Learn Extracted exam questions AP Chemistry 2022 Free Response
2022 Free Response
Source PDF on the left, extracted YAML on the right. Compare numbering, marks, options and text.
A student reacts 0.300 g of methyl salicylate $(\text{C}_8\text{H}_8\text{O}_3)$ with a stoichiometric amount of a strong base. This product is then acidified to produce salicylic acid crystals $(\text{HC}_7\text{H}_5\text{O}_3)$.
For every 1 mole of $\text{C}_8\text{H}_8\text{O}_3$ (molar mass 152.15 g/mol) reactant used, 1 mole of salicylic acid crystals $(\text{HC}_7\text{H}_5\text{O}_3$, molar mass 138.12 g/mol) is produced. Calculate the maximum mass, in grams, of $\text{HC}_7\text{H}_5\text{O}_3$ that could be produced in this reaction.
As part of the experimental procedure to purify the $\text{HC}_7\text{H}_5\text{O}_3$ crystals after the reaction is complete, the crystals are filtered from the reaction mixture, rinsed with distilled water, and dried. Some physical properties of $\text{HC}_7\text{H}_5\text{O}_3$ are given in the following table.
| Properties of Salicylic Acid $(\text{HC}_7\text{H}_5\text{O}_3)$ | |
|---|---|
| Melting point | 159°C |
| Solubility in $\text{H}_2\text{O}$ at 25°C | 2.2 g/L |
| Specific heat capacity | 1.17 J/(g·°C) |
| Heat of fusion | 27.1 kJ/mol |
The student's experiment results in an 87% yield of dry $\text{HC}_7\text{H}_5\text{O}_3$. The student suggests that some of the $\text{HC}_7\text{H}_5\text{O}_3$ crystals dissolved in the distilled water during the rinsing step. Is the student's claim consistent with the calculated percent yield value? Justify your answer.
Given the physical properties in the table, calculate the quantity of heat that must be absorbed to increase the temperature of a 0.105 g sample of dry $\text{HC}_7\text{H}_5\text{O}_3$ (molar mass 138.12 g/mol) crystals from $25^\circ\text{C}$ to the melting point $159^\circ\text{C}$ and melt the crystals completely.
The structures and melting points for methyl salicylate and salicylic acid are shown.
[Two skeletal/Lewis structural formulas of benzene-ring compounds are shown side by side, drawn with explicit H atoms and lone pairs. Left: Methyl Salicylate, Melting Point: $-9^\circ\text{C}$ — a benzene ring with an ester group $-\text{C}(=\text{O})-\text{O}-\text{CH}_3$ and an adjacent $-\text{O}-\text{H}$ hydroxyl group on the ring. Right: Salicylic Acid, Melting Point: $159^\circ\text{C}$ — the same benzene ring but with a carboxylic acid group $-\text{C}(=\text{O})-\text{O}-\text{H}$ and the adjacent $-\text{O}-\text{H}$ hydroxyl group on the ring.]
The same three types of intermolecular forces (London dispersion forces, dipole-dipole interactions, and hydrogen bonding) exist among molecules of each substance. Explain why the melting point of salicylic acid is higher than that of methyl salicylate.
The student titrates 20.0 mL of $0.0100\ M\ \text{HC}_7\text{H}_5\text{O}_3(aq)$ with $0.0200\ M$ NaOH, using a probe to monitor the pH of the solution. The data are plotted producing the following titration curve.
[Titration curve graph: x-axis "Volume of 0.0200 M NaOH Added (mL)" from 0 to 14; y-axis "pH" from 0 to 14. The curve starts near pH 2-3 at 0 mL, rises gradually and gently through the buffer region, then shows a steep equivalence-point jump at approximately 10 mL (rising from about pH 7 to pH 11), then levels off near pH 12-13 as more NaOH is added.]
Using the information in the graph, estimate the $\text{p}K_a$ of $\text{HC}_7\text{H}_5\text{O}_3$. ______
When the pH of the titration mixture is 4.00, is there a higher concentration of the weak acid, $\text{HC}_7\text{H}_5\text{O}_3$, or its conjugate base, $\text{C}_7\text{H}_5\text{O}_3^-$, in the flask? Justify your answer.
The student researches benzoic acid $(\text{HC}_7\text{H}_5\text{O}_2)$ and finds that it has similar properties to salicylic acid $(\text{HC}_7\text{H}_5\text{O}_3)$. The $K_a$ for benzoic acid is $6.3 \times 10^{-5}$. Calculate the value of $\text{p}K_a$ for benzoic acid.
The student performs a second titration, this time titrating 20.0 mL of a $0.0100\ M$ benzoic acid solution with $0.0200\ M$ NaOH. Sketch the curve that would result from this titration of benzoic acid on the following graph, which already shows the original curve from the titration of $20.0\ \text{mL}$ of $0.0100\ M$ salicylic acid. The initial pH of the benzoic acid solution is 3.11.
