10 Point Questions (College Board AP® Chemistry): Exam Questions

A student investigates a rectangular sheet of copper to estimate the size of a copper atom.

The rectangular sheet of copper has a mass of 2.54 g. Calculate the number of atoms in the sheet.

The sheet has dimensions 10.0 cm × 5.00 cm and a thickness of 5.0 × 10^-3 cm. The density of copper is 8.96 g/cm³.

i) Calculate the volume of the copper sheet, in cm³.

ii) Use your answers from parts (a) and (b)(i) to determine the number of copper atoms per cubic centimeter.

The student models copper atoms as cubes packed in a simple cubic arrangement.

i) Assuming each copper atom occupies an equal volume in the sheet, calculate the volume, in cm³, occupied by a single atom.

ii) Use your answer from part (c)(i) to estimate the length of the cube occupied by one copper atom. Express your answer in picometers (pm).
(1 cm = 10¹⁰ pm)

The accepted atomic radius of copper is 128 pm.

Explain whether the estimate from part (c)(ii) supports the accepted atomic radius of copper? Use a claim–evidence–reasoning structure in your answer.

A simplified representation of part of the copper sheet is shown below. Each sphere represents one atom of copper in a square grid arrangement.

i) Identify one assumption the student is making by modeling the copper sheet with this atomic arrangement.

ii) Explain how this assumption might affect the accuracy of the estimated atomic size compared to the actual atomic radius.

A materials scientist is studying elements across Period 3 to evaluate their suitability for use in safe flame-retardant compounds. The scientist is particularly interested in the relationships between atomic structure and chemical behavior for elements from sodium to chlorine.

The atomic radii of Na, Mg, and Al decrease across Period 3. Explain this trend using nuclear charge and electron shielding.

Chlorine and iodine are both Group 17 elements.

Predict which of the two has a greater first ionization energy, and justify your prediction using Coulomb’s Law and periodic position.

The electron affinity of chlorine is more negative than that of fluorine, even though fluorine is more electronegative. Explain this anomaly using atomic structure and electron repulsion.

A student proposes the following isoelectronic series: O^2-, F^-, Na⁺, Mg²⁺.

Rank the species in order of increasing ionic radius, and justify your reasoning using effective nuclear charge.

Magnesium and nitrogen form the ionic compound Mg₃N₂.
Compare the relative ionic radii of the Mg²⁺ and N^3- ions, and justify your answer using concepts of effective nuclear charge and electron configuration.

Predict whether sodium or phosphorus is more likely to form a basic oxide. Justify your answer.

Two organic compounds, propanal and propan-1-ol, each have the molecular formula C₃H₈O. Their structures are shown below:

The boiling point of propanal is 49 °C, while the boiling point of propan-1-ol is 97 °C.

Identify the strongest type of intermolecular force present between propan-1-ol molecules.

Identify the strongest type of intermolecular force present between propanal molecules.

Justify why hydrogen bonding is considered a particularly strong type of intermolecular force.

Describe how the relative orientation of two propanal molecules affects the strength of their dipole–dipole interaction.

Predict which compound has stronger London dispersion forces. Justify your answer based on molecular structure.

Explain why propan-1-ol has a significantly higher boiling point than propanal.

A student is given the task of determining the 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 Pb(NO₃)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 below.

For the chemical reaction that occurs when the precipitate forms,

i) write a balanced, net-ionic equation for the reaction, and

ii) 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 [K⁺] greater than, less than, or equal to [NO₃⁻] ? Justify your answer.

Calculate the number of moles of precipitate that is produced in the experiment.

Calculate the mass percent of 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 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 I⁻ content of a KI tablet but does not have access to Pb(NO₃)₂ . However, the student does have access to 0.20 M AgNO₃ , which reacts with I⁻(aq) to produce AgI (s). The value of K_sp for AgI is 8.5 × 10⁻¹⁷.

i) Will the substitution of AgNO₃ for Pb(NO₃)₂ result in the precipitation of the I⁻ ion from solution? Justify your answer.

ii) 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.

The reaction between iodide ions (I^-) and persulfate ions (S₂O₈^2-) is often studied using a clock reaction to investigate kinetics. The reaction is given by:

2I^- (aq) + S₂O₈^2- (aq) → I₂ (aq) + 2SO₄^2- (aq)

i) Identify the species being oxidized. Justify your answer using oxidation states.

ii) Identify the species being reduced. Justify your answer in terms of electron transfer.

A study of the reaction produced the following experimental data:

i) Use the experimental data to determine the order of reaction with respect to S₂O₈^2-.

ii) Use the experimental data to determine the order of reaction with respect to I⁻.

iii) Write the overall rate law for this reaction based on your answers to (i) and (ii).

