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

A chemist is investigating the volatility and solubility of three molecular compounds at room temperature: pentane, butan-1-ol, and ethanoic acid. The molecular formulas are shown below:

The chemist observes the following:

• Pentane evaporates quickly and is not very soluble in water.

• Butan-1-ol evaporates more slowly and is moderately soluble.

• Ethanoic acid is very soluble and evaporates the slowest.

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

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

Justify why ethanoic acid forms more extensive hydrogen bonding interactions than butan-1-ol based on molecular structure.

Predict whether butan-1-ol or ethanoic acid exhibits stronger dipole–dipole interactions. Justify your answer based on molecular structure.

Rank the three compounds in order of increasing volatility. Justify your ranking based on the strength of intermolecular forces.

Which compound is most soluble in water? Justify your answer in terms of intermolecular forces.

A student investigates several changes that occur when substances interact in the lab. Each process is analyzed at the macroscopic and molecular level.

A small amount of solid NaHCO₃ is added to vinegar (a dilute solution of CH₃COOH). Bubbling is observed, and a colorless gas is released.

Explain why this reaction is classified as a chemical process. In your answer, refer to both observable evidence and molecular-level changes.

Write a balanced molecular equation for the reaction in part (a).

Write the corresponding complete net ionic equation for the reaction in part (b).

Explain why the reaction between NaHCO₃ and CH₃COOH proceeds to completion, even though both are weak electrolytes.

The student performs a similar experiment dissolving the soluble salt in water, and observes that no gas forms.

Explain why dissolving NaCl in water is classified as a physical change, even though ionic bonds are broken in the process.

The student notices that the reaction mixture feels cooler after NaHCO₃ is added to CH₃COOH.

What does this temperature change suggest about the reaction?

Consider the following reaction mechanism:

1. NO + O₃ → NO₂ + O₂ (fast)

2. NO + NO₂ → N₂O₃ (slow)

The overall reaction releases energy as products form. Write the overall balanced chemical equation for this reaction.

Draw the energy profile for this mechanism, clearly labelling the activation energy (E_a​) for each step, the intermediate, and ΔH.

Justify why the first step has a lower activation energy than the second step.

Explain how the addition of a catalyst changes the energy profile and the reaction rate.

Explain how increasing the concentration of NO₂​ would affect the overall rate of the reaction. Justify your answer.

A student proposes using temperature to speed up the reaction. Predict how increasing the temperature affects the fraction of collisions with energy ≥ E_a​ and explain the molecular basis of this change.

A sealed, rigid 2.00 L container holds solid ammonium carbamate, NH₄CO₂NH₂ (s), at 298 K. Over time, a gas-phase equilibrium is established for the endothermic reaction of the solid forming the gaseous products. The total pressure in the container eventually levels off at 1.28 atm and remains constant.

Write the balanced equation for the decomposition of ammonium carbamate into ammonia and carbon dioxide.

Explain why, even though the system appears unchanged after a certain point, gas-phase reactions are still occurring.

Sketch a graph of total pressure (y-axis) vs. time (x-axis) for this system. Label the axes and show the shape of the curve.

On your graph in part (c), indicate the point at which equilibrium is established and explain how you identified it.

In terms of the relative rates of the forward and reverse reactions, explain why the system reaches a constant pressure over time.

Explain how a catalyst would change the time taken to reach equilibrium and how the final equilibrium pressure would be affected.

Suppose the temperature is increased while keeping the container volume constant.

Predict and explain how the pressure would change once a new equilibrium is reached.

The solubilty of three fluoride salts, LiF, CaF₂ and MgF₂, in water at 25 ^oC is investigated. The following data is collected:

Write a balanced equation for the dissolution of CaF₂ (s) in water.

State the mathematical expression that relates the solubility (S) of CaF₂ to its K_sp.

Determine which of the three salts has the highest molar solubility.
Justify your answer.

An acidic solution is added to a saturated solution of CaF₂. Explain how this affects the solubility of CaF₂, considering that HF is a weak acid.

The dissolution of CaF₂ is endothermic (ΔH^o_soln = +16.6 kJ mol^-1).
Explain how increasing the temperature would affect the solubility of CaF₂.

A saturated solution of CaF₂ is mixed with a saturated solution of MgF₂.

Predict whether a precipitate will form when the two solutions are combined. Justify your answer.

Answer the following questions relating to Fe and its ions, Fe²⁺ and Fe³⁺.

Write the ground-state electron configuration of the Fe²⁺ ion.

The radii of the ions are given in the table above. Using principles of atomic structure, explain why the radius of the Fe²⁺ ion is larger than the radius of the Fe³⁺ ion.

Fe³⁺ ions interact more strongly with water molecules in aqueous solution than Fe²⁺ ions do.

