Acid-Base Titrations (College Board AP® Chemistry)

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

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

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.

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.

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) .

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.

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.

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

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.

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

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.

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.

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.

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.

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

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.

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.

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

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.

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

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.