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

A student is studying the bonding and reactivity of several phosphorus-containing molecules: PF₃, PCl₅, and POCl₃. These molecules illustrate key concepts such as formal charge, expanded octets, molecular polarity, and bonding limitations.

Draw the complete Lewis structure for PF₃.

Predict the molecular geometry of PF₃. Justify your answer.

Draw the Lewis diagram for PCl₅.

Explain why PCl₅ is an exception to the octet rule.

Represent a complete Lewis diagram for POCl₃. Minimize formal charges and show all bonding.

Determine the formal charge on each atom in the Lewis structure of POCl₃ in your answer to part (e) and explain why this is the stable resonance structure.

Predict whether PF₃ or POCl₃ is more polar. Justify your answer.

Seawater contains dissolved salts like NaCl, MgCl₂, and CaCl₂. These ions affect properties such as conductivity and salinity, which are important in environmental and marine chemistry.

Explain, in terms of interactions at the molecular level, why ionic compounds such as NaCl, MgCl₂, and CaCl₂ dissolve readily in water.

Explain how these interactions allow seawater to conduct electricity.

A seawater sample contains the following concentrations:

• 0.550 M NaCl

• 0.250 M MgCl₂

• 0.100 M CaCl₂

Calculate the total concentration of ions (all dissolved particles) in the solution.

Explain why a 1.0 M solution of MgCl₂ conducts electricity better than a 1.0 M solution of NaCl.

The diagram below shows a simple distillation setup used to separate pure water from seawater.

i) Label the component where salt remains, and the component where pure water collects.

ii) Explain why this method successfully separates water from dissolved salts.

A student starts with 100.0 mL of seawater containing dissolved salts from part (c).

i) Assuming this residue consists of a mixture of NaCl, MgCl₂, and CaCl₂ in the same mole ratio as in part (c), calculate the percentage by mass of NaCl in the total dissolved salts.

ii) After distillation of the 100.0 mL sample, 4.92 g of solid residue is recovered. Assuming this residue retains the same composition as in part (i), calculate the mass of NaCl present in the solid.

Answer the following questions about the isomers fulminic acid and isocyanic acid.

Two possible Lewis electron-dot diagrams for fulminic acid , HCNO, are shown below.

Explain why the diagram on the left is the better representation for the bonding in fulminic acid. Justify your choice based on formal charges.

Fulminic acid can convert to isocyanic acid according to the equation below.

   HCNO(g)        HNCO(g)   

fulminic acid    isocyanic acid

Using the Lewis electron-dot diagrams of fulminic acid and isocyanic acid shown in the boxes above and the table of average bond enthalpies below, determine the value of ΔH° for the reaction of  HCNO (g) to form HNCO (g).

A student claims that ΔS°  for the reaction is close to zero. Explain why the student's claim is accurate.

Which species, fulminic acid  (HCNO) or isocyanic acid  (HNCO), is present in higher concentration at equilibrium at 298 K ? Justify your answer in terms of thermodynamic favorability and the equilibrium constant.

The ammonium salt of isocyanic acid is a product of the decomposition of urea, CO(NH₂)₂, represented  below.

CO(NH₂)₂ (aq) NH₄⁺ (aq) + OCN⁻ (aq)

A student studying the decomposition reaction runs the reaction at 90°C. The student collects data on the concentration of urea as a function of time, as shown by the data table and the graph below.

The student proposes that the rate law is rate = k[CO(NH₂)₂].

i) Explain how the data support the student's proposed rate law

ii) Using the proposed rate law and the student's results, determine the value of the rate constant, k.

Include units with your answer.

The student learns that the decomposition reaction was run in a solution with a pH of 13. Briefly describe an experiment, including the initial conditions that you would change and the data you would gather, to determine whether the rate of the reaction depends on the concentration of OH⁻ (aq).

The reaction between methane and chlorine forms chloromethane and hydrogen chloride:

CH₄ (g) + Cl₂ (g) → CH₃Cl (g) + HCl (g)

The enthalpy change for this reaction can be estimated in different ways.

The average bond enthalpies for the relevant bonds are shown in the table below.

Calculate the enthalpy change (ΔH) for the reaction using these data.

The standard enthalpies of formation of the reactants and products are given in the table below.

Calculate the standard enthalpy change (ΔH^o) for the reaction.

Suggest one reason why the value calculated in part (a) differs from the value calculated in part (b).

Explain how a negative enthalpy change affects the thermodynamic favorability of a reaction.

The overall reaction is reversed in a later step of a thermochemical cycle.

CH₃Cl (g) + HCl (g) → CH₄ (g) + Cl₂ (g)

Explain how this affects the sign and magnitude of ΔH.

