Learn Extracted exam questions AP Chemistry 2021 Free Response
2021 Free Response
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Methanoic acid, HCOOH, ionizes according to the equation above.
Write the expression for the equilibrium constant, $K_a$, for the reaction.
Calculate the pH of a 0.25 $M$ solution of HCOOH.
In the box below, complete the Lewis electron-dot diagram for HCOOH. Show all bonding and nonbonding valence electrons.
[Box containing the skeletal atom arrangement of HCOOH (no bonds or lone pairs drawn), laid out as:
O
|
H - C - O - H
i.e. a central C bonded to: an O atom above it (drawn separated, no bond line shown), an H atom to its left, and an O atom to its right which is in turn bonded to a further H atom on the far right. The student must add all bonding and nonbonding valence electrons to complete the Lewis structure.]
In aqueous solution, the compound $\text{H}_2\text{NNH}_2$ reacts according to the equation above. A 50.0 mL sample of 0.25 $M$ $\text{H}_2\text{NNH}_2(aq)$ is combined with a 50.0 mL sample of 0.25 $M$ HCOOH$(aq)$.
Write the balanced net ionic equation for the reaction that occurs when $\text{H}_2\text{NNH}_2$ is combined with HCOOH.
Is the resulting solution acidic, basic, or neutral? Justify your answer.
When a catalyst is added to a solution of HCOOH$(aq)$, the reaction represented by the following equation occurs.
Is the reaction a redox reaction? Justify your answer.
[Graph: Total Pressure (atm) on the y-axis (from 0 to above 24, with a dashed horizontal line at 24) vs. Time (hours) on the x-axis. The curve rises steeply from the origin, curving and leveling off asymptotically to a plateau at a total pressure of 24 atm.]
The reaction occurs in a rigid 4.3 L vessel at 25°C, and the total pressure is monitored, as shown in the graph above. The vessel originally did not contain any gas. Calculate the number of moles of $\text{CO}_2(g)$ produced in the reaction. (Assume that the amount of $\text{CO}_2(g)$ dissolved in the solution is negligible.)
After the reaction has proceeded for several minutes, does the amount of catalyst increase, decrease, or remain the same? Justify your answer.
Answer the following questions about the element Si and some of its compounds.
The mass spectrum of a pure sample of Si is shown below.
[Bar graph: Relative Abundance (y-axis, 0 to 100) vs. Mass (amu) (x-axis, 27 to 32). A tall bar of relative abundance ~92 at mass 28; a very small bar at mass 29 (~3); a very small bar at mass 30 (~4); negligible/no bars at 27, 31, 32.]
How many protons and how many neutrons are in the nucleus of an atom of the most abundant isotope of Si?
Write the ground-state electron configuration of Si.
Two compounds that contain Si are $\text{SiO}_2$ and $\text{SiH}_4$.
At 161 K, $\text{SiH}_4$ boils but $\text{SiO}_2$ remains as a solid. Using principles of interparticle forces, explain the difference in boiling points.
At high temperatures, $\text{SiH}_4$ decomposes to form solid silicon and hydrogen gas.
Write a balanced equation for the reaction.
A table of absolute entropies of some substances is given below.
| Substance | $S°$ (J/(mol · K)) |
|---|---|
| $\text{H}_2(g)$ | 131 |
| $\text{Si}(s)$ | 18 |
| $\text{SiH}_4(g)$ | 205 |
Explain why the absolute molar entropy of $\text{Si}(s)$ is less than that of $\text{H}_2(g)$.
Calculate the value, in J/(mol · K), of $\Delta S°$ for the reaction.
The reaction is thermodynamically favorable at all temperatures. Explain why the reaction occurs only at high temperatures.
A partial photoelectron spectrum of pure Si is shown below. On the spectrum, draw the missing peak that corresponds to the electrons in the $3p$ sublevel.
[Photoelectron spectrum: Relative Number of Electrons (y-axis) vs. Binding Energy (MJ/mol) (x-axis, logarithmic scale decreasing left to right: 1,000, 100, 10, 1, 0.1). Peaks shown (from left to right, i.e. highest to lowest binding energy): a small peak near ~180 (between 1,000 and 100), a small peak near ~20 (between 100 and 10), a tall peak near ~10, and a small peak near ~1.5 (between 1 and 0.1). No peak is shown corresponding to the $3p$ sublevel — the student must draw it in at the appropriate position and relative height.]
Using principles of atomic structure, explain why the first ionization energy of Ge is lower than that of Si.