[A second copy of the titration curve graph, titled "Titration of Salicylic Acid with NaOH," showing the same original salicylic-acid curve as in part (d) (x-axis "Volume of 0.0200 M NaOH Added (mL)" from 0 to 14; y-axis "pH" from 0 to 14; gradual rise from about pH 2-3 through a buffer region, then a steep jump near 10 mL up to about pH 11, leveling off near pH 12-13), onto which the student is to sketch the new benzoic-acid titration curve.]
Methanol vapor decomposes to form carbon monoxide gas and hydrogen gas at high temperatures in the presence of a platinum catalyst, as represented by the balanced chemical equation given.
Are the hydrogen atoms oxidized or are they reduced in the forward reaction? Justify your answer in terms of oxidation numbers.
In the following box, draw the complete Lewis electron-dot diagram for the carbon monoxide molecule in which every atom obeys the octet rule. Show all bonding and nonbonding valence electrons.
[A blank box is provided containing the unconnected atomic symbols "C" and "O", for the student to complete into a full Lewis structure.]
The values of the standard molar entropies of the compounds involved in the reaction are given in the following table.
| Substance | S° (J/(K·mol)) |
|---|---|
| $\text{CH}_3\text{OH}(g)$ | 240. |
| $\text{CO}(g)$ | 198 |
| $\text{H}_2(g)$ | 131 |
Use the data in the table to calculate the value of the standard entropy change, $\Delta S^\circ$, in $\text{J/(K·mol}_{rxn})$, for the reaction.
Calculate the value of $\Delta G^\circ$, in $\text{kJ/mol}_{rxn}$, for the reaction at 375 K. Assume that $\Delta H^\circ$ and $\Delta S^\circ$ are independent of temperature.
The following particle-level diagram shows a representative sample of the equilibrium mixture represented by the equation given.
[A rectangular box (the "container") contains a mixture of particles at equilibrium: several two-linked-open-circle pairs (H₂), and several three-part particles each made of one gray sphere + one black sphere + one small open circle (CO), plus one larger cluster shown as CH₃OH. A key box beside it identifies: a cluster of one gray sphere + one black sphere + three small open circles = $\text{CH}_3\text{OH}$; one gray sphere + one black sphere = CO; two small open circles joined = $\text{H}_2$. Counting the container: there appear to be 1 CH₃OH particle, 4 CO particles, and 8 H₂ particles.]
Use information from the particle diagram to calculate the partial pressure of CO at equilibrium when the total pressure of the equilibrium mixture is 12.0 atm.
Write the expression for the equilibrium constant, $K_p$, for the reaction.
The reaction system represented by the equation is allowed to achieve equilibrium at a different temperature. The following table gives the partial pressure of each species in the equilibrium mixture.
| Substance | Partial Pressure at Different Temperature |
|---|---|
| $\text{CH}_3\text{OH}(g)$ | 2.7 atm |
| $\text{CO}(g)$ | 4.2 atm |
| $\text{H}_2(g)$ | 8.4 atm |
Use the information in the table to calculate the value of the equilibrium constant, $K_p$, at the new temperature.
The volume of the container is rapidly doubled with no change in temperature. As equilibrium is re-established, does the number of moles of $\text{CH}_3\text{OH}(g)$ increase, decrease, or remain the same? Justify your answer by comparing the value of the reaction quotient, $Q$, with the value of the equilibrium constant, $K_p$.
Answer the following questions relating to the element aluminum, Al.
Write the complete ground-state electron configuration of an Al atom.
Based on principles of atomic structure, explain why the radius of the Al atom is larger than the radius of the $\text{Al}^{3+}$ ion.
A student plans to combine solid aluminum with an aqueous solution of silver ions. The student determines the mass of solid $\text{AgNO}_3$ needed to prepare the solution with a specific concentration.
In the following table, briefly list the steps necessary to prepare 200.0 mL of an aqueous solution of $\text{AgNO}_3$ using only equipment selected from the choices given. Assume that all appropriate safety measures are already in place. Not all equipment or lines in the table may be needed.
Available equipment: Solid $\text{AgNO}_3$ · Weighing paper and scoop · 250 mL beakers · Distilled water · 200.00 mL volumetric flask · Pipet · Balance · 50.0 mL graduated cylinder
| Step | Step Description |
|---|---|
| 1. | Use weighing paper to measure the determined mass of solid $\text{AgNO}_3$ on a balance. |
| 2. | |
After preparing the solution, the student places some of the solution into a beaker and adds a sample of aluminum. The reaction represented by the following equation occurs.
