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

A researcher is investigating the structure and properties of acetate ion (CH₃COO⁻), phosphorus pentachloride (PCl₅), and xenon tetrafluoride (XeF₄). These substances demonstrate key bonding patterns and structural effects on physical properties.

Draw two valid resonance structures of the acetate ion, CH₃COO⁻, and explain how resonance affects the bond lengths in the ion.

Determine the hybridization of each carbon atom in the acetate ion. Justify your answer based on electron domains.

Predict the molecular geometry of phosphorus pentachloride (PCl₅) and explain why axial and equatorial bonds experience different repulsions.

i) Predict the molecular geometry of xenon tetrafluoride (XeF₄). Justify your answer based on electron domain geometry.

ii) Explain why XeF₄ has no overall dipole moment.

Compare the polarity of CH₃COOH (acetic acid) and CH₃COO⁻ (acetate ion), and explain how their molecular structures contribute to their relative polarity.

A student is given the task of determining the molar concentration of a CuSO₄ solution using two different procedures, precipitation and spectrophotometry.

For the precipitation experiment, the student adds 20.0 mL of 0.200 M Ba(NO₃)₂ to 50.0 mL of the CuSO₄ (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.

Write a balanced net ionic equation for the precipitation reaction.

Calculate the number of moles of precipitate formed.

Calculate the molarity of the original CuSO₄  solution.

For the spectrophotometry experiment, the student first makes a standard curve. The student uses a 0.1000 M solution of CuSO₄ (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 CuSO₄ (aq) needed to make 50.00 mL of 0.0500 M CuSO₄ (aq).

Briefly describe the procedure the student should follow to make 50.00 mL of 0.0500 M CuSO₄ (aq) using 0.1000 M CuSO₄ (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.

The absorbance of the CuSO₄ 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 CuSO₄ (aq) solution of unknown concentration. Will this result in a CuSO₄ (aq) concentration for the unknown that is greater than, less than, or equal to the concentration determined in part (f) ? Justify your answer.

A chemical reaction between gaseous hydrogen (H₂) and iodine (I₂) is represented by the following equation:

H₂ (g) + I₂ (g) → 2HI (g)

The reaction occurs in a closed container at constant temperature.

Explain how the frequency and energy of collisions affect the rate of reaction according to the collision model.

The temperature of the reaction is decreased.

Explain, using kinetic molecular theory, how this change affects the number of successful collisions per unit time.

The activation energy for this reaction is 167 kJ/mol.

Sketch a potential energy diagram for this reaction, labeling the reactants, products, activation energy (E_a), and enthalpy change (ΔH).

A powdered catalyst is added to the reaction mixture.

Explain how the catalyst affects both the activation energy and the overall rate of the reaction.

The initial concentration of hydrogen gas is 0.80 mol/L. After 4.0 seconds, the concentration decreases to 0.20 mol/L.

Calculate the average rate of reaction with respect to hydrogen over this time period. Include units.

A sealed container initially contains only NO₂(g), a brown gas. Over time, a colorless gas forms, and the intensity of the brown color decreases. Eventually, the mixture appears pale brown in color.

The overall chemical equation is shown below:

2 NO₂ (g) ⇌ N₂O₄ (g)

A student records the concentrations of both gases in the container at regular intervals. A graph of the concentration of NO₂ (g) and N₂O₄ (g) over time is shown.

Identify one observable feature of the system that supports the conclusion that the reaction is reversible.

Define dynamic equilibrium and explain why the system appears unchanged once equilibrium is reached.

Compare the rates of the forward and reverse reactions before equilibrium is reached and at equilibrium.

Using the graph, estimate the time at which equilibrium is first established. Justify your estimate.

Predict how the concentration vs. time graph would change, if at all, and explain whether the final equilibrium concentrations would be affected if a catalyst is added to the system at t = 0.

A student investigates the equilibrium established when yellow chromate ions (CrO₄²⁻) react with hydrogen ions in solution:

2CrO₄²⁻ (aq) + 2H⁺ (aq) ⇌ Cr₂O₇²⁻ (aq) + H₂O (l)

The dichromate ion, Cr₂O₇²⁻, is orange.

