AP Classroom (Chemistry Unit 3, Ch. 1,5,9,10: Intermolecular Forces and Properties)

(Cobalt II ion molarity) A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration. Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration. The original solution used to make the solutions for the standard curve was prepared by dissolving 2.60g of CoCl2 (molar mass 130.g/mol) in enough water to make 100.mL of solution. What is the molar concentration of the solution? A. 0.200M0.200M B. 0.500M0.500M C. 1.00M1.00M D. 5.00M

A. 0.200M

(Preparation of calcium chloride solution) How many grams of CaCl2 (molar mass =111g/mol) are needed to prepare 100.mL of 0.100MCl−(aq) ions? A. 0.555g0.555g B. 1.11g1.11g C. 2.22g2.22g D. 5.55g

A. 0.555g

(Solubility of compounds from Lewis structures) Based on their Lewis diagrams, which of the following pairs of liquids are most soluble in each other?

A. H H H-C-C-O-H & H-O-H H H

(Microwave vs infrared) Which statement correctly compares what occurs when molecules absorb photons in the microwave region with what occurs when molecules absorb photons in the infrared region? A. Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause the molecules to increase their vibrational energy states. B. Microwave photons cause electrons in the molecules to increase their electronic energy states, whereas infrared photons cause the molecules to increase their rotational energy states. C. Microwave photons cause the molecules to increase their vibrational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states. D. Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states.

A. Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause the molecules to increase their vibrational energy states.

(Nonpolar gas closer to ideal behavior) Which of the following best helps explain why the pressure of a sample of CH4(g) (molar mass 16g/mol) is closer to the pressure predicted by the ideal gas law than a sample of NH3(g) (molar mass 17g/mol) ? A. NH3NH3 molecules are polar while CH4CH4 molecules are not, and the greater attractions between NH3NH3 molecules cause the molecules to collide with the walls of the container with less force. B. NH3NH3 molecules have a greater molar mass than CH4CH4 molecules, so the NH3NH3 molecules collide with the walls of the container with more force. C. CH4CH4 molecules have more hydrogen atoms than NH3NH3 molecules, so CH4CH4 molecules have more hydrogen bonding and greater intermolecular forces. D. CH4CH4 molecules are larger than NH3NH3 molecules, so the actual CH4CH4 molecules take up a significant portion of the volume of the gas.

A. NH3NH3 molecules are polar while CH4CH4 molecules are not, and the greater attractions between NH3NH3 molecules cause the molecules to collide with the walls of the container with less force.

(Ar deviates from ideal gas at high pressure) Ar(g) deviates more from ideal behavior at extremely high pressures than Ne(g) does. Which of the following is one reason for this difference? A. The particle volume of ArAr is greater than that of NeNe. B. ArAr atoms have more valence electrons than NeNe atoms have, so ArAr atoms have greater interparticle forces. C. The intermolecular forces between NeNe atoms are greater than those between ArAr atoms. D. ArAr atoms are more attracted to the walls of the container than NeNe atoms are.

A. The particle volume of ArAr is greater than that of NeNe.

( Cobalt II ion calibration curve) A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration. Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration. The student made the standard curve above. Which of the following most likely caused the error in the point the student plotted at 0.050MCo2+(aq) ? A. There was distilled water in the cuvette when the student put the standard solution in it. B. There were a few drops of the 0.100MCo2+(aq)0.100MCo2+(aq) standard solution in the cuvette when the student put the 0.050 M0.050 ⁢M standard solution in it. C. The student used a cuvette with a longer path length than the cuvette used for the other standard solutions. D. The student did not run a blank between the 0.050MCo2+(aq)0.050MCo2+(aq) solution and the one before it.

A. There was distilled water in the cuvette when the student put the standard solution in it.

(Distribution of molecular speeds oxygen) The diagram above shows the distribution of speeds for a sample of N2(g) at 25°C. Which of the following graphs shows the distribution of speeds for a sample of O2(g) at 25°C (dashed line) ?

A. peak of O2 graph is to the left b/c slower

(Structure c-BN) The structure of one form of boron nitride is represented above. This form of boron nitride is one of the hardest substances known. Which of the following best helps explain why boron nitride is so hard? A. Boron ions and nitrogen ions are held together by ionic bonds. B. Boron nitride is a network solid of atoms connected by covalent bonds with fixed bond angles. C. Boron nitride is an alloy, and alloys are typically harder than the elements used to make them. D. Boron nitride is a polymer made of long chains of boron atoms and nitrogen atoms held together by dispersion forces.

