APChemistryCollege BoardTopic QuestionsUnit 8: Acids & Bases8.3 Weak Acid & Base Equilibria

Weak Acid & Base Equilibria (College Board AP Chemistry)

Topic Questions

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1a1 mark

CH3CH2COOH(aq) + H2O(l)  rightwards arrow over leftwards arrow CH3CH2COO−(aq) + H3O+(aq)

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

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1b4 marks

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

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1c1 mark

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.

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1d2 marks

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.

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1e2 marks

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 pKa 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.

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2a1 mark

HF(aq) + H2O(l)  rightwards arrow over leftwards arrow with blank on top 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.)

q5-june-2018---ap-chemistry

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2b2 marks

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

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2c1 mark

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.

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3a1 mark

HCOOH(aq) rightwards arrow over leftwards arrow H2O(l) rightwards arrow over leftwards arrow H3O+(aq) rightwards arrow over leftwards arrow HCOO−(aq)                 Ka = 1.8 × 10−4

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

Write the expression for the equilibrium constant, Ka, for the reaction.

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3b2 marks

Calculate the pH of a 0.25 M solution of HCOOH.

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3c1 mark

In the box below, complete the Lewis electron-dot diagram for HCOOH. Show all bonding and nonbonding valence electrons.

O       

H     C     O     H

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3d2 marks

H2NNH2(aq) + H2O(l) rightwards arrow over leftwards arrow H2NNH3 + (aq) + OH−(aq)                 Kb  = 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 H2 NNH2  is combined with HCOOH.

 

ii)
Is the resulting solution acidic, basic, or neutral? Justify your answer.

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3e1 mark

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. 

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3f2 marks

-T7C6k-u_q1e-june-2021--ap-chemistry

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.)

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3g1 mark

After the reaction has proceeded for several minutes, does the amount of catalyst increase, decrease, or remain the same? Justify your answer.

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