A single photon with a wavelength of $4.00 \times 10^{-7}$ m is absorbed by the Si sample. Calculate the energy of the photon in joules.
A student is given the task of determining the molar concentration of a $\text{CuSO}_4$ solution using two different procedures, precipitation and spectrophotometry.
For the precipitation experiment, the student adds 20.0 mL of 0.200 $M$ $\text{Ba(NO}_3)_2$ to 50.0 mL of the $\text{CuSO}_4(aq)$. The reaction goes to completion, and a white precipitate forms. The student filters the precipitate and dries it overnight. The data are given in the following table.
| Mass of dry filter paper | 0.764 g |
| Volume of $\text{CuSO}_4(aq)$ | 50.0 mL |
| Volume of 0.200 $M$ $\text{Ba(NO}_3)_2$ | 20.0 mL |
| Mass of filter paper and dried precipitate | 1.136 g |
Write a balanced net ionic equation for the precipitation reaction.
Calculate the number of moles of precipitate formed.
Calculate the molarity of the original $\text{CuSO}_4$ solution.
For the spectrophotometry experiment, the student first makes a standard curve. The student uses a 0.1000 $M$ solution of $\text{CuSO}_4(aq)$ to make three more solutions of known concentration (0.0500 $M$, 0.0300 $M$, and 0.0100 $M$) in 50.00 mL volumetric flasks.
Calculate the volume of 0.1000 $M$ $\text{CuSO}_4(aq)$ needed to make 50.00 mL of 0.0500 $M$ $\text{CuSO}_4(aq)$.
Briefly describe the procedure the student should follow to make 50.00 mL of 0.0500 $M$ $\text{CuSO}_4(aq)$ using 0.1000 $M$ $\text{CuSO}_4(aq)$, a 50.00 mL volumetric flask, and other standard laboratory equipment. Assume that all appropriate safety precautions will be taken.
The standard curve is given below.
[Graph: Absorbance (y-axis, 0 to 0.800, gridlines every 0.200) vs. Concentration ($M$) (x-axis, 0 to 0.1000, gridlines every 0.0200). Four data points plotted approximately at (0.0100, 0.090), (0.0300, 0.190), (0.0500, 0.330), and (0.1000, 0.610), with a best-fit straight line drawn through them from near the origin up through the highest point.]
The absorbance of the $\text{CuSO}_4$ solution of unknown concentration is 0.219. Determine the molarity of the solution.
A second student performs the same experiment. There are a few drops of water in the cuvette before the second student adds the $\text{CuSO}_4(aq)$ solution of unknown concentration. Will this result in a $\text{CuSO}_4(aq)$ concentration for the unknown that is greater than, less than, or equal to the concentration determined in part (f)? Justify your answer.
A student investigates a reaction used in hand warmers, represented above. The student mixes Fe$(s)$ with a catalyst and sand in a small open container. The student measures the temperature of the mixture as the reaction proceeds. The data are given in the following table.
| Time (min) | Temperature of Mixture (°C) |
|---|---|
| 0 | 22.0 |
| 1 | 25.1 |
| 2 | 34.6 |
| 3 | 37.3 |
| 4 | 39.7 |
| 5 | 39.4 |
The mixture (Fe$(s)$, catalyst, and sand) has a total mass of 15.0 g and a specific heat capacity of 0.72 J/(g·°C). Calculate the amount of heat absorbed by the mixture from 0 minutes to 4 minutes.
Calculate the mass of Fe$(s)$, in grams, that reacted to generate the amount of heat calculated in part (a).
In a second experiment, the student uses twice the mass of iron as that calculated in part (b) but the same mass of sand as in the first experiment. Would the maximum temperature reached in the second experiment be greater than, less than, or equal to the maximum temperature in the first experiment? Justify your answer.
[Diagram: An electrolytic cell. A container labeled "Molten $\text{MgCl}_2$" holds the molten liquid. Two electrodes dip into the liquid from above, each connected by a wire up to a box labeled "Power Source" at the top. The left electrode sits at the surface where a shaded region labeled "Mg $(l)$" pools on top of the liquid at that electrode. The right electrode has clusters of small gas bubbles rising around it, labeled "$\text{Cl}_2(g)$".]
| Half-Reaction | $E°$ (V) |
|---|---|
| $\text{Mg}^{2+} + 2e^- \rightarrow \text{Mg}$ | $-2.37$ |
| $\text{Cl}_2 + 2e^- \rightarrow 2\text{Cl}^-$ | $+1.36$ |
Molten $\text{MgCl}_2$ can be decomposed into its elements if a sufficient voltage is applied using inert electrodes. The products of the reaction are liquid Mg (at the cathode) and $\text{Cl}_2$ gas (at the anode). A simplified representation of the cell is shown above. The reduction half-reactions related to the overall reaction in the cell are given in the table.
