mol sample of H2 (g) are pumped into a rigid, previously evacuated 2.0L reaction vessel at 483K. Which of the following must be true at equilibrium?

[H2] < [CO]

A mixture of NO2 (g) and N2O4 (g) is at equilibrium in a rigid reaction vessel. If the temperature of the mixture is decreased then

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

A 1.0 L sample of a pure gas is found to have a lower pressure than that predicted by the ideal gas law. The best explanation for the observation is that the molecules of the gas...

are attracted to each other and do not exert as much force on the container walls as they would if they had no mutual attractions

The amount of H2 (g) present in a reaction mixture at equilibrium can be maximized by

increasing the temperature and decreasing the pressure by increasing the volume