Acid-Base Titrations (College Board AP® Chemistry)

Exam Questions

33 mins8 questions
1a1 mark

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 Ka for sorbic acid, HC6H7O2, is 1.7 × 10−5.

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

1b2 marks

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.

1c2 marks

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.

Indicator

pKa

Phenolphthalein

9.3

Bromothymol blue

7.0

Methyl red

5.0

Thymol blue

2.0

Methyl violet

0.80

1d1 mark

Calculate the pH at the half-equivalence

1e3 marks

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.

q3e-june-2015---ap-chemistry
1f1 mark

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.

Did this page help you?

2a1 mark

Answer the following questions relating to Fe and its ions, Fe2+ and Fe3+.

Write the ground-state electron configuration of the Fe2+ ion.

2b1 mark

Ion

Ionic Radius (pm)

Fe2+

92

Fe3+

79

The radii of the ions are given in the table above. Using principles of atomic structure, explain why the radius of the Fe2+ ion is larger than the radius of the Fe3+ ion.

2c1 mark

Fe3+ ions interact more strongly with water molecules in aqueous solution than Fe2+ ions do.

Give one reason for this stronger interaction, and justify your answer using Coulomb’s law.

2d1 mark

A student obtains a solution that contains an unknown concentration of Fe2+(aq). To determine the concentration of Fe2+(aq) in the solution, the student titrates a sample of the solution with MnO4 (aq), which converts Fe2+(aq) to Fe3+(aq) , as represented by the following equation.

5 Fe2+(aq) + MnO4  (aq) + 8 H+ (aq) →  5 Fe3+ (aq) + Mn2+ (aq) + 4 H2O(l)

Write the balanced equation for the half-reaction for the oxidation of Fe2+(aq) to Fe3+(aq) .

2e2 marks

The student titrates a 10.0 mL sample of the Fe2+(aq) solution . Calculate the value of [Fe2+] in the solution if it takes 17.48 mL of added 0.0350 M KMnO4(aq) to reach the equivalence point of the titration.

2f1 mark

To deliver the 10.0 mL sample of the Fe2+(aq) solution in part (e), the student has the choice of using one of the pieces of glassware listed below.

  • 25 mL buret 

  • 25 mL beaker

  • 25 mL graduated cylinder

  • 25 mL volumetric flask

Explain why the 25 mL volumetric flask would be a poor choice to use for delivering the required volume of the Fe2+(aq) solution.

2g1 mark

In a separate experiment, the student is given a sample of powdered Fe(s) that contains an inert impurity. The student uses a procedure to oxidize the Fe(s) in the sample to Fe2O3(s) . The student collects the following data during the experiment.

Mass of Fe(s) with inert impurity

6.724 g

Mass of Fe2O3(s) produced

7.531 g

Calculate the number of moles of Fe in the Fe2O3(s) produced.

Did this page help you?

3a1 mark

A student reacts 0.300 g of methyl salicylate (C8H8O3) with a stoichiometric amount of a strong base. This product is then acidified to produce salicylic acid crystals (HC7H5O3).

For every 1 mole of C8H8O3 (molar mass 152.15 g/mol) reactant used, 1 mole of salicylic acid crystals (HC7H5O3, molar mass 138.12 g/mol) is produced. Calculate the maximum mass, in grams, of HC7H5O3 that could be produced in this reaction.

3b1 mark

As part of the experimental procedure to purify the HC7H5O3 crystals after the reaction is complete, the crystals are filtered from the reaction mixture, rinsed with distilled water, and dried. Some physical properties of HC7H5O3 are given in the following table.

Properties of Salicylic Acid (HC7H5O3)

 Melting point

159°C

Solubility in H2O at 25°C  

2.2 g/L

 Specific heat capacity

1.17 J/(g·°C)

 Heat of fusion

27.1 kJ/mol

The student’s experiment results in an 87% yield of dry HC7H5O3. The student suggests that some of the HC7H5O3 crystals dissolved in the distilled water during the rinsing step. Is the student’s claim consistent with the calculated percent yield value? Justify your answer.

3c2 marks

Given the physical properties in the table, calculate the quantity of heat that must be absorbed to increase the temperature of a 0.105 g sample of dry HC7H5O3 (molar mass 138.12 g / mol) crystals from 25° C to the melting point of 159° C and melt the crystals completely.

3d1 mark

The structures and melting points for methyl salicylate and salicylic acid are shown.

q1c-june-2022---ap-chemistry

The same three types of intermolecular forces (London dispersion forces, dipole-dipole interactions, and hydrogen bonding) exist among molecules of each substance. Explain why the melting point of salicylic acid is higher than that of methyl salicylate.

3e1 mark

The student titrates 20.0  mL of 0.0100 M  HC7H5O3(aq) with 0.0200 M  NaOH, using a probe to monitor the pH of the solution. The data are plotted producing the following titration curve.

q1d-june-2022---ap-chemistry

Using the information in the graph, estimate the pKa  of HC7H5O3.                                 

3f1 mark

When the pH of the titration mixture is 4.00, is there a higher concentration of the weak acid, HC7H5O3, or its conjugate base, C7H5O3, in the flask? Justify your answer.

3g1 mark

The student researches benzoic acid (HC7H5O2) and finds that it has similar properties to salicylic acid (HC7H5O3). The Ka  for benzoic acid is 6.3 × 10−5. Calculate the value of pKa  for benzoic acid.

3h2 marks

The student performs a second titration, this time titrating 20.0 mL of a 0.0100 M benzoic acid solution with 0.0200 M NaOH. Sketch the curve that would result from this titration of benzoic acid on the following graph, which already shows the original curve from the titration of 20.0 mL of 0.0100 M salicylic acid. The initial pH of the benzoic acid solution is 3.11.

q1h-june-2022---ap-chemistry

Did this page help you?