AP Chemistry | Unit 3 Progress Check: MCQ

In an experiment on the photoelectric effect, a photon with a frequency of 7×1014s−1 was absorbed by a sample. What is the approximate energy of this photon?

A) 5×10^−19 J WHY? E=hν (energy = Planck's constant × frequency), so E=(6.626×10−34Js)×(7×1014s−1)=5×10−19J.

Which of the following best explains what happens as photons of visible light are absorbed by dye molecules?

A) Certain electrons in the dye molecule move to a higher energy level, with the difference in energy between the lower and higher energy levels being the same as the energy of the absorbed photons. WHY? Absorption of photons in the visible range will cause transition of electrons between electron energy levels.

Which of the diagrams above best represents the interactions that are responsible for the relatively large solubility of KCl crystals in water, and why?

A) Diagram 1, because strong ion-dipole interactions between KCl and water help to dissociate the solute. WHY? As KCl dissolves in water, the strong ion-dipole forces between K+ ions and H2O molecules and Cl− ions and H2O molecules help dissociate the ionic solid and overcome the hydrogen bonding forces between H2O molecules.

The infrared spectrum above represents the absorption of certain wavelengths of radiation by molecules of CO2. Which of the following best explains what occurs at the molecular level as the CO2 molecules absorb photons of the infrared radiation?

A) The atoms in the CO2 molecules increase their vibration as the bonds between the atoms bend and stretch. WHY? The absorption of photons of infrared radiation is associated with transitions in molecular vibrational levels.

A student adds a 1g sample of an unknown, brittle solid to distilled water, stirs the mixture, and then measures its conductivity. The student repeats this procedure with more samples of the unknown solid and then produces the graph above. Which of the following statements about the graph and the properties of the solid is correct?

A) The increase in conductivity indicates that the unknown is an ionic solid that dissociates into ions when it dissolves in water. WHY? Since the solid is brittle and the solution conducts electricity, the unknown solid must be an ionic solid. When an ionic solid dissolves in water, it dissociates and the ions are free to move around in the solution, resulting in the conduction of electricity.

A student uses a spectrophotometer to analyze a solution of blue food dye. The student first rinses a cuvette with distilled water. Then the student adds the blue dye solution to the cuvette, forgetting to rinse the cuvette with the blue dye solution first. The student places the cuvette in the spectrophotometer and measures the absorbance of the solution. Assuming that some distilled water droplets were still in the cuvette when the blue dye solution was added, how would the measured absorbance be affected?

A) The measured absorbance would be too low, because the distilled water left in the cuvette would slightly dilute the solution. WHY? The dilution of the dye solution would lower the concentration of food dye. Less dye in the solution means less absorbance of light; thus the measured absorbance would be too low.

The absorption spectrum of a certain red dye is shown above. If a student analyzing the same concentration of this dye neglected to wipe fingerprints off the cuvette before placing it in the spectrophotometer, how would the absorption curve be affected?

A) The peak of the curve would be higher because more light would be absorbed. WHY? Fingerprints scatter light, so less light would pass through the solution and reach the detector inside the instrument. Less light detected implies more light absorbed. Therefore the peak of the curve would be higher, erroneously indicating that the absorbance is larger than it really is.

One type of organic molecule can be converted to another type of organic molecule through an oxidation-reduction process, as represented in the diagram above. Which of the following best explains why infrared spectroscopy is an appropriate method to confirm that the product contains a carbonyl?

B) The absorption of infrared radiation leads to an increase in molecular vibrational level associated with the stretching of the carbonyl. WHY? The presence of a carbonyl functional group results in an intense peak within a narrow range in an infrared spectrum that corresponds to the carbonyl stretching vibration; thus infrared spectroscopy can be used to confirm that the product contains a carbonyl group.

The frequency and energy ranges of photons in some parts of the electromagnetic spectrum are given in the table above. Which of the following could be the energy of a photon in the visible range?

C) 4×10^19 J WHY? The frequency and energy of light in the visible range are between those in the infrared and ultraviolet ranges. The upper limit for the IR region is 3×10−19 J, and the lower limit for the ultraviolet region is 5×10−19 J.

What volume of a 0.100MHCl stock solution should be used to prepare 250.00mL of 0.0250MHCl?