B. Boron nitride is a network solid of atoms connected by covalent bonds with fixed bond angles.

(Chromatography reduce retention factor) The diagram above shows a thin-layer chromatogram of a mixture of products from a chemical reaction. The separation was performed using 50% ethyl acetate in hexane as the solvent (mobile phase) and silica gel as the polar stationary phase. On the basis of the chromatogram and the information about solvents in the table above, which of the following would be the best way to decrease the distance that the products travel up the plate? A. Use pentane instead of hexane in the solvent. B. Decrease the percentage of ethyl acetate in the solvent. C. Increase the percentage of ethyl acetate in the solvent. D. Add up to 5%5% methanol to the solvent.

B. Decrease the percentage of ethyl acetate in the solvent.

(NaCl and MgS compared) The ionic compounds NaCl and MgS are represented by the diagrams above. Which statement correctly identifies diagram 1 and identifies the compound with the lower melting point, explaining why? A. Diagram 1 represents NaClNaCl; it has a lower melting point than MgSMgS has because the coulombic attractions between the singly charged Na+Na+ ions and the Cl−Cl− ions in NaClNaCl are stronger than those between the ions in MgSMgS. B. Diagram 1 represents NaClNaCl; it has a lower melting point than MgSMgS because the coulombic attractions between its singly charged Na+Na+ ions and the Cl−Cl− ions are weaker than those between the ions in MgSMgS. C. Diagram 1 represents MgSMgS; it has a lower melting point than NaClNaCl because the coulombic attractions between its doubly charged Mg2+Mg2+ ions and the S2−S2− ions are stronger than those between the ions in NaClNaCl. D. Diagram 1 represents MgSMgS; it has a lower melting point than NaClNaCl because the coulombic attractions between the doubly charged Mg2+Mg2+ ions and the S2−S2− ions are weaker than those between the ions in NaClNaCl.

B. Diagram 1 represents NaClNaCl; it has a lower melting point than MgSMgS because the coulombic attractions between its singly charged Na+Na+ ions and the Cl−Cl− ions are weaker than those between the ions in MgSMgS.

(Solubility related to intermolecular interactions) A student places a piece of I2(s) in 50.0 mL of H2O(l), another piece of I2(s) of the same mass in 50.0 mL of C6H14(l), and shakes the mixtures. The results are shown above. What do the results indicate about the intermolecular interactions of the substances? A. I2I2 and H2OH2O have similar intermolecular interactions, and I2I2 and C6H14C6H14 do not. B. I2I2 and C6H14C6H14 have similar intermolecular interactions, and I2I2 and H2OH2O do not. C. I2I2, H2OH2O, and C6H14C6H14 all have similar intermolecular interactions. D. I2I2, H2OH2O, and C6H14C6H14 have three completely different types of intermolecular interactions.

B. I2I2 and C6H14C6H14 have similar intermolecular interactions, and I2I2 and H2OH2O do not.

(Wavelength in photoelectric effect) The diagram above represents the photoelectric effect for a metal. When the metal surface is exposed to light with increasing frequency and energy of photons, electrons first begin to be ejected from the metal when the energy of the photons is 3.3×10^−19J. Using the wavelength information provided above, what is the color of the light? A. Red B. Orange C. Yellow D. Blue

B. Orange

(IR as a tool to investigate structure of molecules) Infrared spectroscopy is a useful tool for scientists who want to investigate the structure of certain molecules. Which of the following best explains what can occur as the result of a molecule absorbing a photon of infrared radiation? A. The energies of infrared photons are in the same range as the energies associated with changes between different electronic energy states in atoms and molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of electronic transitions within the molecules. B. The energies of infrared photons are in the same range as the energies associated with different vibrational states of chemical bonds. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the types and strengths of different bonds in the molecules. C. The energies of infrared photons are in the same range as the energies associated with different rotational states of molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of transition between different rotational energy states of the molecules. D. The energies of infrared photons are in the same range as the total bond energies of bonds within molecules. Chemical bonds can be completely broken as they absorb infrared photons of characteristic wavelengths, thus revealing the energies of the bonds within the molecules.

