Acids & Bases (College Board AP® Chemistry): Exam Questions

Propanoic acid, CH₃CH₂COOH, is a carboxylic acid that reacts with water according to the equation below.

CH₃CH₂COOH(aq) + H₂O(l)   CH₃CH₂COO⁻(aq) + H₃O⁺(aq)

At 25°C the pH of a 50.0 mL sample of 0.20 M CH₃CH₂COOH is 2.79.

Identify a Brønsted-Lowry conjugate acid-base pair in the reaction. Clearly label which is the acid and which is the base.

 Determine the value of K_a for propanoic acid at 25°C. 

For each of the following statements, determine whether the statement is true or false. In each case, explain the reasoning that supports your answer.

i) The pH of a solution prepared by mixing the 50.0 mL sample of 0.20 M CH₃CH₂COOH with a 50.0 mL sample of 0.20 M NaOH is 7.00 .

ii) If the pH of a hydrochloric acid solution is the same as the pH of a propanoic acid solution, then the molar concentration of the hydrochloric acid solution must be less than the molar concentration of the propanoic acid solution.

A student is given the task of determining the concentration of a propanoic acid solution of unknown concentration. A 0.173 M NaOH solution is available to use as the titrant. The student uses a 25.00 mL volumetric pipet to deliver the propanoic acid solution to a clean, dry flask. After adding an appropriate indicator to the flask, the student titrates the solution with the 0.173 M NaOH , reaching the end point after 20.52 mL of the base solution has been added.

Calculate the molarity of the propanoic acid solution.

The student is asked to redesign the experiment to determine the concentration of a butanoic acid solution instead of a propanoic acid solution. For butanoic acid the value of pK_a is 4.83. The student claims that a different indicator will be required to determine the equivalence point of the titration accurately. Based on your response to part (b), do you agree with the student’s claim? Justify your answer.

HF (aq) + H₂O (l) F⁻ (aq) + H₃O⁺ (aq)

The ionization of HF (aq) in water is represented by the equation above. In a 0.0350 M HF (aq) solution, the percent ionization of HF is 13.0 percent.

Two particulate representations of the ionization of HF molecules in the 0.0350 M HF (aq) solution are shown below in Figure 1 and Figure 2. Water molecules are not shown. Explain why the representation of the ionization of HF molecules in water in Figure 1 is more accurate than the representation in Figure 2. (The key below identifies the particles in the representations.)

Use the percent ionization data above to calculate the value of K_a for HF.

If 50.0 mL of distilled water is added to 50.0 mL of 0.035 M HF (aq) , will the percent ionization of HF (aq) in the solution increase, decrease, or remain the same? Justify your answer with an explanation or calculation.

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.

A 0.100 M solution of lactic acid (HC₃H₅O₃) has a K_a = 1.38 × 10^-4.

Define percentage ionization of a weak acid.

Calculate the percentage ionization of lactic acid in this solution.

If another solution of acetic acid has a concentration of 0.50 M, predict whether its percentage ionization will be greater than, less than, or equal to that of the 0.10 M solution.

Explain your answer.

Explain how the strength of an acid is related to the stability of its conjugate base

The following acids are carboxylic acids with varying substituents:

• CH₃COOH (Acetic acid)

• ClCH₂COOH (Chloroacetic acid)

• Cl₂CHCOOH (Dichloroacetic acid)

• Cl₃CCOOH (Trichloroacetic acid)

i) Identify which of these acids has the highest K_a value and justify your answer in terms of molecular structure.

ii) Write the balanced chemical equation for the dissociation of Cl₃CCOOH in water.

Methylamine (CH₃NH₂) and aniline (C₆H₅NH₂) are both weak bases.

Identify which base is stronger and justify your answer in terms of electron density and molecular structure.

C₆H₅OH (aq) + H₂O (l)  C₆H₅O⁻ (aq) + H₃O⁺ (aq), K_a = 1.12 × 10⁻¹⁰

Phenol is a weak acid that partially dissociates in water according to the equation

What is the pH of a 0.75 M C₆H₅OH (aq) solution?

For a certain reaction involving C₆H₅OH(aq) to proceed at a significant rate, the phenol must be primarily in its deprotonated form,  C₆H₅O⁻(aq). In order to ensure that the  C₆H₅OH(aq)  is deprotonated,  the reaction must be conducted in a buffered solution. On the number scale below, circle each pH for which more than 50 percent of the phenol molecules  are in the deprotonated  form (C₆H₅O⁻(aq)). Justify your answer.

Formic acid (HCOOH) is a weak acid that partially ionizes in water. The pK_a of formic acid is 3.75.

A 0.150 M solution of formic acid is prepared. Calculate the pH of the solution.

i) Write the balanced equation for the reaction of the conjugate base of formic acid with water.

ii) Determine its K_b value at 298 K.

A 100.0 mL solution of 0.250 M formic acid (HCOOH) is mixed with 40.0 mL of 0.100 M barium hydroxide (Ba(OH)₂). The pK_a of methanoic acid at 25°C is 3.75.

Write the balanced chemical equation for the reaction that occurs when Ba(OH)₂ is added to formic acid.

Calculate the number of moles of excess reactant after the reaction has occured.

Calculate the pH of the resulting solution. Show your work.