2019 AP ChemistryExam
Of the following diagrams, which best represents the structure of solid KFKF ?
+-+-+- -+-+-+ +-+-+- -+-+-+ +-+-+- -+-+-+ Alternating, same size
The concentration of F^−(aq) in drinking water that is considered to be ideal for promoting dental health is 44.0×10^−5M. Based on the information above, the maximum concentration of Ca^2+(aq) that can be present in drinking water without lowering the concentration of F^−(aq) below the ideal level is closest to
0.0025 M
After a certain pesticide compound is applied to crops, its decomposition is a first-order reaction with a half-life of 56 days. What is the rate constant, k, for the decomposition reaction?
0.012day^−1
The value of Kw at 40°C is 3.0×10−14. What is the pH of pure water at 40°C?
6.8
Ionization Energy(kJ/mol) First 578 Second 1,817 Third 2,745 Fourth 11,577 Fifth 14,842 The first five ionization energies of an unknown element are listed in the table above. Which of the following statements correctly identifies the element and cites the evidence supporting the identification?
Al, because of the large difference between the third and fourth ionization energies
The saturated CuSO4(aq) shown above is left uncovered on a lab bench at a constant temperature. As the solution evaporates, 1.0mL1.0mL samples of the solution are removed every three days and the [SO42−] in the samples is measured. It is observed that the [SO42−] in the solution did not change over time. Which of the following best helps to explain the observation?
As water evaporates, more CuSO4(s)CuSO4(s) precipitates out of the solution in the beaker.
The structural formula of the glycinium cation is shown above. Arrows indicate the pKa values for the labile protons in the molecule. Which of the following is true about the geometry of the glycinium cation?
Both C atoms and both O atoms lie in the same plane
Which of the following is the molecular formula of the unknown compound?
C4 H8 O2
In which of the following processes will ΔS° be negative?
Cl2(g)→Cl2(l)
As a sample of KNO3(s) is stirred into water at 25°C, the compound dissolves endothermically. Which of the following best helps to explain why the process is thermodynamically favorable at 25°C?
Dissolving the salt increases the entropy of the system.
A solution is prepared by mixing equal volumes of 0.20M HC2H3O and 20.40M NaC2H3O2. Which of the following correctly describes what occurs if a small amount of HCl(aq)HCl(aq) or NaOH(aq)NaOH(aq) is added?
If HCl(aq) is added, the pH will decrease only slightly because the H+ ions will react with C2H3O2− ions.
The structural formula of the glycinium cation is shown above. Arrows indicate the pKa values for the labile protons in the molecule. Which of the following statements is true about sodium glycinate, represented above? (images above question)
It is an ionic solid at room temperature.
The formation of Fe(s) and O2(g) from FeO(s) is not thermodynamically favorable at room temperature. In an effort to make the process favorable,C(s) is added to the FeO(s) at elevated temperatures. Based on the information above, which of the following gives the value of Keq and the sign of ΔG° for the reaction represented by the equation below at 1000K?
Keq = 1×10^26 ΔG° = Negative
Which of the following is a list of the minimum amount of data needed for determining the molar enthalpy of solution of KCl(s) in pure H2O(l) ? (Assume that the KCl(aq) has the same specific heat capacity as pure water and that the initial temperatures of the KCl(s) and the water are the same.)
Mass of KCl(s), mass of H2O, initial temperature of the water, and final temperature of the solution
Samples of NaF(s) and NH4Cl(s) are dissolved in separate beakers that each contain 100mL of water. One of the salts produces a slightly acidic solution. Which of the following equations best represents the formation of the slightly acidic solution?
NH4^+(aq) + H2O(l) ⇄ NH3(aq) + H3O^+(aq)
Based on the structural formulas, which of the following identifies the compound that is more soluble in water and best helps to explain why?
Propanol, because its molecules can form hydrogen bonds with water molecules but those of ethane cannot.
The data from a study of the decomposition of NO2(g) to form NO2(g) and O2(g) are given in the table above. Which of the following rate laws is consistent with the data?
Rate=k[NO2]^2
Which of the following can be inferred from the diagram above that shows the dependence of potential energy on the internuclear distance between two atoms?
The atoms form a bond with a bond length of 75pm
A sealed 10.0L flask at 400K contains equimolar amounts of ethane and propanol in gaseous form. Which of the following statements concerning the average molecular speed of ethane and propanol is true?
The average molecular speed of ethane is greater than the average molecular speed of propanol.
Which of the following best helps to explain why Na(s) is more reactive with water than Mg(s) is?
The first ionization energy of Na is less than that of Mg.
