17.3.4 Polyprotic Acid-Strong Base Titration

Question 1

Polyprotic Acid-Strong Base Titration

Answer 1

• A diprotic weak acid will have a different acid-dissociation constant (K a ) for the first and the second deprotonation.
• The two individual deprotonations are clearly visible in the titration curve of a polyprotic acid.

Question 2

note

Answer 2

• A diprotic weak acid will have a different acid-dissociation constant (K a ) for the first and the second deprotonation.
• In the example of oxalic acid (H 2 (Ox)), if one of the hydrogen atoms is removed, the remaining hydrogen atom will feel the resulting negative charge. The remaining hydrogen atom will be more tightly bound to the molecule than it was before the loss of the first hydrogen atom.
• It does not matter which of the hydrogen atoms is removed first. Whichever one remains will be more tightly bound to the remaining molecule.
• The K a values indicate that oxalic acid is a relatively strong weak acid and that bioxalate ion (H(Ox)–) is considerably weaker. Bioxalate ion is not as likely to give up a proton as oxalic acid.
• As the titration begins, one of the protons dissociates. At the half-equivalence point the system is in the first buffer region and there should be equals amounts of oxalic acid and oxalate ion.
• At the first equivalence point there is no oxalic acid left—only bioxalate ion. The K a of bioxalate ion is 6.4 x 10 –5 , so most of it does not continue to dissociate to oxalate ion.
• As the titration continues a second buffer region and
  half-equivalence point is reached. At this point, there are equal amounts of bioxalate ion and oxalate ion in the solution.
• At the second equivalence point, there is a dramatic pH
  change because the titration is no longer in a buffer region.

Question 3

Look again at the four reactions for the titration of oxalic acid.
Which statement about these reactions is not correct?

Answer 3

The forward reaction for Reaction 3 is more inclined to proceed than the forward reaction for Reaction 2.

Question 4

Which statement about the titration of the diprotic oxalic acid is not correct?

Answer 4

We can calculate the pKa values from the pH values that we determine at the equivalence points.

Question 5

Look again at Point I of this reaction. Oxalate anion (Ox)2− is in solution. It is a weak base. Two of the reactions can occur at this point.

Assume that Reaction 3 is the dominant equilibrium to consider when determining the solution’s pH. What is the pH of the solution if we start with 100 mL of 0.12 M H2(Ox) and titrate it with 0.12 M NaOH? Use the plot for the amounts of NaOH added during the reaction.

Answer 5

8.4

Question 6

Look again at these two reactions.

Which statement about these reactions is not correct?

Answer 6

Not all of these reactions actually occur during the double deprotonation of oxalic acid.

Question 7

Reaction 1 shows the deprotonation of oxalic acid. Reaction 2 shows the deprotonation of bioxalate.

Which statement about these reactions is not correct?

Answer 7

The H2O acts as a proton donor in reaction 1.

Question 8

When oxalic acid, H2(Ox), is titrated with NaOH, two deprotonation events occur. This is because oxalic acid is a diprotic acid; it has two protons to donate. For each deprotonation event, the Ka value is different.

Which statement about these reactions is not correct?

Answer 8

The forward reaction for Reaction 3 favors the products much more than the forward reaction for Reaction 2.

Question 9

Look at Point I of the reaction. At this point, oxalate anion, (Ox2− ), in solution. It is a weak base. Two of the reactions can occur at this point.

Which statement about the reaction at Point I is not correct?

Answer 9

The OH − in solution comes equally from the NaOH and the dissociation of oxalate anion (Reaction 3).

Question 10

Look at the plot (pH vs. volume of NaOH) for the titration of diprotic oxalic acid. The two deprotonation reactions that occur in this titration event are:

Which statement correctly describes point D in the plot?

Answer 10

Point D is the equivalence point for the first deprotonation event. At this point, it is assumed that all of the H2(Ox) has been consumed via the first deprotonation.

Question 11

At Point I of the reaction, oxalate anion (Ox2− ) is in solution. It is a weak base. Two of the reactions can occur at this point.

Which statement about the reaction at Point I is not correct?

Answer 11

The final concentration of (Ox)2−, when the system in Reaction 3 (at Point I) reaches final equilibrium will change significantly because (Ox)2− is combining with water to form H(Ox)− (aq) and OH − (aq).