B. The energies of infrared photons are in the same range as the energies associated with different vibrational states of chemical bonds. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the types and strengths of different bonds in the molecules.

(Nonideal behavior in Xe gas) The diagrams above represent two samples of Xe gas in containers of equal volume at 280K. Which of the following correctly compares the two samples in terms of their deviation from ideal gas behavior and explains why? A. The gas in sample 1 would deviate more from ideal behavior because the average distance an XeXe atom travels before colliding with another XeXe atom is greater. B. The gas in sample 2 would deviate more from ideal behavior because the XeXe atoms are closer together, leading to an increase in intermolecular attractions. C. The gas in sample 2 would deviate more from ideal behavior because the average speed of the XeXe atoms is less, leading to an increase in intermolecular attractions. D. The gases in both sample 1 and sample 2 would show the same deviation from ideal behavior.

B. The gas in sample 2 would deviate more from ideal behavior because the XeXe atoms are closer together, leading to an increase in intermolecular attractions.

(Molarity of sodium ions) How many moles of Na+ ions are in 100.mL of 0.100MNa3PO4(aq) ? A. 0.300mol0.300mol B. 0.100mol0.100mol C. 0.0300mol0.0300mol D. 0.0100mol

C. 0.0300mol

(Frequency in photoelectric effect) The diagram above represents the photoelectric effect for a metal. When the metal surface is exposed to light with increasing frequency and energy of photons, electrons first begin to be ejected from the metal when the energy of the photons is 3.3×10−19J. Which of the following is closest to the frequency of the light with photon energy of 3.3×10−19J? A. 5.0×10^−53 s−1 B. 5.0×10^−16 s−1 C. 5.0×10^14 s−1 D. 5.0×10^52 s−1

C. 5.0×10^14 s−1

(Partial pressure gas in mixture) A gas mixture at 0°C and 1.0atm contains 0.010mol of H2, 0.015mol of O2, and 0.025mol of N2. Assuming ideal behavior, what is the partial pressure of hydrogen gas (H2) in the mixture? A. About 0.010atm0.010atm, because there is 0.010mol0.010mol of H2H2 in the sample. B. About 0.050atm0.050atm, because there is 0.050mol0.050mol of gases at 0°C0°C and 1.0atm1.0atm. C. About 0.20atm0.20atm, because H2H2 comprises 20%20% of the total number of moles of gas. D. About 0.40atm0.40atm, because the mole ratio of H2:O2:N2H2:O2:N2 is 0.4:0.6:10.4:0.6:1.

C. About 0.20atm0.20atm, because H2H2 comprises 20%20% of the total number of moles of gas.

(O2 in H2O) The survival of aquatic organisms depends on the small amount of O2 that dissolves in H2O. The diagrams above represent possible models to explain this phenomenon. Which diagram provides the better particle representation for the solubility of O2 in H2O, and why? A. Diagram 1, because O2O2 molecules can form hydrogen bonds with the H2OH2O molecules. B. Diagram 1, because O2O2 and H2OH2O are polar molecules that can interact through dipole-dipole forces. C. Diagram 2, because the polar H2OH2O molecules can induce temporary dipoles on the electron clouds of O2O2 molecules. D. Diagram 2, because the nonpolar O2O2 molecules can induce temporary dipoles on the electron clouds of H2OH2O molecules.

C. Diagram 2, because the polar H2OH2O molecules can induce temporary dipoles on the electron clouds of O2O2 molecules.

(Comparison of IMFs in NH3 and PH3) Which statement best helps to explain the observation that NH3(l) boils at −28°C, whereas PH3(l) boils at −126°C? A. The dispersion forces in NH3NH3 are weaker than the dispersion forces in PH3PH3. B. The dispersion forces in NH3NH3 are stronger than the dipole-dipole forces in PH3PH3. C. NH3NH3 has hydrogen bonding that is stronger than the dipole-dipole forces in PH3PH3. D. NH3NH3 has hydrogen bonding that is weaker than the dipole-dipole forces in PH3PH3.