This reaction step occurs in a thermochemical cycle:

HCl (g) → H (g) + Cl (g)

Explain how this step affects the total enthalpy change for the overall reaction.

A sealed 5.00 L container is used to study the following gas-phase equilibrium at constant temperature:

2NO₂ (g) ⇌ 2NO (g) + O₂ (g)

NO₂ is a brown gas. NO and O₂ are colorless gases.

The system reaches equilibrium with the following partial pressures:

After equilibrium is established, the system is subjected to changes in volume, concentration, and temperature.

Calculate the value of the reaction quotient, Q, for the system.

If the equilibrium constant K_p = 0.0625, determine whether the system is at equilibrium. Justify your answer.

The volume of the container is suddenly decreased to 2.50 L. Predict the direction in which the system will shift to re-establish equilibrium. Justify your answer.

Immediately after the volume change in part (c), the brown color of the gas mixture briefly intensifies and then fades. What does this suggest about the concentration of NO₂ during and after the shift?

After equilibrium is re-established, a small amount of NO (g) is injected into the container. Predict how the concentrations of all three species will change as the system shifts to a new equilibrium.

i) The forward reaction is exothermic. If the temperature is increased, will the new equilibrium mixture contain more or less NO₂ than before the temperature change? Justify your answer.

ii) Explain how increasing the temperature affects the value of the equilibrium constant, K_p​.

Methanoic acid, HCOOH, ionizes according to the equation below.

HCOOH (aq) + H₂O (l) H₃O⁺ (aq) + HCOO⁻ (aq),     K_a = 1.8 × 10⁻⁴

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.

H₂NNH₂ (aq) + H₂O (l) H₂NNH₃⁺ (aq) + OH⁻ (aq), K_b = 1.3 × 10⁻⁶

In aqueous solution, the compound H₂NNH₂ reacts according to the equation above. A 50.0 mL sample of 0.25 M H₂NNH₂ (aq) is combined with a 50.0 mL sample of 0.25 M HCOOH (aq).

i) Write the balanced net ionic equation for the reaction that occurs when H₂NNH₂  is combined with HCOOH.

ii) 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.

HCOOH (aq)  →  H₂ (g) + CO₂ ( g)

Is the reaction a redox reaction? Justify your answer. 

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 CO₂ (g) produced in the reaction. (Assume that the amount of CO₂ (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.

i) How many protons and how many neutrons are in the nucleus of an atom of the most abundant isotope of Si ?

ii) Write the ground-state electron configuration of Si.

Two compounds that contain Si are SiO₂ and SiH₄.

At 161 K, SiH₄  boils but SiO₂ remains as a solid. Using principles of interparticle forces, explain the difference in boiling points.

At high temperatures, SiH₄  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.

Explain why the absolute molar entropy of Si (s) is less than that of H₂ (g).

Calculate the value, in J/(mol·K), of ∆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.

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 × 10⁻⁷ m is absorbed by the Si sample. Calculate the energy of the photon in joules.

Answer the following questions relating to the element aluminum, Al.

Write the complete ground-state electron configuration of an Al atom.

Based on principles of atomic structure, explain why the radius of the Al atom is larger than the radius of the Al³⁺ ion.

A student plans to combine solid aluminum with an aqueous solution of silver ions. The student determines the mass of solid AgNO₃ needed to prepare the solution with a specific concentration.

In the following table, briefly list the steps necessary to prepare 200.0 mL of an aqueous solution of AgNO₃ using only equipment selected from the choices given. Assume that all appropriate safety measures are already in place. Not all equipment or lines in the table may be needed.

• Solid AgNO₃

• Weighing paper and scoop

• Distilled water

• 200.00 mL volumetric flask

• Balance

• 50.0 mL graduated cylinder

• 250 mL beakers

• Pipet 

After preparing the solution, the student places some of the solution into a beaker and adds a sample of aluminum. The reaction represented by the following equation occurs.

Al (s) + 3Ag⁺ (aq) → Al³⁺ (aq) + 3Ag (s)   

The following diagram gives an incomplete particulate representation of the reaction. The beaker on the left represents the system before the mixture reacts. Complete the drawing on the right to represent the system after the reaction has occurred. Be sure to include 1) the correct type and number of particles based on the number shown on the left and 2 ) the relative spacing to depict the appropriate phases.

The student finds the standard reduction potentials given in the table, which are related to the reaction that occurs.

 Using the standard reduction potentials, calculate the value of E ° for the reaction.

Based on the value of E °, would the standard free energy change of the reaction under standard conditions, ∆G°, be positive, negative, or zero? Justify your answer.

Once the reaction appears to stop progressing, would the change in free energy, ∆G, be positive, negative, or zero? Justify your answer.