An aqueous solution contains small but equal concentrations of both chloroacetic and fluoroacetic acids. Which statement comparing the percent ionizations of the two acids in the solution is true?
The percent ionization of chloroacetic acid is less than that of fluoroacetic acid.
A mixture of two gases, 0.01mol of C4H10(g) and 0.065mol of O2(g), is pumped into a cylinder with a movable piston, as shown above. The mixture, originally at 200°C and 1.0atm, is sparked and the reaction represented below occurs. Which of the following is true after the product gases return to the original temperature and pressure, and why will the change occur? (Assume all gases behave ideally.)
The piston will be higher than its original position, because the cylinder will contain a greater number of gas molecules.
H2(g) + I2(g) ⇄ 2HI(g) Hydrogen gas reacts with iodine gas at constant temperature in a sealed rigid container. The gases are allowed to reach equilibrium according to the equation above. Which of the following best describes what will happen to the reaction immediately after additional iodine gas is added to the system?
The rate of the forward reaction becomes greater than the rate of the reverse reaction.
C(diamond)→C(graphite) ΔG°=−2.9kJ/molrxn Which of the following best explains why the reaction represented above is not observed to occur at room temperature?
The reaction has an extremely large activation energy due to strong three-dimensional bonding among carbon atoms in diamond.
The structural formulas for two isomers of 1,2-dichloroetheneare shown above. Which of the two liquids has the higher equilibrium vapor pressure at 20°C, and why?
The trans-isomer, because it has only London dispersion forces, whereas the cis-isomer also has dipole-dipole interactions
An experiment is performed to measure the mass percent of CaCO3(s) in eggshells. Five different samples of CaCO3(s) of known mass react with an excess of 2.0MHCl(aq) in identical sealed, rigid reaction vessels. The pressure of the gas produced is measured with a pressure gauge attached to the reaction vessel. Since the reaction is exothermic, the reaction system is cooled to its original temperature before the pressure is recorded. The experimental data are used to create the calibration line below. Which of the following modifications will increase the rate of the reaction the most?
Using eggshells that are more finely powdered than those used in the original experiment
C9 H8 O4(aq) + NaOH(aq) ⇄ C9 H7 O4^−(aq) + Na^+(aq) + H2O(l) The reaction represented above occurs when 2.00×10−4mol of pure acetylsalicylic acid. 15.00mL of water in a flask and titrated to the equivalence point with 0.100M NaOH(aq). Which of the following statements about the titration is true at the equivalence point?
[C9H8O4] is less than [C9H7O4−].
To catalyze a biochemical reaction, an enzyme typically
binds temporarily to reactant molecules to lower the activation energy of the reaction
Solid carbon tetrachloride, CCl4(s), is represented by the diagram above. The attractions between the CCl4 molecules that hold the molecules together in the solid state are best identified as
intermolecular attractions resulting from temporary dipoles
0.1MHC2H3O2(aq),0.1MKI(aq),0.1MCH3OH(aq).Of the three solutions listed in the table above, which one, if any, has the greatest electrical conductivity and why?
0.1MKI(aq) because KI completely dissociates in water to produce ions.
An experiment is performed to measure the mass percent of CaCO3(s) in eggshells. Five different samples of CaCO3(s) of known mass react with an excess of 2.0MHCl(aq) in identical sealed, rigid reaction vessels. The pressure of the gas produced is measured with a pressure gauge attached to the reaction vessel. Since the reaction is exothermic, the reaction system is cooled to its original temperature before the pressure is recorded. The experimental data are used to create the calibration line below. Another sample of eggshell reacts completely with 4.0mL of an HCl(aq) solution of unknown concentration. If the reaction produced 0.095atm of gas, the concentration of the HCl(aq) solution was at least
1.0 M
Standard reduction potentials for the half-reactions associated with the electrochemical cell shown above are given in the table below. What is the value of E° for the cell?
1.56V
The structural formula of the glycinium cation is shown above. Arrows indicate the pKa values for the labile protons in the molecule. What is the approximate H−O−C bond angle in the glycinium cation?
105 degrees
Ge(g) + 2Cl2(g) ⇄ GeCl4(g) The value of the equilibrium constant for the reaction represented above is 1×1010. What is the value of the equilibrium constant for the following reaction? 2GeCl4 (g) ⇄ 2Ge(g) + 4Cl2(g)
1×10^−20
At 25°C, enough distilled water is added to a 30.0mL sample of HNO3(aq) with a pH of 4.20 so that the final pH of the diluted solution is 5.20. The volume of distilled water added to the original solution is closest to
270 mL
Standard reduction potentials for the half-reactions associated with the electrochemical cell shown above are given in the table below. Which of the following is the net ionic equation for the overall reaction that occurs as the cell operates?