The following diagram gives an incomplete particulate representation of the reaction. The beaker on the left represents the system before the mixture reacts. Complete the drawing on the right to represent the system after the reaction has occurred. Be sure to include 1) the correct type and number of particles based on the number shown on the left and 2) the relative spacing to depict the appropriate phases.
[Two beaker diagrams side by side connected by a rightward arrow. Left beaker ("before reaction"): shown containing 4 "Al" (shaded circles, representing solid Al atoms clustered at the bottom) and 6 "Ag$^+$" (open circles, representing dissolved silver ions distributed throughout the liquid). Right beaker ("after reaction," to be completed by the student): shown only with 2 "Al" solid particles remaining at the bottom, with the rest of the beaker left blank for the student to draw in the products. A key box below states: "Al" (shaded circle) = Al atom; "Ag" (open circle) = Ag atom; "$\text{Al}^{3+}$" (circle) = $\text{Al}^{3+}$ ion; "$\text{Ag}^+$" (open circle) = $\text{Ag}^+$ ion.]
The student finds the standard reduction potentials given in the table, which are related to the reaction that occurs.
| Half-Reaction | $E^\circ$ |
|---|---|
| $\text{Ag}^+(aq) + e^- \rightarrow \text{Ag}(s)$ | 0.80 V |
| $\text{Al}^{3+}(aq) + 3\,e^- \rightarrow \text{Al}(s)$ | $-1.66$ V |
Using the standard reduction potentials, calculate the value of $E^\circ$ for the reaction.
Based on the value of $E^\circ$, would the standard free energy change of the reaction under standard conditions, $\Delta G^\circ$, be positive, negative, or zero? Justify your answer.
Once the reaction appears to stop progressing, would the change in free energy, $\Delta G$, be positive, negative, or zero? Justify your answer.
Answer the following questions about the compounds $\text{NH}_2\text{Cl}$ and $\text{NCl}_3$. The Lewis electron-dot diagrams of the two compounds are shown.
[Two Lewis electron-dot structures shown side by side. Left ($\text{NH}_2\text{Cl}$): a central N atom bonded to two H atoms and one Cl atom, with one lone pair on N and three lone pairs on Cl — drawn as H–N(–H)–Cl with the lone pair over N and three lone pairs around Cl. Right ($\text{NCl}_3$): a central N atom bonded to three Cl atoms, with one lone pair on N and three lone pairs on each Cl atom — drawn as Cl–N(–Cl)–Cl with a lone pair over N and three lone pairs around each Cl.]
Calculate the number of moles of $\text{NH}_2\text{Cl}$ (molar mass 51.48 g/mol) present in 1.0 L of a solution in which the concentration of $\text{NH}_2\text{Cl}$ is 0.0016 g/L.
$\text{NH}_2\text{Cl}$ is highly soluble in water, whereas $\text{NCl}_3$ is nearly insoluble. Explain this observation in terms of the types and relative strengths of the intermolecular forces between each of the solutes and water.
The value of $\Delta H^\circ_{vaporization}$ for $\text{NCl}_3(l)$ is 32.9 kJ/mol. Calculate the amount of energy required to vaporize a 15.0 g sample of $\text{NCl}_3$ (molar mass 120.36 g/mol).
The following equation represents the decomposition of $\text{N}_2\text{O}_5$, for which the rate law is $\text{rate} = k[\text{N}_2\text{O}_5]$.
A sample of pure $\text{N}_2\text{O}_5(g)$ is placed in an evacuated container and allowed to decompose at a constant temperature of 300 K. The concentration of $\text{N}_2\text{O}_5(g)$ in the container is measured over a period of time, and the measurements are recorded in the following table.
| Time (hr) | $[\text{N}_2\text{O}_5]\ (M)$ |
|---|---|
| 0 | 0.160 |
| 1.67 | 0.0800 |
| 3.33 | 0.0400 |
| 5.00 | 0.0200 |
Determine the value of the rate constant, $k$, for the reaction. Include units in your answer.
The following mechanism is proposed for the decomposition of $\text{N}_2\text{O}_5(g)$.
Step 1: $\text{N}_2\text{O}_5(g) \rightarrow \text{NO}_2(g) + \text{NO}_3(g)$
Step 2: $\text{NO}_2(g) + \text{NO}_3(g) \rightarrow \text{NO}_2(g) + \text{NO}(g) + \text{O}_2(g)$
Step 3: $\text{N}_2\text{O}_5(g) + \text{NO}(g) \rightarrow 3\,\text{NO}_2(g)$
Identify which step of the proposed mechanism (1, 2, or 3) is the rate-determining step. Justify your answer in terms of the rate law given.
If this experiment was repeated at the same temperature but with twice the initial concentration of $\text{N}_2\text{O}_5$, would the value of $k$ increase, decrease, or remain the same? Explain your reasoning.
