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

Compound X has the molecular formula C₄H₈O₂ and exists as a white crystalline solid at room temperature. Compound X has a melting point of 135  ^oC and dissolves well in water. The Lewis diagram of a molecule of compound X is shown below.

Explain the type of bonding present in compound X.

Is the C=O bond in compound X nonpolar covalent, polar covalent, or ionic? Justify your answer.

Predict whether compound X is polar or nonpolar. Justify your answer.

State the approximate bond angle around the carbon atom in the –COOH group. Explain your answer in terms of electron domain geometry.

Identify the strongest intermolecular force between molecules of compound X. Explain how this affects the melting point.

Explain why compound X dissolves readily in water.

A materials scientist compares the structures and properties of three different solids:

• NaCl (s) – an ionic compound

• SiO₂ (s) – a covalent network solid

• Cu (s) – a metallic solid

Explain why copper conducts electricity as a solid, but NaCl does not.

Despite both being solids with strong interactions, SiO₂ is significantly harder than NaCl. Explain this difference in terms of their bonding and structure.

The scientist heats equal masses of NaCl (s) and Cu (s). Explain which solid will melt first in terms of particle motion and structure.

Draw a particulate-level representation of the structure of NaCl (s).
Use labeled circles to represent Na⁺ and Cl⁻ ions. Show an appropriate arrangement in the lattice and indicate any interactions clearly.

A polymer material shows similar conductivity to SiO₂ and is flexible like a soft metal. Suggest a structural explanation for these observed properties.

A student investigates the thermal decomposition of sodium bicarbonate in a sealed flask at 398 K.

2NaHCO₃ (s) → Na₂CO₃ (s) + H₂O (g) + CO₂ (g)

The student heats 8.40 g of NaHCO₃ in an evacuated 2.00 L container. After the reaction reaches completion, the total pressure of the gaseous products is measured to be 1.22 atm at 398 K.

Calculate the number of moles of NaHCO₃ placed in the container.

Determine the theoretical number of moles of gas produced from complete decomposition.

Use the ideal gas law to calculate the total number of moles of gas produced in the reaction.

Compare your answers to parts (b) and (c). Calculate the percent yield of the decomposition reaction.

Suggest one possible reason why the percent yield is less than 100%, even though all of the NaHCO₃ was heated.

After the decomposition of Na₂CO₃, the solid residue remaining in the container is dissolved in water and reacted with excess HCl (aq).

Write a balanced molecular equation for the reaction that occurs between Na₂CO₃ and HCl.

The student bubbles the gas produced from part (f) through limewater, (Ca(OH)₂ (aq)).

Describe the expected observation and identify the gas responsible for the change.

Instead of collecting the gas, the student titrates the resulting solution with silver nitrate (AgNO₃).

Write the balanced net ionic equation for the reaction, and
explain how this titration could be used to confirm the amount of Na₂CO₃ present in the original sample.

A student investigates the energy changes that occur when potassium chloride (KCl) dissolves in water. The student adds 4.00 g of solid KCl to 100.0 g of distilled water in a calorimeter and records the temperature change over time. The calorimeter has negligible heat loss, and the specific heat capacity of water is 4.18 J g^{-1 o}C^-1.

The temperature of the water changes from 24.6 ^oC to 21.3 ^oC after the KCl dissolves completely.

Write the balanced chemical equation, including state symbols, that represents the dissolution of solid KCl in water.

Is the dissolution of KCl endothermic or exothermic? Justify your answer using the data from the experiment.

Using the data from the experiment, calculate the amount of heat energy transferred (q) as the KCl dissolves in water. Assume the solution has the same specific heat capacity and mass as the water.

Use your answer from part (c) to calculate the molar enthalpy change (ΔH, in kJ mol^-1) for the dissolution of KCl.

Explain the thermodynamic reason why the dissolution of KCl occurs, even though the temperature of the solution decreases.

If the same amount of KCl were dissolved in only 50.0 g of water, would the temperature change of the solution be greater, smaller, or the same? Justify your answer.

Answer the following questions relating to the chemistry of the halogens.

The molecular formulas of diatomic bromine, chlorine, fluorine, and iodine are written below. Circle the formula of the molecule that has the longest bond length. Justify your choice in terms of atomic structure.

Br₂                            Cl₂                            F₂                              I₂

A chemistry teacher wants to prepare Br₂. The teacher has access to the following three reagents: NaBr (aq) , Cl₂ (g) , and I₂ (s).

Using the data in the table above, write the balanced equation for the thermodynamically favorable reaction that will produce Br₂ when the teacher combines two of the reagents. Justify that the reaction is thermodynamically favorable by calculating the value of E° for the reaction.

Br₂ and Cl₂ can react to form the compound BrCl .

The boiling point of Br₂ is 332 K, whereas the boiling point of BrCl is 278 K. Explain this difference in boiling point in terms of all the intermolecular forces present between molecules of each substance.

The compound BrCl can decompose into Br₂ and Cl₂ , as represented by the balanced chemical equation below.

2 BrCl (g)  ⇌ Br₂ (g) + Cl₂ (g)               ΔH° = 1.6 kJ/mol_rxn

A 0.100 mole sample of pure BrCl (g) is placed in a previously evacuated, rigid 2.00 L container at 298 K. Eventually the system reaches equilibrium according to the equation above.

