Learn Extracted exam questions AP Chemistry 2014 Free Response
2014 Free Response
Source PDF on the left, extracted YAML on the right. Compare numbering, marks, options and text.
[Table of experimental data:]
| Mass of KI tablet | 0.425 g |
|---|---|
| Mass of thoroughly dried filter paper | 1.462 g |
| Mass of filter paper + precipitate after first drying | 1.775 g |
| Mass of filter paper + precipitate after second drying | 1.699 g |
| Mass of filter paper + precipitate after third drying | 1.698 g |
A student is given the task of determining the $\text{I}^-$ content of tablets that contain KI and an inert, water-soluble sugar as a filler. A tablet is dissolved in 50.0 mL of distilled water, and an excess of 0.20 $M$ $\text{Pb(NO}_3\text{)}_2(aq)$ is added to the solution. A yellow precipitate forms, which is then filtered, washed, and dried. The data from the experiment are shown in the table above.
For the chemical reaction that occurs when the precipitate forms, write a balanced, net-ionic equation for the reaction.
For the chemical reaction that occurs when the precipitate forms, explain why the reaction is best represented by a net-ionic equation.
Explain the purpose of drying and weighing the filter paper with the precipitate three times.
In the filtrate solution, is $[\text{K}^+]$ greater than, less than, or equal to $[\text{NO}_3{}^-]$? Justify your answer.
Calculate the number of moles of precipitate that is produced in the experiment.
Calculate the mass percent of $\text{I}^-$ in the tablet.
In another trial, the student dissolves a tablet in 55.0 mL of water instead of 50.0 mL of water. Predict whether the experimentally determined mass percent of $\text{I}^-$ will be greater than, less than, or equal to the amount calculated in part (e). Justify your answer.
A student in another lab also wants to determine the $\text{I}^-$ content of a KI tablet but does not have access to $\text{Pb(NO}_3\text{)}_2$. However, the student does have access to 0.20 $M$ $\text{AgNO}_3$, which reacts with $\text{I}^-(aq)$ to produce $\text{AgI}(s)$. The value of $K_{sp}$ for AgI is $8.5 \times 10^{-17}$.
Will the substitution of $\text{AgNO}_3$ for $\text{Pb(NO}_3\text{)}_2$ result in the precipitation of the $\text{I}^-$ ion from solution? Justify your answer.
The student only has access to one KI tablet and a balance that can measure to the nearest 0.01 g. Will the student be able to determine the mass of AgI produced to three significant figures? Justify your answer.
Propanoic acid, $\text{CH}_3\text{CH}_2\text{COOH}$, is a carboxylic acid that reacts with water according to the equation above. At $25^\circ\text{C}$ the pH of a 50.0 mL sample of 0.20 $M$ $\text{CH}_3\text{CH}_2\text{COOH}$ is 2.79.
Identify a Brønsted-Lowry conjugate acid-base pair in the reaction. Clearly label which is the acid and which is the base.
Determine the value of $K_a$ for propanoic acid at $25^\circ\text{C}$.
For each of the following statements, determine whether the statement is true or false. In each case, explain the reasoning that supports your answer.
The pH of a solution prepared by mixing the 50.0 mL sample of 0.20 $M$ $\text{CH}_3\text{CH}_2\text{COOH}$ with a 50.0 mL sample of 0.20 $M$ NaOH is 7.00.
For each of the following statements, determine whether the statement is true or false. In each case, explain the reasoning that supports your answer.
If the pH of a hydrochloric acid solution is the same as the pH of a propanoic acid solution, then the molar concentration of the hydrochloric acid solution must be less than the molar concentration of the propanoic acid solution.
A student is given the task of determining the concentration of a propanoic acid solution of unknown concentration. A 0.173 $M$ NaOH solution is available to use as the titrant. The student uses a 25.00 mL volumetric pipet to deliver the propanoic acid solution to a clean, dry flask. After adding an appropriate indicator to the flask, the student titrates the solution with the 0.173 $M$ NaOH, reaching the end point after 20.52 mL of the base solution has been added.
Calculate the molarity of the propanoic acid solution.
The student is asked to redesign the experiment to determine the concentration of a butanoic acid solution instead of a propanoic acid solution. For butanoic acid the value of $\text{p}K_a$ is 4.83. The student claims that a different indicator will be required to determine the equivalence point of the titration accurately. Based on your response to part (b), do you agree with the student's claim? Justify your answer.
