AP Chemistry Unit 3: Properties of Substances and Mixtures Practice Exam

Assessment for Unit 3: Properties of Substances and Mixtures

Estimated Time: 60 minutes

Exam Details

Questions: 31 total (30 multiple choice, 1 short answer)
Time: 60 minutes (recommended)
Topics Covered: Unit 3: Properties of Substances and Mixtures
Format: Multiple Choice, Short Answer questions matching real exam difficulty
Scoring: Instant results with detailed explanations

A. Multiple-Choice Questions (30 Questions)

Select the one best answer for each question.

1. **1.** [Skill: 3.A | Topic: 3.1] A student measures the normal boiling points of three nonpolar molecules that have the same molar mass (72.15 g/mol). The results are shown. | Substance | Structural description | Normal boiling point (°C) | |---|---|---| | X | straight-chain hydrocarbon | 36.1 | | Y | moderately branched hydrocarbon | 27.8 | | Z | highly branched, nearly spherical hydrocarbon | 9.5 | Which explanation best accounts for the trend in boiling point from X to Z?

(A)As branching increases, the molecules have greater surface contact area, so London dispersion forces become stronger and boiling point increases.(B)As branching increases, the molecules have less surface contact area, so London dispersion forces become weaker and boiling point decreases.(C)As branching increases, the molecules become more polar, so dipole-dipole forces become stronger and boiling point decreases.(D)As branching increases, hydrogen bonding becomes possible, so boiling point decreases.

2. **2.** [Skill: 1.B | Topic: 3.1] A student compares the miscibility of several substances with water at room temperature. - Substance I: $NaCl(s)$ dissolves readily in water. - Substance II: $C_6H_{14}(l)$ (hexane) does not mix with water. Which type of interaction is primarily responsible for the dissolution of $NaCl$ in water?

(A)London dispersion forces between $Na^+$ ions and water molecules(B)Dipole-dipole attractions between $Cl^-$ ions and water molecules(C)Ion-dipole attractions between the ions ($Na^+$ and $Cl^-$) and polar water molecules(D)Hydrogen bonding between $NaCl$ and water molecules

3. **3.** [Skill: 4.A | Topic: 3.1] A biochemistry student investigates why a particular protein loses its functional shape (denatures) when moved from water into a nonpolar solvent. The student notes that the protein contains many -OH and -NH groups on its surface when folded in water. Which claim best explains why noncovalent interactions are crucial for the protein’s shape and why changing the solvent can cause denaturation?

(A)Covalent bonds within the protein are replaced by hydrogen bonds in nonpolar solvents, changing the primary structure of the protein.(B)Hydrogen bonding and other intermolecular attractions between polar groups and surrounding water help stabilize a specific folded shape; removing water disrupts these interactions and can change the folding.(C)Ion-dipole forces within a single protein molecule are stronger in nonpolar solvents, causing the protein to unfold.(D)London dispersion forces only occur in polar solvents, so the protein cannot maintain any attractions in a nonpolar solvent.

4. [Skill: 3.A | Topic: 3.10] A student investigates the solubility of several substances in two solvents at 25°C. The results are shown. Solubility results (25°C): - $NaNO_3$: soluble in water; insoluble in hexane - sucrose ($C_{12}H_{22}O_{11}$): soluble in water; insoluble in hexane - $I_2$: slightly soluble in water; soluble in hexane - octane ($C_8H_{18}$): insoluble in water; soluble in hexane Which of the following best explains the overall pattern in the data?

(A)Substances dissolve best in solvents that can form similar types of intermolecular attractions; water stabilizes ions and polar molecules, while hexane stabilizes nonpolar molecules via dispersion forces.(B)Substances dissolve best in solvents with the strongest intermolecular forces; because water has hydrogen bonding, it should dissolve all substances better than hexane.(C)Nonpolar solvents dissolve ionic compounds well because ions induce dipoles in nonpolar molecules, making ion-induced dipole interactions stronger than ion-dipole interactions.(D)Solubility is determined primarily by molar mass; because $I_2$ and octane have larger molar masses than $NaNO_3$, they dissolve better in hexane than in water.