C. NH3NH3 has hydrogen bonding that is stronger than the dipole-dipole forces in PH3PH3.

(Partial-total pressure of gas mixture) Diagram 1 above shows equimolar samples of two gases inside a container fitted with a removable barrier placed so that each gas occupies the same volume. The barrier is carefully removed as the temperature is held constant. Diagram 2 above shows the gases soon after the barrier is removed. Which statement describes the changes to the initial pressure of each gas and the final partial pressure of each gas in the mixture and also indicates the final total pressure? A. The partial pressure of each gas in the mixture is double its initial pressure; the final total pressure is half the sum of the initial pressures of the two gases. B. The partial pressure of each gas in the mixture is double its initial pressure; the final total pressure is twice the sum of the initial pressures of the two gases. C. The partial pressure of each gas in the mixture is half its initial pressure; the final total pressure is half the sum of the initial pressures of the two gases. D. The partial pressure of each gas in the mixture is half its initial pressure; the final total pressure is the same as the sum of the initial pressures of the two gases.

C. The partial pressure of each gas in the mixture is half its initial pressure; the final total pressure is half the sum of the initial pressures of the two gases.

(Hydrocarbon IMFs and VP) Four different liquid compounds in flasks at 20°C are represented above. The table below identifies the compounds. Flask C shows the most particles in the vapor phase. Which of the following is not shown in the model but best helps to explain why flask C must contain pentane? A. The random motion of the particles within the liquids B. The relative speeds of the vapor particles in each flask C. The strength of the intermolecular forces between the particles in the liquids D. The structural formula of the molecules of the liquid and vapor in each flask

C. The strength of the intermolecular forces between the particles in the liquids

(Acetone interference) The diagrams above show the ultraviolet absorption spectra for two compounds. Diagram 1 is the absorption spectrum of pure acetone, a solvent used when preparing solutions for an experiment. Diagram 2 is the absorption spectrum of the solute for which the absorbance needs to be measured to determine its concentration. When the student reads the absorbance of the solution at 280nm, the result is too high. Which of the following is most likely responsible for the error in the measured absorbance? A. The student added too little solute to the acetone before measuring its absorbance. B. The student rinsed the cuvette with the solution before filling the cuvette with the solution. C. The student forgot to calibrate the spectrophotometer first by using a cuvette containing only acetone. D. The wavelength setting was accidentally changed from 280nm280nm to 300nm300nm before the student made the measurement.

C. The student forgot to calibrate the spectrophotometer first by using a cuvette containing only acetone.

(Spectrum of beta-carotene) Beta-carotene is an organic compound with an orange color. The diagram above shows the ultraviolet spectrum of beta-carotene. Which of the following statements is true about the absorption bands in the spectrum? A. The absorption band between 250250 and 320 nm320 ⁢⁢nm is due to transitions in electronic energy levels, and the absorption band between 380380 and 520 nm520 nm is due to transitions in molecular vibrational levels. B. The absorption band between 250250 and 320 nm320 nm is due to transitions in molecular vibrational levels, and the absorption band between 380380 and 520 nm520 nm is due to transitions in molecular rotational levels. C. The two main absorption bands are associated with transitions in electronic energy levels. The band in the region corresponding to shorter wavelengths shows a lower absorbance than the band in the region corresponding to longer wavelengths. D. The two main absorption bands are associated with transitions in molecular vibrational levels. The band in the region corresponding to shorter wavelengths shows a lower absorbance than the band in the region corresponding to longer wavelengths.

C. The two main absorption bands are associated with transitions in electronic energy levels. The band in the region corresponding to shorter wavelengths shows a lower absorbance than the band in the region corresponding to longer wavelengths.

(Total pressure of gas mixture) An equimolar mixture of N2(g) and Ar(g) is kept inside a rigid container at a constant temperature of 300 K. The initial partial pressure of Ar in the mixture is 0.75atm. An additional amount of Ar was added to the container, enough to double the number of moles of Ar gas in the mixture. Assuming ideal behavior, what is the final pressure of the gas mixture after the addition of the Ar gas? A. 0.75atm0.75atm, because increasing the partial pressure of ArAr decreases the partial pressure of N2N2. B. 1.13atm1.13atm, because 33%33% of the moles of gas are N2N2. C. 1.50atm1.50atm, because the number of moles of N2N2 did not change. D. 2.25atm2.25atm, because doubling the number of moles of ArAr doubles its partial pressure.