Give one reason for this stronger interaction, and justify your answer using Coulomb’s law.

A student obtains a solution that contains an unknown concentration of Fe²⁺ (aq). To determine the concentration of Fe²⁺ (aq) in the solution, the student titrates a sample of the solution with MnO₄^– (aq), which converts Fe²⁺ (aq) to Fe³⁺ (aq) , as represented by the following equation.

5 Fe²⁺ (aq) + MnO₄^– (aq) + 8 H⁺ (aq) →  5 Fe³⁺ (aq) + Mn²⁺ (aq) + 4H₂O(l)

Write the balanced equation for the half-reaction for the oxidation of Fe²⁺ (aq) to Fe³⁺ (aq) .

The student titrates a 10.0 mL sample of the Fe²⁺ (aq) solution . Calculate the value of [Fe²⁺] in the solution if it takes 17.48 mL of added 0.0350 M KMnO₄ (aq) to reach the equivalence point of the titration.

To deliver the 10.0 mL sample of the Fe²⁺ (aq) solution in part (e), the student has the choice of using one of the pieces of glassware listed below.

Explain why the 25 mL volumetric flask would be a poor choice to use for delivering the required volume of the Fe²⁺ (aq) solution.

In a separate experiment, the student is given a sample of powdered Fe (s) that contains an inert impurity. The student uses a procedure to oxidize the Fe(s) in the sample to Fe₂O₃ (s) . The student collects the following data during the experiment.

Calculate the number of moles of Fe in the Fe₂O₃(s) produced.

Calculate the percent by mass of Fe in the original sample of powdered Fe (s) with the inert impurity.

If the oxidation of the Fe (s) in the original sample was incomplete so that some of the 7 .531 g of product was FeO (s) instead of Fe₂O₃, would the calculated mass percent of Fe (s) in the original sample be higher, lower, or the same as the actual mass percent of Fe (s)? Justify your answer.

A student investigates the reactions of nitrogen oxides. One of the reactions in the investigation requires an equimolar mixture of NO(g) and NO₂(g) , which the student produces by using the reaction represented below.

2 NO (g) + O₂ (g) →  2 NO₂ (g)

The particle-level representation of the equimolar mixture of NO (g) and NO₂ (g) in the flask at the completion of the reaction between NO (g) and O₂ (g) is shown below in the box on the right. In the box below on the left, draw the particle-level representation of the reactant mixture of NO (g) and O₂ (g) that would yield the product mixture shown in the box on the right. In your drawing, represent oxygen atoms and nitrogen atoms as indicated below.

The student reads in a reference text that NO(g) and NO₂(g) will react as represented by the equation below. Thermodynamic data for the reaction are given in the table below the equation.

NO (g) + NO₂ (g)  N₂O₃ (g)

 The student begins with an equimolar mixture of NO (g) and NO₂ (g) in a rigid reaction vessel and the mixture reaches equilibrium at 298 K.

i) Calculate the value of the equilibrium constant, K, for the reaction at 298 K.

 ii) If both P_NO  and in the vessel are initially 1.0 atm, will at equilibrium be equal to 1.0 atm? Justify your answer.

The student hypothesizes that increasing the temperature will increase the amount of N₂O₃ (g) in the equilibrium mixture. Indicate whether you agree or disagree with the hypothesis. Justify your answer.

N₂O₃ (g) reacts with water to form nitrous acid, HNO₂ (aq), a compound involved in the production of acid rain. The reaction is represented below.

N₂O₃ (g) + H₂O (l) →  2 HNO₂ (aq)

The skeletal structure of the HNO₂ molecule is shown in the box below.

i) Complete the Lewis electron-dot diagram of the HNO₂ molecule in the box below, including any lone pairs of electrons.

ii) Based on your completed diagram above, identify the hybridization of the nitrogen atom in the HNO₂ molecule.

To produce an aqueous solution of HNO₂, the student bubbles N₂O₃ (g) into distilled water. Assume that the reaction goes to completion and that HNO₂ is the only species produced. To determine the concentration of HNO₂ (aq) in the resulting solution, the student titrates a 100. mL sample of the solution with 0.100 M KOH (aq). The neutralization reaction is represented below.

HNO₂ (aq) + OH⁻  (aq) →  NO₂⁻ (aq) + H₂O (l)

The following titration curve shows the change in pH of the solution during the titration.

Use the titration curve and the information above to

i) determine the initial concentration of the HNO₂ (aq) solution

ii) estimate the value of pK_a for HNO₂ (aq)

During the titration, after a volume of 15 mL of 0.100 M KOH (aq) has been added , which species, HNO₂ (aq) or NO₂^– (aq) , is present at a higher concentration in the solution? Justify your answer.