5. [Skill: 1.A | Topic: 3.10] A student compares the solubility of an ionic compound in three solvents at 25°C. The solvents have the following properties: - Water: very polar; strong hydrogen-bonding; high dielectric constant - Acetone ($C_3H_6O$): polar; cannot donate hydrogen bonds; lower dielectric constant than water - Hexane ($C_6H_{14}$): nonpolar The student observes that $NaCl(s)$ dissolves readily in water, dissolves only very slightly in acetone, and does not dissolve in hexane. Which statement best explains why $NaCl(s)$ is much more soluble in water than in acetone?

(A)Water molecules can form strong ion-dipole interactions that stabilize $Na^+$ and $Cl^-$ in solution more effectively than acetone can.(B)Acetone is nonpolar, so it cannot interact with ions, whereas water is polar and can interact with ions.(C)Acetone molecules form stronger hydrogen bonds with each other than water molecules do, preventing acetone from dissolving ionic solids.(D)In water, $NaCl(s)$ reacts chemically to form new covalent bonds, but in acetone no reaction occurs.

Refer to the figure below.

6. [Skill: 1.B | Topic: 3.10] Four molecular substances are shown: 1. hexane ($C_6H_{14}$) 2. dimethyl ether ($CH_3OCH_3$) 3. ethanol ($CH_3CH_2OH$) 4. ethylene glycol ($HOCH_2CH_2OH$) Assuming similar amounts of each substance are added separately to water at 25°C and stirred, which substance is expected to be most soluble in water?

(A)hexane ($C_6H_{14}$)(B)dimethyl ether ($CH_3OCH_3$)(C)ethanol ($CH_3CH_2OH$)(D)ethylene glycol ($HOCH_2CH_2OH$)

Refer to the figure below.

7. **1. [Skill: 2.A | Topic: 3.11]** A student analyzes a sample of hydrogen chloride gas, $HCl(g)$, using different regions of the electromagnetic spectrum. [Image Cue]: Spectrum (absorbance vs. wavenumber), titled "Infrared Absorption Spectrum of HCl". x-axis: wavenumber (cm$^{-1}$) from 400 to 4000; y-axis: absorbance. Key feature: a strong absorption band centered near 2900 cm$^{-1}$. Which of the following best describes the molecular-level change responsible for the strong absorption band near 2900 cm$^{-1}$?

(A)Promotion of an electron in HCl to a higher-energy electronic state(B)Vibration of the H–Cl bond (change in bond length/energy levels associated with vibrational motion)(C)Rotation of the HCl molecule about its center of mass (change in rotational energy levels)(D)Ionization of HCl to form $H^+$ and $Cl^-$

8. **2. [Skill: 2.A | Topic: 3.11]** A scientist studies a polar diatomic gas in the gas phase using spectroscopy. The scientist observes absorption lines when the sample is exposed to radiation with frequencies on the order of $10^{10}$ Hz. Which of the following best identifies the type of transition occurring in the molecules at these frequencies?

(A)Rotational transitions of the molecules(B)Vibrational transitions of the chemical bonds(C)Electronic transitions involving valence electrons(D)Nuclear transitions involving changes in the nucleus

9. **3. [Skill: 1.B | Topic: 3.11]** A compound appears blue to the human eye. A UV-visible spectrometer shows that the compound strongly absorbs light with wavelengths in the range 590–620 nm. Which statement best explains why the compound appears blue and what molecular-level transition is associated with the absorption in this wavelength range?

(A)The compound absorbs blue light, and the absorption corresponds to rotational transitions(B)The compound absorbs orange/red light, so it transmits/reflicts the complementary color (blue); the absorption corresponds to electronic transitions(C)The compound absorbs orange/red light, so it emits blue light; the absorption corresponds to vibrational transitions(D)The compound absorbs orange/red light due to ionization, which makes the compound appear blue

10. **1.** [Skill: 2A | Topic: 3.12] In a flame test, a sample containing excited atoms emits light with a wavelength of 486 nm. Assume the emission corresponds to a single electronic transition in the atom. Which of the following is the energy of one emitted photon? (Use $h = 6.626\times10^{-34}\ \text{J·s}$ and $c = 3.00\times10^8\ \text{m·s}^{-1}$.)