C) 62.5mL WHY? When using a stock solution to prepare a solution of lower concentration, only solvent is added and the number of moles of solute remain constant. In these situations, the volume of stock solution can be obtained by rearranging the equation: M1V1=M2V2. Therefore, V1=M2V2/M1= 0.0250M×250.00mL/0.100M=62.5mL. Note that when working with ratios, the units of volume will be the same as those of the volume given.

The structures of two allotropes of carbon are represented above. Which of the following statements best helps explain why diamond is much harder than graphite?

C) Carbon atoms in diamond have four covalent bonds, whereas graphite is made of layers that are held together by relatively weak dispersion forces. WHY? Diamond is a covalent network solid with each carbon atom bonded to four other carbon atoms. Graphite has a layered structure with each carbon atom bonded to three other carbon atoms. Dispersion forces hold the layers together, but the forces are relatively weak, and the layers can slide past each other.

(two diagrams shown above) Which particle diagram shown above best represents the strongest intermolecular force between two ethanol, C2H6O, molecules?

C) Diagram 2, because it shows the formation of a hydrogen bond between an H atom bonded to an O atom with an O atom from another molecule. WHY? Hydrogen bonding is the strongest attractive force between ethanol molecules. It requires an H atom to be bonded to a small atom with a high electronegativity, like N, O, or F. The hydrogen bond forms between atoms with partial positive and partial negative charges. In the O−H bond, the H atom has a partial positive charge, and the Oatom has a partial negative charge. The hydrogen bond is formed between O−H---O−H bonds from different molecules, as shown.

The two gas samples represented in the graph above are at the same temperature. Which of the following statements about the gases is correct?

C) Gas Z has a smaller molar mass than gas X. WHY? Gases at the same temperature have the same average kinetic energy of their molecules. Since KE=12mXvX2=12mZvZ2, the gas with the higher average molecular speed has the lower molar mass (mX/mZ=vZ2/vX2). The curve for gas Z shows that the molecules have a higher average speed than the molecules in gas X, so gas Z has a smaller molar mass.

The diagram above shows thin-layer chromatograms of the same mixture of two compounds. Based on the chromatograms, which solvent would be most effective at separating the two compounds if the same stationary phase is used for column chromatography?

C) Solvent C WHY? For the two compounds in the mixture, the difference in affinity for solvent C is the largest of the four solvents. Therefore, the most effective separation of the compounds can be achieved in a column by using solvent C as the mobile phase.

A solid compound of a group 1 (alkali) metal and a group 17 (halogen) element dissolves in water. The diagram above represents one type of solute particle present in the solution. Which of the following identifies the solute particle and best helps explain how the solute particle interacts with water molecules?

C) The particle is a positive ion, and the interactions are ion-dipole attractions. WHY? The water molecules are all oriented the same way with respect to the solute particle, with the negative ends of the water molecule dipoles directed toward the solute particle. This can only be the case if the solute particle has a positive charge. The major attractive forces between the polar water molecules and the positive ion are characterized as ion-dipole attractions.

The diagram above shows the distribution of speeds for a sample of O2(g). Which of the following graphs shows the distribution of speeds for the same sample at a higher temperature (dashed line) ?

C) [graph with dashed line's peak at center and below original line] WHY? When the temperature increases, the average speed of the gas molecules increases. The peak is to the right of the original peak, and the relative number of molecules at the highest speeds is greater.

Methanol, CH3OH, dissolves completely in water to form a solution that does not conduct electricity. Which of the following diagrams best shows the major type of attractive force that exists between the particles in the solution?

C) [the diagram shows Methanol attracted to a water atom with a dashed line; the Oxygen atom from Methanol is attracted to a Hydrogen atom from water] WHY? Water and methanol exhibit hydrogen bonding. Hydrogen bonding is a dipole-dipole interaction that occurs between an H atom bonded to a highly electronegative atom (O, F, or N) and an electronegative atom in a nearby molecule. Methanol, being a molecular compound, does not ionize in water and will not form aqueous solutions that conduct electricity.

A 500.mL aqueous solution of Na3PO4 (molarmass=164g/mol) was prepared using 82gof the solute. What is the molarity of Na3PO4 in the resulting solution?