2Ag(aq) + Zn(s) → 2Ag(s) + Zn2(aq)
An experiment is performed to measure the mass percent of CaCO3(s) in eggshells. Five different samples of CaCO3(s) of known mass react with an excess of 2.0MHCl(aq) in identical sealed, rigid reaction vessels. The pressure of the gas produced is measured with a pressure gauge attached to the reaction vessel. Since the reaction is exothermic, the reaction system is cooled to its original temperature before the pressure is recorded. The experimental data are used to create the calibration line below. Which of the following equations best represents the species that react and the species that are produced when CaCO3(s) and HCl(aq) are combined?
2H^+(aq) + CaCO3(s) → Ca^2+(aq) + H2O(l) + CO2(g)
An experiment is performed to measure the mass percent of CaCO3(s) in eggshells. Five different samples of CaCO3(s) of known mass react with an excess of 2.0MHCl(aq) in identical sealed, rigid reaction vessels. The pressure of the gas produced is measured with a pressure gauge attached to the reaction vessel. Since the reaction is exothermic, the reaction system is cooled to its original temperature before the pressure is recorded. The experimental data are used to create the calibration line below. The mass percent of CaCO3(s) in the eggshell sample is closest to
75%
In a one-person spacecraft, an astronaut exhales 880g of CO2(g) (molar mass 44g/mol) per day. To prevent the buildup of CO2(g) in the spacecraft, a device containing LiOH(s) is used to remove the CO2(g), as represented by the equation above. What mass of LiOH(s) (molar mass 24g/mol) is needed to react with all of the CO2(g) produced by an astronaut in one day?
960g
A student titrates 10.0mL samples of 1.0M solutions of each of the haloacetic acids with a standard solution of NaOH. Which of the following statements correctly predicts the volume of NaOH(aq) needed to reach the equivalence point?
All of the acids will need the same volume of NaOH(aq) to reach the equivalence point.
A 1.0mol1.0mol sample of He(g) at 25°C is mixed with a 1.0mol sample of Xe(g) at 50°C. Which of the following correctly predicts the changes in average kinetic energy and the average speed of the Xe(g) atoms that will occur as the mixture approaches thermal equilibrium?
Average Kinetic Energy of Xe Atoms Will decrease Average Speed of Xe Atoms Will decrease
The structure of haloacetic acids, XCH2COOH (where X is either F, Cl, Br, or I), is shown above. The dissociation constants and molar masses of four haloacetic acids are listed in the table below. Which compound, chloroacetic acid or iodoacetic acid, most likely has the lower boiling point, and why?
Chloroacetic acid, because the London dispersion forces among its molecules are weaker.
A student obtains a mixture of the liquids hexane and octane, which are miscible in all proportions. Which of the following techniques would be best for separating the two components of the mixture, and why?
Distillation, because the liquids would boil at different temperatures owing to the difference in strength of their intermolecular forces.
The photoelectron spectra for H and He are represented above. Which of the following statements best accounts for the fact that the peak on the He spectrum is farther to the left and higher than the peak on the H spectrum?
He has a greater nuclear charge than H and an additional electron in the same energy level.
A mixture of H2(g) and O2(g) is placed in a container as represented above. The H2(g) and O2(g) react to form H2O(g). Which of the following best represents the container after the reaction has gone to completion?
TWO WHITE DOTS ON LEFT TWO WHITE DOTS TOP RIGHT WHITE AND BLACK MOLECULES IN MIDDLE
Which of the following best explains why the combustion reactions represented in the table are exothermic?
The energy required to break the bonds in the reactants is less than the energy released in forming the bonds in the products.
The diagram above represents a particle in aqueous solution. Which of the following statements about the particle is correct?
The particle must be an anion because the positive end of each water molecule is pointed toward it.
A mixture of X2(g), Y2(g), and XY(g) is placed in a previously evacuated, rigid container and allowed to reach equilibrium at a constant temperature, as shown above. Which of the following sets of initial concentrations would lead to the formation of more product as the system moves toward equilibrium?
[X2]initial 0.40M [Y2]initial 0.40M [XY]initial 0.20M
In an experiment, 30.0g of ethane and 30.0g of propanol are placed in separate reaction vessels. Each compound undergoes complete combustion with excess O2(g). Which of the following best compares the quantity of heat released in each combustion reaction?
q ethane > q propanol
A student observes that the equilibrium constant for a reaction is greater than 1.0 at temperatures below 500K but less than 1.0 at temperatures above 500K. What can the student conclude about the values of ΔH° and ΔS° for the reaction? (Assume that ΔH° and ΔS° are independent of temperature.)
ΔH°<0 and ΔS°<0