D. 2.25atm2.25atm, because doubling the number of moles of ArAr doubles its partial pressure.

(Photon-energy calculation) A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration. Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration. A wavelength of 510nm corresponds to an approximate frequency of 6×1014s−1. What is the approximate energy of one photon of this light? A. 9×10^47J B. 3×10^17J C. 5×10^−7J D. 4×10^−19J

D. 4×10^−19J

(Fractional distillation hydrocarbons) A student performed a fractional distillation of a mixture of two straight-chain hydrocarbons, C7H16 and C8H18. Using four clean, dry flasks, the student collected the distillate over the volume ranges (A, B, C, and D) shown in the graph above. Over what volume range should the student collect the distillate of the compound with the stronger intermolecular forces? A AA B BB C CC D D

D. D

(Dispersion forces between diatomic atoms) The diagram above is a molecular model of a gaseous diatomic element that is just above its boiling point. Intermolecular forces between the gas molecules will cause them to condense into the liquid phase if the temperature is lowered. Which of the following best describes how the model is limited in its depiction of the phenomenon? A. It does not show how hydrogen bonds are constantly forming, breaking, and reforming, which results in a net force of attraction between the molecules. B. It does not show how the interactions between ions and the induced molecular dipoles result in a net force of attraction between the molecules. C. It does not show how the interacting permanent dipoles of the molecules result in a net force of attraction between the molecules. D. It does not show how the temporary fluctuating dipoles of the molecular electron clouds result in a net force of attraction between the molecules.

D. It does not show how the temporary fluctuating dipoles of the molecular electron clouds result in a net force of attraction between the molecules.

(HF hydrogen bonding) The electron cloud of HF is smaller than that of F2 , however, HF has a much higher boiling point than F2 has. Which of the following explains how the dispersion-force model of intermolecular attraction does not account for the unusually high boiling point of HF? A. F2F2 is soluble in water, whereas HFHF is insoluble in water. B. The F2F2 molecule has a greater mass than the HFHF molecule has. C. Liquid F2F2 has weak dispersion force attractions between its molecules, whereas liquid HFHF has strong ionic interactions between H+H+ and F−F− ions. D. Liquid F2F2 has weak dispersion force attractions between its molecules, whereas liquid HFHF has both weak dispersion force attractions and hydrogen bonding interactions between its molecules.

D. Liquid F2F2 has weak dispersion force attractions between its molecules, whereas liquid HFHF has both weak dispersion force attractions and hydrogen bonding interactions between its molecules.

(Mixing gases does not change speed) A 1L sample of helium gas at 25°C and 1atm is combined with a 1L sample of neon gas at 25°C and 1atm. The temperature is kept constant. Which of the following statements about combining the gases is correct? A. The average speed of the helium atoms increases when the gases are combined. B. The average speed of the neon atoms increases when the gases are combined. C. The average kinetic energy of the helium atoms increases when the gases are combined. D. The average kinetic energy of the helium atoms and neon atoms do not change when the gases are combined.

D. The average kinetic energy of the helium atoms and neon atoms do not change when the gases are combined.

(Spectrometry data analysis) Using a spectrophotometer, a student measures the absorbance of four solutions of CuSO4 at a given wavelength. The collected data is given in the table above. Which of the following is the most likely explanation for the discrepant data in trial 4 ? A. The solution was at a lower temperature than the solutions in the other trials. B. The measurement was made using a different spectrophotometer that uses a cell with a longer path length. C. The solution was saturated and the flow of light through the solution was restricted. D. The concentration of the solution was actually lower than 0.150M0.150M.

D. The concentration of the solution was actually lower than 0.150M0.150M.

(T from particle-speed graph) The graph above shows the distribution of molecular speeds for four different gases at the same temperature. What property of the different gases can be correctly ranked using information from the graph, and why? A. The densities of the gases, because as the density of a gas increases, the average speed of its molecules decreases. B. The pressures of the gases, because the pressure exerted by a gas depends on the average speed with which its molecules are moving. C. The volumes of the gases, because at a fixed temperature the volume of a gas can be calculated using the equation PV=nRT. D. The molecular masses of the gases, because the gas molecules have the same average kinetic energy and mass can be calculated using the equation KEavg=12mv2.

D. The molecular masses of the gases, because the gas molecules have the same average kinetic energy and mass can be calculated using the equation KEavg=12mv2.

(Food dye separation) A student obtains a liquid sample of green food coloring that is known to contain a mixture of two solid pigments, one blue and one yellow, dissolved in an aqueous solution of ethanol. Which of the following laboratory setups is most appropriate for the student to use in order to separate and collect a substantial sample of each of the two pigments?

D. chromatography column