D) 1.0M WHY? To calculate the molarity of a solution, the number of moles of solute is divided by the volume of the solution in liters. Therefore, 82g/164g/mol/500.mL/1000mL/L= 1.0M.

How many grams of NaCl (molarmass=58g/mol) are needed to prepare 100.mL of a 0.25M NaCl solution?

D) 1.5g WHY? When the concentration of a solution is expressed as molarity, the number of moles of solute needed can be determined by the moles of solute if the volume of the solution is known. Moles of NaCl=M× (volume in L). Therefore, 0.25M×100.mL/1000mL/L=0.025mol×58g/mol= 1.5g.

Electromagnetic radiation with a maximum wavelength of 540nm (5.4×10−7 m ) is needed for the study of the photoelectric effect in potassium atoms. What is the approximate frequency that corresponds to this wavelength?

D) 5.6×10^14 s−1 WHY? c=λν (speed of light = wavelength × frequency), so ν=c/λ. ν=(2.998×108m/s)/(5.4×10−7m)=5.6×1014s−1.

The gases CO2(g) and NH3(g) can be liquefied at 20°C by compressing them to sufficiently high pressures. A student claims that NH3(g) can be liquefied at a lower pressure than CO2(g) can be liquefied. Which of the following is the best justification for this claim?

D) CO2 is a nonpolar molecule that has London dispersion intermolecular forces that are weaker than the dipole-dipole and London dispersion forces between the polar NH3 molecules. WHY? Because polar NH3 molecules have stronger intermolecular forces than CO2, its molecules can condense to a liquid at a lower pressure than CO2.

The crystal structure of NaBr is represented in the diagram above. Which statement correctly compares crystalline NaBr(s) to molten NaBr(l) in terms of electrical conductivity?

D) Crystalline NaBr contains no freely moving electrons to conduct electricity, but molten NaBr is composed of freely moving Na+ and Br− ions, which allows it to be a good conductor of electricity. WHY? In crystalline NaBr, there are no free electrons (or any other charged particles) that can move to carry an electrical current. But when molten, NaBr has freely moving ions that can carry an electric current.

The diagrams above use arrows to represent the speed of a gas particle. Which of the diagrams best represents the speed of the particles of a gas at a fixed temperature, and why?

D) Diagram 2, because the particles have a variety of different speeds. WHY? Based on kinetic molecular theory, gas particles have a distribution of speeds that is dependent on the temperature, which is represented in diagram 2.

Which of the following methods is most appropriate to use to determine the number of different-colored components in a sample of black ink?

D) Paper chromatography using different solvents with a range of polarities as the mobile phase WHY? Paper chromatography is appropriate to use to determine the number of components, because it can be used to separate very small quantities of a mixture into its components. Trying different solutions with a range of polarities as the mobile phase is helpful for identifying the solution that results in the greatest difference in retention factors among the components.

The graph above shows how a particular real gas deviates from ideal behavior at very high pressures. Based on this information, which of the following is most likely the gas and gives the reason based on kinetic molecular theory?

D) SO2, because it has the largest molecular volume. WHY? SO2 has the largest molecular volume. At very high pressures, the space occupied by the molecules is more significant and will result in the largest deviation from the volume predicted by the ideal gas law. According to kinetic molecular theory, ideal gas molecules have negligible volumes, so the larger its molecules are, the more the behavior of the gas will deviate from that of an ideal gas.

When methanol and water are mixed together, they form a homogeneous mixture. Based on the information in the table above, which of the following would be the best procedure for separating a mixture of methanol and water?

B) Distillation WHY? Distillation is a separation technique that takes advantage of differences in boiling points. Methanol has a lower boiling point than water and can be separated from water using a distillation apparatus.

(Li+, Mg2+, Na+, and Ca2+ cations shown above) The diagram above represents four cations, all shown to the same scale. Which cation would be predicted by Coulomb's law to have the strongest ion-dipole attraction to water, and why?

B) Mg2+, because it has the largest charge-to-size ratio. WHY? Because water is polar, the O atom has a partial negative charge that can interact with cations through coulombic forces. Coulomb's law predicts that the attractive force between the partial negative charge in the O atom and the cation will be directly proportional to the charge of the ion and inversely proportional to the size or ionic radius. Mg2+ is a relatively small ion and has the most positive charge among the cations shown.