A student wants to determine the concentration of permanganate, $\text{MnO}_4^-(aq)$, in a solution. The student plans to use colorimetric analysis because solutions containing $\text{MnO}_4^-(aq)$ have a purple color.
[Graph: "Absorbance" (y-axis, 0.00 to 1.00) vs. "Wavelength (nm)" (x-axis, 400 to 750). The curve begins near 0 absorbance at 400 nm, rises to a local peak near 0.80 around 510-520 nm, dips slightly to about 0.75 near 530-540 nm, rises again to a second peak of about 0.85 near 545-555 nm, then falls steeply back toward 0 absorbance by about 580-600 nm, remaining near 0 from 600-750 nm. Vertical gridlines mark 400, 450, 500, 550, 600, 650, 700, 750 nm.]
To determine the optimum wavelength for an experiment that measures the concentration of $\text{MnO}_4^-(aq)$, the student takes a sample of the solution and measures the amount of light absorbed by the sample over a range of wavelengths. The data are plotted in the graph shown. Identify the optimum wavelength that the student should use for the experimental procedure.
The student uses a stock solution of $2.40 \times 10^{-3}\ M\ \text{KMnO}_4(aq)$ to prepare the standard solutions of $\text{MnO}_4^-(aq)$ that are needed to construct a calibration curve.
[Diagram of a 100.0 mL graduated cylinder with a magnified circular inset showing the liquid meniscus between the "100" and "90" mL gradation marks; the shaded liquid level in the inset sits a little above the "90" mark, roughly at the 92-93 mL region.]
The student uses a 100.0 mL graduated cylinder to measure a certain volume of $\text{KMnO}_4(aq)$ stock solution, as shown in the diagram given. What volume should the student record?
Calculate the volume, in mL, of $2.40 \times 10^{-3}\ M\ \text{KMnO}_4(aq)$ that is required to produce 100.0 mL of a standard $1.68 \times 10^{-3}\ M\ \text{MnO}_4^-(aq)$ solution.
The student designs the following procedure to produce a calibration curve.
Step 1: Prepare several standard solutions that have known $\text{MnO}_4^-(aq)$ concentrations by dilution of the stock solution.
Step 2: Rinse the cuvette with distilled water.
Step 3: Rinse the cuvette with the standard solution and fill the cuvette with the standard solution.
Step 4: Measure the absorbance of the standard solution with the colorimeter.
Step 5: Repeat steps 2-4 for each of the standard solutions.
The data are plotted in the calibration curve shown. One of the data points (indicated with an arrow) on the calibration curve is below the line of best fit.
[Calibration curve graph: x-axis "$[\text{MnO}_4^-]$" from 0 to 0.0025 (gridlines at 0.0005, 0.0010, 0.0015, 0.0020, 0.0025); y-axis "Absorbance" from 0 to 0.9 (gridlines every 0.1). Data points rise roughly linearly from (0,0) to about (0.0024, 0.85), lying close to a straight line of best fit drawn through them. One data point near $x \approx 0.0011$, $y \approx 0.32$ lies noticeably below the line of best fit (the line at that x-value would be around 0.40-0.42); this point is indicated with an arrow pointing to it.]
Assuming that all lab equipment is functioning properly, identify which one of the procedural steps the student could have executed incorrectly that would explain why the marked data point is below the line of best fit. Justify your answer.
A Lewis electron-dot diagram of the oxalate ion, $\text{C}_2\text{O}_4^{2-}$, is shown.
[A Lewis electron-dot structure of the oxalate ion $\text{C}_2\text{O}_4^{2-}$: two carbon atoms singly bonded to each other, each carbon also double-bonded to one O atom and singly bonded to another O atom bearing a negative charge, with appropriate lone pairs shown on all oxygen atoms, consistent with the resonance-averaged/standard depiction of oxalate.]
Identify the hybridization of the valence orbitals of either carbon atom in the oxalate ion.
Silver oxalate, $\text{Ag}_2\text{C}_2\text{O}_4(s)$, is slightly soluble in water. The value of $K_{sp}$ for $\text{Ag}_2\text{C}_2\text{O}_4$ is $5.40 \times 10^{-12}$.
Write the expression for the solubility-product constant, $K_{sp}$, for $\text{Ag}_2\text{C}_2\text{O}_4$.
Calculate the molar solubility of $\text{Ag}_2\text{C}_2\text{O}_4$ in neutral distilled water.
The molar solubility of $\text{Ag}_2\text{C}_2\text{O}_4$ increases when it is dissolved in $0.5\ M\ \text{HClO}_4(aq)$ instead of neutral distilled water. Write a balanced, net-ionic equation for the process that occurs between species in solution that contributes to the increased solubility of $\text{Ag}_2\text{C}_2\text{O}_4(aq)$ in $\text{HClO}_4(aq)$.