(A)$4.09\times10^{-19}\ \text{J}$(B)$2.31\times10^{-19}\ \text{J}$(C)$1.36\times10^{-45}\ \text{J}$(D)$1.46\times10^{-6}\ \text{J}$

11. **2.** [Skill: 4A | Topic: 3.12] A student studies two events involving the same gaseous atom: • Event 1: The atom absorbs a photon with frequency $6.0\times10^{14}\ \text{s}^{-1}$. • Event 2: The atom emits a photon with frequency $5.0\times10^{14}\ \text{s}^{-1}$. Which statement best compares the energy change of the atom in Event 1 and Event 2?

(A)In Event 1 the atom’s energy increases by a larger amount than it decreases in Event 2.(B)In Event 1 the atom’s energy increases by a smaller amount than it decreases in Event 2.(C)In both events the magnitude of the atom’s energy change is the same because both involve one photon.(D)In Event 1 the atom’s energy decreases, and in Event 2 the atom’s energy increases.

Refer to the figure below.

12. **3.** [Skill: 1B | Topic: 3.12] A spectroscope is used to observe the emission spectrum from excited atoms of an element. Several bright lines are observed. [Image Cue]: Line emission spectrum, "Emission Spectrum of Element X". Horizontal axis labeled Wavelength (nm) from 350 to 700. Vertical axis labeled Relative Intensity (arbitrary units). Four narrow peaks at 410 nm, 434 nm, 486 nm, and 656 nm. Which observed line corresponds to the photon with the greatest energy (and thus the largest magnitude change in the atom’s electronic energy during emission)?

(A)410 nm(B)434 nm(C)486 nm(D)656 nm

13. [Skill: 5.A | Topic: 3.13] A student uses a spectrophotometer to determine the concentration of an aqueous dye solution. All absorbance measurements are taken using the same cuvette (path length held constant) and at the wavelength of maximum absorbance for the dye. The student prepares standard solutions and obtains the best-fit line for a calibration curve: $A = 250\,c$ where $A$ is absorbance (unitless) and $c$ is the dye concentration in M. An unknown dye solution gives an absorbance of 0.625 under the same conditions. What is the concentration of the dye in the unknown solution?

(A)0.00250 M(B)0.00156 M(C)0.00500 M(D)0.250 M

Refer to the figure below.

14. [Skill: 6.A | Topic: 3.13] A student is designing a procedure to determine the concentration of an aqueous solution of an ion that absorbs visible light. The student collects an absorption spectrum (absorbance vs. wavelength) for the ion at a fixed concentration and fixed path length. Based on the spectrum, the student chooses to collect all calibration-curve data at the wavelength of maximum absorbance, $\lambda_{\max}$. Which of the following best justifies this choice?

(A)At $\lambda_{\max}$, molar absorptivity is greatest, so absorbance changes most strongly with concentration, improving sensitivity.(B)At $\lambda_{\max}$, the path length of the cuvette is larger, causing the calibration curve to be more accurate.(C)At $\lambda_{\max}$, the solution transmits nearly all incident light, so absorbance measurements have the smallest uncertainty.(D)At $\lambda_{\max}$, absorbance no longer depends on concentration, so the calibration curve is not needed.

15. [Skill: 2.B | Topic: 3.2] A student tests four pure solids (W, X, Y, and Z). The results are shown. - W: melting point 1200°C; does not conduct as a solid; conducts when molten; brittle when struck - X: melting point 45°C; does not conduct as a solid; does not conduct when melted; soft - Y: melting point 1500°C; conducts as a solid; malleable (can be hammered into sheets) - Z: melting point 3500°C; does not conduct as a solid; extremely hard; does not readily melt under a laboratory burner Which choice correctly classifies W, X, Y, and Z based on the data?

(A)W is ionic, X is molecular, Y is metallic, and Z is a covalent network solid.(B)W is metallic, X is ionic, Y is molecular, and Z is a covalent network solid.(C)W is ionic, X is metallic, Y is covalent network, and Z is molecular.(D)W is covalent network, X is molecular, Y is ionic, and Z is metallic.

Refer to the figure below.

16. [Skill: 5.A | Topic: 3.2] Two liquids, Substance A and Substance B, have similar molar masses. Their vapor pressures are measured at several temperatures, and the results are plotted. In addition, the student is told that Substance A is $CH_3OCH_3$ (dimethyl ether) and Substance B is $CH_3CH_2OH$ (ethanol). Which choice best uses the vapor-pressure information to explain a difference in boiling point between A and B?