[Diagram of a standard galvanic cell: a Cu electrode in a beaker of 1.0 $M$ $\text{Cu(NO}_3\text{)}_2$ and a Sn electrode in a beaker of 1.0 $M$ $\text{Sn(NO}_3\text{)}_2$, connected by a KNO₃ salt bridge; wires from each electrode run up to a switch and a voltmeter.]
A student is given a standard galvanic cell, represented above, that has a Cu electrode and a Sn electrode. As current flows through the cell, the student determines that the Cu electrode increases in mass and the Sn electrode decreases in mass.
Identify the electrode at which oxidation is occurring. Explain your reasoning based on the student's observations.
As the mass of the Sn electrode decreases, where does the mass go?
In the expanded view of the center portion of the salt bridge shown in the diagram below, draw and label a particle view of what occurs in the salt bridge as the cell begins to operate. Omit solvent molecules and use arrows to show the movement of particles.
[Diagram: a dashed circle showing an expanded view of the KNO₃ salt bridge's center portion, drawn as a horizontal tube (empty, for the student to fill in with a particle diagram).]
When heated, calcium carbonate decomposes according to the equation above. In a study of the decomposition of calcium carbonate, a student added a 50.0 g sample of powdered $\text{CaCO}_3(s)$ to a 1.00 L rigid container. The student sealed the container, pumped out all the gases, then heated the container in an oven at 1100 K. As the container was heated, the total pressure of the $\text{CO}_2(g)$ in the container was measured over time. The data are plotted in the graph below.
[Graph of Pressure (atm) vs. Time (min); x-axis Time (min) 0 to 20 (gridlines at 0, 5, 10, 15, 20), y-axis Pressure (atm) 0 to 1.25 (gridlines at 0, 0.25, 0.50, 0.75, 1.00, 1.25); curve starts at (0, 0), rises steeply and roughly linearly to about (5, 0.65), continues rising with decreasing slope through about (10, 0.95), levels off and becomes essentially flat at a pressure of about 1.0-1.05 atm from about $t=12$ min to $t=20$ min.]
The student repeated the experiment, but this time the student used a 100.0 g sample of powdered $\text{CaCO}_3(s)$. In this experiment, the final pressure in the container was 1.04 atm, which was the same final pressure as in the first experiment.
Calculate the number of moles of $\text{CO}_2(g)$ present in the container after 20 minutes of heating.
The student claimed that the final pressure in the container in each experiment became constant because all of the $\text{CaCO}_3(s)$ had decomposed. Based on the data in the experiments, do you agree with this claim? Explain.
After 20 minutes some $\text{CO}_2(g)$ was injected into the container, initially raising the pressure to 1.5 atm. Would the final pressure inside the container be less than, greater than, or equal to 1.04 atm? Explain your reasoning.
Are there sufficient data obtained in the experiments to determine the value of the equilibrium constant, $K_p$, for the decomposition of $\text{CaCO}_3(s)$ at 1100 K? Justify your answer.
[Table:]
| Nonmetal | C | N | O | Ne | Si | P | S | Ar |
|---|---|---|---|---|---|---|---|---|
| Formula of Compound | $\text{CF}_4$ | $\text{NF}_3$ | $\text{OF}_2$ | No compound | $\text{SiF}_4$ | $\text{PF}_3$ | $\text{SF}_2$ | No compound |
Some binary compounds that form between fluorine and various nonmetals are listed in the table above. A student examines the data in the table and poses the following hypothesis: the number of F atoms that will bond to a nonmetal is always equal to 8 minus the number of valence electrons in the nonmetal atom.
Based on the student's hypothesis, what should be the formula of the compound that forms between chlorine and fluorine?
In an attempt to verify the hypothesis, the student researches the fluoride compounds of the other halogens and finds the formula $\text{ClF}_3$. In the box below, draw a complete Lewis electron-dot diagram for a molecule of $\text{ClF}_3$.
[A blank rectangular box provided for the student to draw the Lewis structure.]
Two possible geometric shapes for the $\text{ClF}_3$ molecule are trigonal planar and T-shaped. The student does some research and learns that the molecule has a dipole moment. Which of the two shapes is consistent with the fact that the $\text{ClF}_3$ molecule has a dipole moment? Justify your answer in terms of bond polarity and molecular structure.
In an attempt to resolve the existence of the $\text{ClF}_3$ molecule with the hypothesis stated above, the student researches the compounds that form between halogens and fluorine, and assembles the following list.
| Halogen | Formula(s) |
|---|---|
| F | $\text{F}_2$ |
| Cl | |
| Br | $\text{BrF}, \text{BrF}_3, \text{BrF}_5$ |
| I | $\text{IF}, \text{IF}_3, \text{IF}_5, \text{IF}_7$ |
Based on concepts of atomic structure and periodicity, propose a modification to the student's previous hypothesis to account for the compounds that form between halogens and fluorine.