Calculate the pressure in the container before equilibrium is established.

Write the expression for the equilibrium constant, K_eq ,for the decomposition of BrCl.

After the system has reached equilibrium, 42 percent of the original BrCl sample has decomposed.

Determine the value of K_eq  for the decomposition reaction of BrCl at 298 K.

Calculate the bond energy of the Br-Cl bond, in kJ/mol, using ΔH° for the reaction (1.6 kJ/mol_rxn) and the information in the following table.

A student investigates how temperature affects the pH of water and how different bases behave in aqueous solution. They perform a series of tests on pure water, 0.0025 M Ba(OH)₂, and a solution of ammonia (NH₃) at 25 °C and 40 °C. The K_b of NH₃ is 1.8 × 10^-5.

At 40 °C, the ionization constant for water is K_w = 2.92 × 10^-14.

Calculate the pH of pure water at this temperature.

At 25 ^oC, the student prepares a solution of 0.0025 M Ba(OH)₂.

Calculate the pH of this solution, assuming complete dissociation.

The student prepares a solution of ammonia (NH₃) at 25 ^oC. The solution is intended to have the same pH as the 0.0025 M Ba(OH)₂ solution from part (b).

Calculate the required concentration of NH₃.

K_b(NH₃) = 1.8 × 10^-5

Assume no change in volume due to ionization.

Explain why the required concentration of NH₃ is higher than that of Ba(OH)₂,
even though the pH values are the same.

At 60 °C, the student measures the pH of pure water to be 6.50.
Does this indicate the water is acidic? Justify your answer.

At high temperatures, N₂ (g) and O₂ (g) can react to produce nitrogen monoxide, NO (g), as represented  by the equation below.

  N₂ (g) + O₂ (g)  2NO (g)

Write the expression for the equilibrium constant, K_P , for the forward reaction.

A student injects N₂ (g) and O₂ (g) into a previously evacuated, rigid vessel and raises the temperature of the vessel to 2000° At this temperature the initial partial pressures of N₂ (g) and O₂ (g) are 6.01 atm and 1.61 atm, respectively. The system is allowed to reach equilibrium. The partial pressure of NO (g)  at equilibrium is 0.122 atm. Calculate the value of K_P.

Nitrogen monoxide, NO (g), can undergo further reactions to produce acids such as HNO₂, a weak acid with a K_a of 4.0 x 10⁻⁴ and a pK_a of 3.40.

A student is asked to make a buffer solution with a pH of 3.40 by using 0.100 M HNO₂ (aq) and 0.100 M NaOH (aq).

i) Explain why the addition of 0.100 M NaOH (aq)  to 0.100 M HNO₂ (aq) can result in the formation of a buffer solution. Include the net ionic equation for the reaction that occurs when the student adds the NaOH (aq)  to the HNO₂ (aq).

ii) Determine the volume, in mL, of 0.100 M NaOH (aq)  the student should add to 100. mL of 0.100 M HNO₂ (aq) to make a buffer solution with a pH of 3.40. Justify your answer.

A second student makes a buffer by dissolving 0.100 mol of NaNO₂ (s) in 100. mL of 1.00 M HNO₂ (aq). Which is more resistant to changes in pH when a strong acid or a strong base is added, the buffer made by the second student or the buffer made by the first student in part (c)? Justify your answer.

A new buffer is made using HNO₂(aq) as one of the A particulate representation of a small representative portion of the buffer solution is shown below. (Cations and water molecules are not shown.)

Is the pH of the buffer represented in the diagram greater than, less than, or equal to 3.40 ? Justify your answer.

Metal-air cells are a relatively new type of portable energy source consisting of a metal anode, an alkaline electrolyte paste that contains water, and a porous cathode membrane that lets in oxygen from the air. A schematic of the cell is shown below.

Reduction potentials for the cathode and three possible metal anodes are given in the table below.

Early forms of metal-air cells used zinc as the Zinc oxide is produced as the cell operates according to the overall equation below.

2Zn (s) + O₂ (g) → 2ZnO (s)

 i) Using the data in the table above, calculate the cell potential for the zinc-air cell.

 ii) The electrolyte paste contains OH⁻. On the diagram of the cell above, draw an arrow to indicate the direction of migration of OH⁻ ions through the electrolyte as the cell operates.

A fresh zinc-air cell is weighed on an analytical balance before being placed in a hearing aid for

i) As the cell operates, does the mass of the cell increase, decrease, or remain the same?

ii) Justify your answer to part (b)(i) in terms of the equation for the overall cell

The zinc-air cell is taken to the top of a mountain where the air pressure is lower

i) Will the cell potential be higher, lower, or the same as the cell potential at the lower elevation?

ii) Justify your answer to part (c)(i) based on the equation for the overall cell reaction and the information.

Metal-air cells need to be lightweight for many In order to transfer more electrons with a smaller mass, Na and Ca are investigated as potential anodes.

A 1.0 g anode of which of these metals would transfer more electrons, assuming that the anode is totally consumed during the lifetime of a cell? Justify your answer with calculations.

The only common oxide of zinc has the formula ZnO.

i) Write the electron configuration for a Zn atom in the ground state.

ii) From which sublevel are electrons removed when a Zn atom in the ground state is oxidized?