(A)Substance A has stronger intermolecular forces than Substance B, so A has a lower vapor pressure at each temperature and a higher boiling point.(B)Substance B has stronger intermolecular forces than Substance A, so B has a lower vapor pressure at each temperature and a higher boiling point.(C)Substance A has weaker intermolecular forces than Substance B, so A has a lower vapor pressure at each temperature and a higher boiling point.(D)Substance B has weaker intermolecular forces than Substance A, so B has a lower vapor pressure at each temperature and a higher boiling point.

Refer to the figure below.

17. [Skill: 1.A | Topic: 3.3] A teacher shows four particulate-level diagrams (not to scale) representing samples of a pure substance at the same temperature. Each circle represents one particle. Which diagram best represents an amorphous solid?

(A)Particles are closely packed in a repeating, ordered 3D arrangement; particles appear fixed in position.(B)Particles are closely packed with no long-range repeating pattern; particles appear fixed in position.(C)Particles are in close contact but disordered; particles are shown moving past one another with some empty space between clusters.(D)Particles are far apart with large empty space; particles are shown moving randomly throughout the container.

18. [Skill: 4.A | Topic: 3.3] Three sealed, frictionless pistons contain equal amounts (in moles) of three different substances at the same temperature and the same initial pressure. The pistons are then compressed until the external pressure is doubled while the temperature is held constant. Which piston will show the greatest decrease in volume, and what is the best particulate-level explanation?

(A)The solid; solids have the largest initial spacing between particles, so their volume decreases the most when pressure increases.(B)The liquid; liquid particles are constantly moving and colliding, so they can be pushed much closer together when pressure increases.(C)The gas; gas particles are far apart, so increased pressure significantly decreases the empty space between particles.(D)All three decrease by the same amount because doubling pressure at constant temperature halves the volume for any phase.

19. **1. [Skill: 2.A | Topic: 3.4]** A sample of gas is contained in a rigid 5.00 L vessel at 300 K. The gas is a mixture of $O_2(g)$ and $Ar(g)$. The total pressure in the vessel is measured to be 2.46 atm. It is known that the vessel contains 0.200 mol of $O_2(g)$. Assuming ideal behavior, how many moles of $Ar(g)$ are in the vessel?

(A)0.100 mol(B)0.300 mol(C)0.500 mol(D)0.700 mol

Refer to the figure below.

20. **2. [Skill: 4.A | Topic: 3.4]** A student investigates the relationship between the pressure and temperature of a fixed amount of gas in a rigid container. The student records several $(T, P)$ pairs and plots pressure versus temperature. Based on the graph shown, which change would make the data representation most consistent with the ideal gas law relationship for a constant-volume sample?

(A)Convert the temperature values from degrees Celsius to kelvins and replot $P$ vs. $T$(B)Convert the pressure values from atm to kPa and replot $P$ vs. $T$(C)Replot the data as $V$ vs. $T$ without changing units(D)Replot the data as $P$ vs. $\dfrac{1}{T}$ using degrees Celsius

Refer to the figure below.

21. **1. [Skill: 2.B | Topic: 3.5]** A sample of an ideal gas is analyzed at two different temperatures, $T_1$ and $T_2$, with $T_2 > T_1$. The Maxwell-Boltzmann speed distributions for the same gas at the two temperatures are shown. Which of the following best describes the changes from $T_1$ to $T_2$?

(A)The distribution shifts to higher speeds and becomes broader, indicating a greater fraction of particles at higher speeds and an increased average kinetic energy.(B)The distribution shifts to lower speeds and becomes narrower, indicating a smaller fraction of particles at higher speeds and an increased average kinetic energy.(C)The peak height increases while the most probable speed decreases, because particles move more slowly at higher temperature due to more collisions.(D)The curve remains the same because the total number of particles is constant, so temperature does not affect the speed distribution.

22. **2. [Skill: 1.B | Topic: 3.5]** Two rigid, sealed containers of equal volume contain the same gas. Each container holds the same number of gas particles. Container 1 is at temperature $T_1$, and Container 2 is at a higher temperature $T_2$. Which of the following best explains how and why the pressure in Container 2 compares to the pressure in Container 1?