Determine the rate constant k using data from Experiment 1. Include units in your answer.

The following mechanisms have been proposed for the reaction:

Mechanism 1:

1. I^- (aq) + I^- (aq) → I₂^2- (aq) (slow)

2.  I₂^2- (aq) + S₂O₈^2- (aq) → I₂ (aq) + 2SO₄^2- (aq) (fast)

Mechanism 2:

1. I^- (aq) + S₂O₈^2- (aq) → S₂O₈I^3- (aq) (slow)

2.  S₂O₈I^3- (aq) + I^- (aq) → I₂ (aq) + 2SO₄^2- (aq) (fast)

Mechanism 3:

1. I^- (aq) + S₂O₈^2- (aq)  → S₂O₈I^3- (aq) (fast)

2.  S₂O₈I^3- (aq) + I^- (aq) → I₂ (aq) + 2SO₄^2- (aq) (slow)

For each mechanism, evaluate whether it is consistent with the experimentally determined rate law. Justify your answers.

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.

CH₃OH (g) ⇌ CO (g) + 2H₂ (g)          ∆H ° = 90.0 kJ / mol_rxn

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.

The values of the standard molar entropies of the compounds involved in the reaction are given in the following table.

i) Use the data in the table to calculate the value of the standard entropy change, ∆S°, in J/(K·mol_rxn), for the reaction.

ii) Calculate the value of ∆G°, in kJ/mol_rxn, for the reaction at 375 K. Assume that ∆H ° and ∆S° are independent of temperature.

The following particle-level diagram shows a representative sample of the equilibrium mixture represented by the equation given.

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.

CH₃OH (g) ⇌ CO (g) + 2 H₂ (g)

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.

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 CH₃OH (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.

A student is investigating the solubility of silver chromate, Ag₂CrO₄, which forms a red precipitate in aqueous solution. The student prepares a saturated solution of Ag₂CrO₄ and uses it in a series of solubility tests at 25 ^oC.

Write a balanced equation, including state symbols, for the dissolution of Ag₂CrO₄ (s) in water at 25 ^oC.

The K_sp for Ag₂CrO₄ at 25 ^oC is 1.1 × 10^-12.

Calculate the molar solubility of Ag₂CrO₄ in pure water at 25 ^oC.

A solution of 0.10 mol L^-1 AgNO₃ is added to a beaker containing the saturated Ag₂CrO₄ solution. Explain whether additional Ag₂CrO₄ will precipitate.

Another beaker contains a saturated solution of Ag₂CrO₄. Potassium chloride (KCl) is slowly added. Justify whether a precipitate will form.

A student claims that the solubility of Ag₂CrO₄ must be greater than that of BaSO₄ because Ag⁺ and CrO₄^2- are soluble ions according to the solubility rules.

Evaluate the validity of this claim using chemical reasoning.

A student reacts 0.300 g of methyl salicylate (C₈H₈O₃) with a stoichiometric amount of a strong base. This product is then acidified to produce salicylic acid crystals (HC₇H₅O₃).

For every 1 mole of C₈H₈O₃ (molar mass 152.15 g/mol) reactant used, 1 mole of salicylic acid crystals (HC₇H₅O₃, molar mass 138.12 g/mol) is produced. Calculate the maximum mass, in grams, of HC₇H₅O₃ that could be produced in this reaction.

As part of the experimental procedure to purify the HC₇H₅O₃ crystals after the reaction is complete, the crystals are filtered from the reaction mixture, rinsed with distilled water, and dried. Some physical properties of HC₇H₅O₃ are given in the following table.

The student’s experiment results in an 87% yield of dry HC₇H₅O₃. The student suggests that some of the HC₇H₅O₃ 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 HC₇H₅O₃ (molar mass 138.12 g / mol) crystals from 25° C to the melting point of 159° C and melt the crystals completely.

The structures and melting points for methyl salicylate and salicylic acid are shown.

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  HC₇H₅O₃(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.

Using the information in the graph, estimate the pK_a  of HC₇H₅O₃.                                 

When the pH of the titration mixture is 4.00, is there a higher concentration of the weak acid, HC₇H₅O₃, or its conjugate base, C₇H₅O₃⁻, in the flask? Justify your answer.

The student researches benzoic acid (HC₇H₅O₂) and finds that it has similar properties to salicylic acid (HC₇H₅O₃). The K_a  for benzoic acid is 6.3 × 10⁻⁵. Calculate the value of pK_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 mL of 0.0100 M salicylic acid. The initial pH of the benzoic acid solution is 3.11.