ChemActivity CA47b
Acid – Base Titrations
Model 1 – Titration of a strong acid with a strong base.
20.00 mL of HNO3 is titrated with 0.10 M NaOH. The acid-base reaction is
HNO3(aq) + NaOH(aq) → H2O(ℓ) + NaNO3(aq)
The net ionic reaction is
H3O+(aq) + OH −(aq) → 2 H2O(ℓ)
The figure below shows the titration at different volumes of sodium hydroxide added.
Critical Thinking Questions
1.
Why aren’t any HNO3 molecules shown in the pictures?
2.
Why are the species in the net ionic reaction in the model different than the acid-base reaction?
3.
What chemical species are shown in the model when:
a) 0.00 mL of base has been added?
b) 10.00 mL of base have been added?
c) 20.00 mL of base have been added?
4.
Which of the pictures in the model is appropriate when all of the hydronium ion present has been
consumed?
5.
If 20.00 mL of 0.10 M NaOH (aq) was used to titrate the 20.00 mL of HNO3, what was the
concentration of the acid? Confirm that the acid’s initial pH was 1.00.
6.
Other than nitrate ions, what ionic species are present in part C of Model 1? Why aren’t these
species shown?
Model 2 – Titration of a weak acid with a strong base.
20.00 mL of acetic acid (HOAc) is titrated with 0.10 M NaOH. The acid-base reaction is
HOAc(aq) + NaOH(aq) → H2O(ℓ) + NaOAc(aq)
The net ionic reaction is
HOAc(aq) + OH −(aq) → H2O(ℓ) + OAc −(aq)
The figure below shows the titration at different volumes of sodium hydroxide added.
Critical Thinking Questions
7.
Why are there HOAc molecules shown in picture A of Model 2?
8.
Why isn’t the net ionic reaction in this acid-base titration the same as the one shown in Model 1?
9.
What chemical species are shown in the model when:
a) 0.00 mL of base has been added?
b) 10.00 mL of base have been added?
c) 20.00 mL of base have been added?
10. Approximately how much of the original HOAc has been consumed after 10.00 mL of sodium
hydroxide was added?
11. The acid dissociation constant for HOAc is 1.8 x 10-5 at 25 °C. Verify that the pH of the solution
in part B of Model 2 is 4.75.
12. Over what approximate range of pH values will this solution behave as a buffer?
13. Which picture in Model 2 represents the equivalence point? Use the titration data for the
equivalence point to determine the concentration of the acetic acid in the original sample.
14. Use the pH data from picture A to confirm your answer to CTQ 13.
15. What is the major species present at the equivalence point? Is the solution acidic or basic? If this
species can react with water, write the reaction that would occur.
16. Determine the value of Kb for OAc − and verify that the pH of the solution at the equivalence point
is 8.72.
17. If you titrated a weak base with a strong acid, would you expect the pH at the equivalence point to
be acidic, basic or neutral? Explain.
Model 3 – Titration curves for the titration of HNO3 (a strong acid and
HOAc (a weak acid) with 0.10 M NaOH.
Critical Thinking Questions
18. Which titration curve (○ or ●) is for HOAc? Which is for HNO3? Explain your choice.
19. Indicate the equivalence point on each curve.
20. On the HOAc curve indicate the following:
a) The major chemical species present at the beginning of the titration.
b) The buffer region and the major species present in this region.
c) The point at which the pH = pKa.
d) The major chemical species present at the equivalence point.
e) The major chemical species present after the equivalence point.
Exercises
1.
What is the pH in a solution containing a weak acid HX and its conjugate base X- when [X-] =
[HX]?
2.
What is the significance of the pH halfway to the equivalence point in a titration of weak acid
with a strong base?
3.
What is the pH when 50.0 mL of 1.50 M HC2H3O2 is titrated with 25.0 mL of 1.50 M NaOH? The
Ka of HC2H3O2 is 1.8 × 10 ‒ 5.
4.
What is the pH when 50.0 mL of 1.50 M NaC2H3O2 is titrated with 25.0 mL of 1.50 M HCl? The
Ka of HC2H3O2 is 1.8 × 10‒5.
5.
What is the significance of the pH halfway to the equivalence point in a titration of weak base
with a strong acid?
6.
The Ka of carbonic acid, H2CO3 is 4.5 × 10 ‒7. What is the Kb of its conjugate base? Sketch a
titration curve (assuming all CO2 stays in solution) of 0.10 M NaHCO3(aq) with 0.10 M HCl.
Label the equivalence point and the point at which [OH ‒] = Kb for HCO3 ‒
Problems
1.
What is the pH when 50.0 mL of 1.50 M HC2H3O2 is titrated with 25.0 mL, 50.0 mL, and
60.0 mL of 1.50 M NaOH? The Ka of HC2H3O2 is 1.8 × 10-5.
2.
What is pH when 50.0 mL of 1.50 M Na C2H3O2 (aq) is titrated with 25.0 mL, 50.0 mL, and
60.0 mL of 1.50 M HCl? The Ka of HC2H3O2 is 1.8 × 10-5.
3.
A 0.3258 g sample of an unknown weak diprotic (i.e., 2 acidic H+) acid is titrated with 43.25 mL
of 0.1087 M NaOH. What is the molecular weight of the unknown acid?