(A)The pressure in Container 2 is higher because particles have greater average kinetic energy at $T_2$, leading to more frequent and/or more forceful collisions with the container walls.(B)The pressure in Container 2 is higher because increasing temperature increases the volume available to the gas particles, which increases collisions with the walls.(C)The pressure in Container 2 is lower because faster particles spend less time in contact with the walls during collisions, reducing force on the walls.(D)The pressure is the same in both containers because pressure depends only on the number of particles and the volume, not on temperature.

23. [Skill: 4.A | Topic: 3.6] A student calculates the compressibility factor, $Z=\dfrac{PV}{nRT}$, for samples of two gases under different conditions. Data: - Sample 1: $CO_2(g)$ at 250 K and 20 atm has $Z=0.88$ - Sample 2: $CO_2(g)$ at 450 K and 20 atm has $Z=1.05$ - Sample 3: $He(g)$ at 250 K and 20 atm has $Z=1.01$ Which statement best explains why Sample 1 deviates from ideal behavior in the observed direction?

(A)At 250 K, attractive intermolecular forces between $CO_2$ particles are significant, lowering the measured pressure relative to the ideal prediction ($Z<1$).(B)At 250 K, the finite volume of $CO_2$ particles is dominant, lowering the measured pressure relative to the ideal prediction ($Z<1$).(C)Because $CO_2$ is heavier than $He$, the particles collide less often, making $Z<1$ at any temperature and pressure.(D)Because the pressure is high, attractions must be negligible, so $Z<1$ indicates the gas is behaving ideally.

Refer to the figure below.

24. [Skill: 2.A | Topic: 3.6] [Image Cue]: Graph, "Compressibility Factor vs. Pressure at Constant Temperature (300 K)", x-axis: Pressure (atm) from 0 to 300, y-axis: $Z$ from 0.7 to 1.3. Two curves: (1) $NH_3(g)$ starts near $Z=1.00$ at low pressure, dips to about $Z=0.80$ around 50 atm, then rises above 1 and reaches about $Z=1.20$ by 300 atm. (2) $Ar(g)$ stays close to 1 at low pressure and rises gradually to about $Z=1.08$ by 300 atm. Which statement best explains the behavior of the $NH_3(g)$ curve across the pressure range?

(A)At intermediate pressures, attractive intermolecular forces in $NH_3$ cause $Z<1$, and at very high pressures, finite particle volume causes $Z>1$.(B)At intermediate pressures, the finite particle volume in $NH_3$ causes $Z<1$, and at very high pressures, attractive forces cause $Z>1$.(C)$NH_3$ has stronger attractions than $Ar$, so $NH_3$ must have $Z<1$ at all pressures shown.(D)Because $NH_3$ is polar, it behaves more ideally than $Ar$ at all pressures shown, so the curve should remain at $Z=1$.

25. [Skill: 2.B | Topic: 3.7] A student prepares a mixture by dissolving 5.85 g of NaCl(s) in water and diluting the mixture to a final volume of 500.0 mL in a volumetric flask. After mixing thoroughly, the student removes a 10.00 mL sample from the top of the solution and a 10.00 mL sample from the bottom. After evaporating the water, the top sample leaves 0.117 g NaCl and the bottom sample leaves 0.118 g NaCl. Which of the following conclusions about the mixture is best supported by the data?

(A)The mixture is heterogeneous, and its concentration is approximately 0.0200 M.(B)The mixture is heterogeneous, and its concentration is approximately 0.200 M.(C)The mixture is a homogeneous solution, and its concentration is approximately 0.200 M.(D)The mixture is a homogeneous solution, and its concentration is approximately 2.00 M.

26. [Skill: 2.B | Topic: 3.7] A bottle is labeled 0.750 M $CaCl_2(aq)$. A student needs a sample that contains $1.20\times10^{23}$ chloride ions, $Cl^-$. What volume of the 0.750 M $CaCl_2(aq)$ should the student measure to obtain this number of chloride ions?

(A)66.3 mL(B)133 mL(C)266 mL(D)0.133 mL

Refer to the figure below.