A student places a mixture of plastic beads consisting of polypropylene (PP) and polyvinyl chloride (PVC) in a 1.0 L beaker containing distilled water. After stirring the contents of the beaker vigorously, the student observes that the beads of one type of plastic sink to the bottom of the beaker and the beads of the other type of plastic float on the water. The chemical structures of PP and PVC are represented by the diagrams below, which show segments of each polymer.
[Structural diagram of PP: a repeating polymer backbone $-\text{C}(\text{H})(\text{CH}_3)-\text{C}(\text{H})(\text{H})-$ shown as three repeat units, the middle one in brackets with subscript $n$, labeled "PP".]
[Structural diagram of PVC: a repeating polymer backbone $-\text{C}(\text{H})(\text{Cl})-\text{C}(\text{H})(\text{H})-$ shown as three repeat units, the middle one in brackets with subscript $n$, labeled "PVC".]
Given that the spacing between polymer chains in PP and PVC is similar, the beads that sink are made of which polymer? Explain.
PP is synthesized from propene, $\text{C}_3\text{H}_6$, and PVC is synthesized from vinyl chloride, $\text{C}_2\text{H}_3\text{Cl}$. The structures of the molecules are shown below.
[Structural diagram of Propene: $\text{H}_2\text{C}=\text{CH}(\text{CH}_3)$, drawn with H and $\text{CH}_3$ on one carbon and two H atoms on the other.]
[Structural diagram of Vinyl Chloride (chloroethene): $\text{H}_2\text{C}=\text{CHCl}$, drawn with H and Cl on one carbon and two H atoms on the other.]
The boiling point of liquid propene (226 K) is lower than the boiling point of liquid vinyl chloride (260 K). Account for this difference in terms of the types and strengths of intermolecular forces present in each liquid.
In a separate experiment, the student measures the enthalpies of combustion of propene and vinyl chloride. The student determines that the combustion of 2.00 mol of vinyl chloride releases 2300 kJ of energy, according to the equation below.
Using the table of standard enthalpies of formation below, determine whether the combustion of 2.00 mol of propene releases more, less, or the same amount of energy that 2.00 mol of vinyl chloride releases. Justify your answer with a calculation. The balanced equation for the combustion of 2.00 mol of propene is $2\,\text{C}_3\text{H}_6(g) + 9\,\text{O}_2(g) \rightarrow 6\,\text{CO}_2(g) + 6\,\text{H}_2\text{O}(g)$.
| Substance | $\text{C}_2\text{H}_3\text{Cl}(g)$ | $\text{C}_3\text{H}_6(g)$ | $\text{CO}_2(g)$ | $\text{H}_2\text{O}(g)$ | $\text{HCl}(g)$ | $\text{O}_2(g)$ |
|---|---|---|---|---|---|---|
| Standard Enthalpy of Formation (kJ/mol) | 37 | 21 | -394 | -242 | -92 | 0 |
[Structural diagrams: cis-2-butene ($\text{H}_3\text{C}$ and $\text{CH}_3$ on the same side of the $\text{C}=\text{C}$ double bond, H atoms on the same side opposite) in equilibrium (double harpoon arrows) with trans-2-butene ($\text{H}_3\text{C}$ and $\text{CH}_3$ on opposite sides of the $\text{C}=\text{C}$ double bond).]
The half-life ($t_{1/2}$) of the catalyzed isomerization of cis-2-butene gas to produce trans-2-butene gas, represented above, was measured under various conditions, as shown in the table below.
| Trial Number | Initial $P_{cis\text{-}2\text{-}butene}$ (torr) | $V$ (L) | $T$ (K) | $t_{1/2}$ (s) |
|---|---|---|---|---|
| 1 | 300. | 2.00 | 350. | 100. |
| 2 | 600. | 2.00 | 350. | 100. |
| 3 | 300. | 4.00 | 350. | 100. |
| 4 | 300. | 2.00 | 365 | 50. |
The reaction is first order. Explain how the data in the table are consistent with a first-order reaction.
Calculate the rate constant, $k$, for the reaction at 350. K. Include appropriate units with your answer.
Is the initial rate of the reaction in trial 1 greater than, less than, or equal to the initial rate in trial 2? Justify your answer.
The half-life of the reaction in trial 4 is less than the half-life in trial 1. Explain why, in terms of activation energy.