Two sealed, rigid 5.0L containers each contain a gas at the same temperature but at a different pressure, as shown above. Also shown are the results of transferring the entire contents of container 1 to container 2. No gases escape during the transfer. Assuming ideal behavior, which statement is correct regarding the total pressure of the gases after they are combined?

A) The total pressure of the gases in the mixture is the sum of the initial pressures of oxygen gas and nitrogen gas because pressure only depends on the total amount of gas when volume and temperature are held constant. WHY? Under conditions of constant T and V, the pressure of a gas or a mixture of gases only depends on the total number of moles of gas, regardless of the identity of the gas. From the ideal gas law, P=nRT/V =(RT/V) n=k×n, thus P is directly proportional to n, since R, T, and V are constant. Since the total amount of gas in the mixture is the sum of the original amounts of each gas, Ptotal=(RT/5)(n oxygen+n nitrogen)=(RT/5)(n oxygen)+(RT/5)(n nitrogen), meaning that the total pressure of the gas mixture is simply the sum of the initial pressures of each gas.

At 10.°C, 20.g of oxygen gas exerts a pressure of 2.1atm in a rigid, 7.0L cylinder. Assuming ideal behavior, if the temperature of the gas was raised to 40.°C, which statement indicates the new pressure and explains why?

B) 2.3atm, because the pressure P increases by the proportion 313/283. WHY? Rearranging the ideal gas law equation yields the equation P=nRT/V. Since the number of moles of oxygen (n), the gas constant (R), and the volume (V) are all constant in this case, the relationship reduces to P=kT, where T is the absolute temperature (in kelvins). Therefore, increasing the temperature to 313 K from 283 K results in a pressure that is larger by the factor 313/283.

A gaseous air‑fuel mixture in a sealed car engine cylinder has an initial volume of 600.mL at 1.0atm. To prepare for ignition of the fuel, a piston moves within the cylinder, reducing the volume of the air‑fuel mixture to 50.mL at constant temperature. Assuming ideal behavior, what is the new pressure of the air‑fuel mixture?

B) About 12atm, because the volume of the gas mixture decreased by a factor of 12. WHY? For a gas or gas mixture that behaves ideally, pressure and volume are inversely proportional. Therefore, since n and T remained constant, a 12-fold decrease in volume should result in a 12-fold increase in pressure.

The diagrams above represent two allotropes of solid phosphorus. Which of the following correctly identifies the allotrope with the higher melting point and explains why?

B) Allotrope II, because it has covalent bonds between the phosphorous atoms that are stronger than the dispersion forces between the P4 molecules in allotrope I. WHY? Allotrope II has a high melting point because its atoms are covalently bonded in a network solid. The covalent bonds are stronger than the dispersion forces between P4 molecules in allotrope I.

Equimolar samples of CH4(g) and C2H6(g) are in identical containers at the same temperature. The C2H6(g) deviates much more from ideal behavior than the CH4(g) does. Which of the following best helps explain this deviation?

B) C2H6 molecules have a larger, more polarizable electron cloud than CH4 molecules do. WHY? The ideal gas law assumes that attractions between gas particles are negligible. Since C2H6(g) molecules have a larger, more polarizable electron cloud than CH4(g) molecules do, their intermolecular forces are greater. Gas pressure is due to collisions of gas particles with the walls of the container. If gas particles about to hit the walls of a container are attracted to nearby gas particles, they will hit the walls of the container with less force than expected.

A student measures the absorbance of a solution containing FeSCN2+ ion using a spectrophotometer. The cuvette used by the student has two frosted walls and two transparent walls. The student properly orients the cuvette so that the path of the light goes through the transparent sides of the cuvette when calibrating the spectrophotometer. How will the measured absorbance of the FeSCN2+ be affected if the student incorrectly orients the cuvette so that the path of the light is through the frosted sides of the cuvette?

B) The measured absorbance of the FeSCN2+ solution will be higher than the actual absorbance. WHY? The measured absorbance of the FeSCN2+ solution will be higher than the actual absorbance. Some of the light will be scattered/reflected by the frosted surfaces of the cuvette; thus, less light will reach the detector in the spectrophotometer, making it appear as if the sample had absorbed more light.