27. [Skill: 1A | Topic: 3.8] A student dissolves a small crystal of sodium chloride in water and then uses a conductivity probe. The solution conducts electricity. Which particulate model best represents the solution and explains why it conducts electricity? [Image Cue]: Particulate diagram, "NaCl(aq) in water", A single container showing many water molecules (bent shapes with O labeled as δ− and H labeled as δ+). The correct model must show Na+ and Cl− ions separated (not paired as NaCl units) and each ion surrounded by oriented water molecules (O atoms pointing toward Na+, H atoms pointing toward Cl−). Incorrect options should include at least one of: undissociated NaCl units, ion pairs stuck together, or incorrect water orientation.

(A)The container shows separated Na+ and Cl− ions dispersed throughout the water, with water molecules oriented so that O faces Na+ and H faces Cl−.(B)The container shows neutral NaCl formula units (paired Na+ and Cl− touching) dispersed throughout the water, with no consistent water orientation.(C)The container shows separated Na+ and Cl− ions, but water molecules are oriented so that H faces Na+ and O faces Cl−.(D)The container shows only neutral NaCl(s) clustered at the bottom of the container, with water molecules above it.

Refer to the figure below.

28. [Skill: 1A | Topic: 3.8] Two solutions, X and Y, are made with the same solvent (water) and the same solute (an ionic compound that dissociates completely). Equal volumes of the two solutions are represented by the particulate diagrams. In the diagrams, each ion is shown as a labeled sphere, and each diagram contains the same number of water molecules. Solution X shows 8 cations and 8 anions. Solution Y shows 4 cations and 4 anions. Which statement best compares the solute concentrations of solutions X and Y? [Image Cue]: Particulate diagram, "Comparing solution concentrations", Two same-sized containers labeled X and Y. Each container shows the same number of water molecules. Container X has 8 separated cations (+) and 8 separated anions (−) dispersed. Container Y has 4 separated cations (+) and 4 separated anions (−) dispersed. (No ion pairs; ions are separated.)

(A)Solution X has twice the solute concentration of solution Y because it contains twice as many dissolved ions (and thus twice as many formula units) in the same volume.(B)Solution X has four times the solute concentration of solution Y because it contains four times as many ions in the same volume.(C)Solution Y has twice the solute concentration of solution X because a smaller number of ions indicates stronger attractions to water.(D)The two solutions have the same solute concentration because both contain equal numbers of water molecules.

29. [Skill: 4.A | Topic: 3.9] A student uses thin-layer chromatography (TLC) to separate three dissolved, colorless organic compounds (X, Y, and Z) in a mixture. The stationary phase is silica gel (very polar, capable of hydrogen bonding). The mobile phase is a mostly nonpolar solvent. After developing the TLC plate, the student marks the origin line, the solvent front, and the centers of the three spots. The measured distances are shown. Distance from origin to solvent front: 8.0 cm Distance from origin to spot X: 1.6 cm Distance from origin to spot Y: 4.0 cm Distance from origin to spot Z: 6.4 cm Which statement best explains the result?

(A)Compound Z experiences the strongest intermolecular attractions to the silica gel, so it travels farthest with the mobile phase.(B)Compound X experiences the strongest intermolecular attractions to the silica gel, so it travels the least distance from the origin.(C)Compound Y has the highest boiling point, so it travels an intermediate distance on the TLC plate.(D)All three compounds must have identical intermolecular forces because they were dissolved in the same solvent.

30. [Skill: 4.B | Topic: 3.9] A student performs a simple distillation on a homogeneous solution containing two miscible liquids, A and B, at 1 atm. The student is provided the information below. Liquid A: boiling point = 56 °C; molecules are polar but cannot hydrogen bond to each other Liquid B: boiling point = 100 °C; molecules are polar and can hydrogen bond to each other Assuming no chemical reaction occurs, which statement best predicts what happens early in the distillation and why?

(A)The first distillate is enriched in B because hydrogen bonding increases vapor pressure, making B evaporate more readily.(B)The first distillate is enriched in A because A has weaker intermolecular forces, giving it a higher vapor pressure at a given temperature.(C)The first distillate contains equal amounts of A and B because miscible liquids must evaporate in the same ratio as in the solution.(D)The first distillate is enriched in B because stronger intermolecular forces cause a lower boiling point, so B boils first.

B. Short Answer Questions (1 Question)

Answer all parts of each question. Answers must be in essay form. Outlines or lists alone are not acceptable.

Question 31:

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