Hydrochloric acid is added dropwise to the solution. Describe the expected color change and explain the shift in terms of Le Châtelier’s Principle.

Sodium hydroxide solution is added to the system after the addition of HCl. Predict the new color and justify the change based on pH and equilibrium shift.

The student adds solid potassium chromate (K₂CrO₄) to the solution. Explain the effect on the equilibrium.

The student adds Ba(NO₃)₂ to the system. A yellow precipitate of BaCrO₄ (s) forms, and the solution becomes lighter.

Explain, using equilibrium reasoning, why the precipitate forms and why the solution color changes.

After the addition of Ba(NO₃)₂, describe how the system re-establishes equilibrium in terms of the value of K_c​.

Potassium sorbate, KC₆H₇O₂ (molar mass 150 g/mol) is commonly added to diet soft drinks as a preservative. A stock solution of KC₆H₇O₂(aq) of known concentration must be prepared. A student titrates 45.00 mL of the stock solution with 1.25 M HCl(aq) using both an indicator and a pH meter. The value of K_a for sorbic acid, HC₆H₇O₂, is 1.7 × 10⁻⁵.

Write the net-ionic equation for the reaction between KC₆H₇O₂(aq) and HCl(aq).

A total of 29.95 mL of 1.25 M HCl(aq) is required to reach the equivalence point. Calculate [KC₆H₇O₂] in the stock solution.

The pH at the equivalence point of the titration is measured to be 2.54. Which of the following indicators would be the best choice for determining the end point of the titration? Justify your answer.

Calculate the pH at the half-equivalence

The initial pH and the equivalence point are plotted on the graph below. Accurately sketch the titration curve on the graph below. Mark the position of the half-equivalence point on the curve with an X.

The pH of the soft drink is 3.37 after the addition of the KC₆H₇O₂(aq). Which species, HC₆H₇O₂ or C₆H₇O₂⁻, has a higher concentration in the soft drink? Justify your answer.

A student designs an experiment to study the reaction between NaHC0₃ and HC₂H₃O₂. The reaction is represented by the equation below.

NaHCO₃ (s) + HC₂H₃O₂ (aq) → NaC₂H₃O₂ (aq) + H₂O (l) + CO₂ (g)

The student places 2.24 g of NaHCO₃ in a flask and adds 60.0 mL of 0.875 M HC₂H₃O₂. The student observes the formation of bubbles and that the flask gets cooler as the reaction proceeds.

Identify the reaction represented above as an acid-base reaction, precipitation reaction, or redox reaction. Justify your answer.

Based on the information above, identify the limiting reactant. Justify your answer with calculations.

The student observes that the bubbling is rapid at the beginning of the reaction and gradually slows as the reaction. Explain this change in the reaction rate in terms of the collisions between reactant particles.

In thermodynamic terms, a reaction can be driven by enthalpy, entropy, or both.

i) Considering that the flask gets cooler as the reaction proceeds, what drives the chemical reaction between NaHCO₃ (s) and HC₂H₃O₂ (aq) ? Answer by drawing a circle around one of the choices.

Enthalpy only / Entropy only / Both enthalpy and entropy

ii) Justify your selection in part (d)(i) in terms of ΔG°

The HCO₃⁻ ion has three carbon-to-oxygen bonds. Two of the carbon-to-oxygen bonds have the same length, and the third carbon-to-oxygen bond is longer than the other two. The hydrogen atom is bonded to one of the oxygen atoms. In the box below, draw a Lewis electron-dot diagram (or diagrams) for the HCO₃⁻ ion that is (are) consistent with the given information.

A student prepares a solution containing equimolar amounts of HC₂H₃O₂ and NaC₂H₃O₂. The pH of the solution is measured to be 4.7. The student adds two drops of 3.0 M HNO₃ (aq) and stirs the sample, observing that the pH remains at 4.7. Write a balanced, net-ionic equation for the reaction between HNO₃ (aq) and the chemical species in the sample that is responsible for the pH remaining at 4.7.