Draw an arrow on the diagram to show the direction of electron flow through the external circuit as the cell operates.
Would an applied voltage of 2.0 V be sufficient for the reaction to occur? Support your claim with a calculation as part of your answer.
If the current in the cell is kept at a constant 5.00 amps, how many seconds does it take to produce 2.00 g of Mg$(l)$ at the cathode?
A student is studying the properties of $\text{CaSO}_4$ and $\text{PbSO}_4$. The student has samples of both compounds, which are white powders.
The student tests the electrical conductivity of each solid and observes that neither solid conducts electricity. Describe the structures of the solids that account for their inability to conduct electricity.
The student places excess $\text{CaSO}_4(s)$ in a beaker containing 100 mL of water and places excess $\text{PbSO}_4(s)$ in another beaker containing 100 mL of water. The student stirs the contents of the beakers and then measures the electrical conductivity of the solution in each beaker. The student observes that the conductivity of the solution in the beaker containing the $\text{CaSO}_4(s)$ is higher than the conductivity of the solution in the beaker containing the $\text{PbSO}_4(s)$.
Which compound is more soluble in water, $\text{CaSO}_4(s)$ or $\text{PbSO}_4(s)$? Justify your answer based on the results of the conductivity test.
The left side of the diagram below shows a particulate representation of the contents of the beaker containing the $\text{CaSO}_4(s)$ from the solution conductivity experiment.
[Diagram: two beakers side by side, each containing liquid. Left beaker, labeled "Higher Conductivity Solution": scattered small circles marked "+" (labeled $\text{Ca}^{2+}$) and "−" (labeled $\text{SO}_4^{2-}$) floating individually in the liquid (several of each, dispersed), plus a small cluster of undissolved solid at the bottom made of alternating "+" and "−" circles packed together. Right beaker, labeled "Lower Conductivity Solution": empty/blank, to be filled in by the student, with the same legend $\text{+} = \text{Pb}^{2+}$, $\text{−} = \text{SO}_4^{2-}$.]
Draw a particulate representation of $\text{PbSO}_4(s)$ and the ions dissolved in the solution in the beaker on the right in the diagram. Draw the particles to look like those shown to the right of the beaker. Draw an appropriate number of dissolved ions relative to the number of dissolved ions in the beaker on the left.
The student attempts to increase the solubility of $\text{CaSO}_4(s)$ by adding 10.0 mL of 2 $M$ $\text{H}_2\text{SO}_4(aq)$ to the beaker, and observes that additional precipitate forms in the beaker. Explain this observation.
[Diagram: A rigid cylindrical container standing upright, fitted with a "Movable Piston" (shown as a shaded bar) near the top that can slide up and down. Below the piston, the container holds "$\text{O}_2(g)$". A "Valve" is shown on the side of the container near the piston.]
A student investigates gas behavior using a rigid cylinder with a movable piston of negligible mass, as shown in the diagram above. The cylinder contains 0.325 mol of $\text{O}_2(g)$.
The cylinder has a volume of 7.95 L at 25°C and 1.00 atm. Calculate the density of the $\text{O}_2(g)$, in g/L, under these conditions.
Attempting to change the density of the $\text{O}_2(g)$, the student opens the valve on the side of the cylinder, pushes down on the piston to release some of the gas, and closes the valve again. The temperature of the gas remains constant at 25°C. Will this action change the density of the gas remaining in the cylinder? Justify your answer.
The student tries to change the density of the $\text{O}_2(g)$ by cooling the cylinder to $-55°$C, which causes the volume of the gas to decrease. Using principles of kinetic molecular theory, explain why the volume of the $\text{O}_2(g)$ decreases when the temperature decreases to $-55°$C.
The student further cools the cylinder to $-180°$C and observes that the measured volume of the $\text{O}_2(g)$ is substantially smaller than the volume that is calculated using the ideal gas law. Assume all equipment is functioning properly. Explain why the measured volume of the $\text{O}_2(g)$ is smaller than the calculated volume. (The boiling point of $\text{O}_2(l)$ is $-183°$C.)