Determination of pKa and pKb Using Acid-Base Titrations Lab Report


Objectives:
This experiment aims at determining the concentration of acid-base equilibrium constants for acetic acid and aqueous ammonia using titration curves. 

Introduction:
The main objective in the volumetric titration of an acid with a base is the determination of the equivalence point. The equivalence point is a point where the acid has been fully neutralized by the base. One can determine the completeness of the reaction by adding an indicator that changes when the reaction is approaching an equivalence point. Furthermore, the completeness of a reaction can be determined by a pH meter. A pH meter measures the actual pH of the solution during titration. 

pH titration curves:
In titration curves of a weak acid or a weak base, the equivalence point lies at the midpoint of particular two points. Equivalent point is the inflection point on the titration curve between these points. Once the equivalence point is established, the volume of a titrant used can be directly read from the curve. Additionally, pKa and pKb are also read directly from the graph. pKa and pKb are the same as the pH of the half-equivalence. The titrant volume together with stoichiometry are used to find the concentration an acid or a base. 

Available Materials and Equipment:

  • Burets
  • Volumetric flask
  • Erlenmeyer flasks
  • Volumetric pipettes
  • Beakers
  • 0.100M NaOH
  • 0.100M HCl
  • Acetic acid
  • Aqueous ammonia
  • Graduated cylinders

Procedures:
(1)Preparation of Acetic Acid
20ml of concentrated acetic was obtained and added into a 50ml beaker. A graduated cylinder was used to add 10ml of distilled water into a 100ml volumetric flask. 5.0ml concentrated acetic acid was pipetted into a 100ml volumetric which already had 10ml of distilled water. After adding acetic acid, the volumetric was swirled to thoroughly mix the solution. The volumetric flask was then topped to the mark with distilled water and covered. 

(2) Titration of Acetic Acid with Sodium Hydroxide
10ml of distilled water was added into a 250ml Erlenmeyer flask. 20ml of diluted acetic acid was then added into it, followed by setting up a titration system with 0.100M of NaOH in buret. A pH meter was used to monitor the pH of the solution as the base (NaOH) was being added. Sodium hydroxide was added in an increment of 1ml until the solution pH reached 4.8. After the solution pH reached, the base was added in an increment of 0.2ml until the equivalence point was passed. After the equivalence point, the base was added in an internal of 1ml until there was an insignificant change in the solution pH. The volume of the base and pH of the solution were recorded and used to draw a titration curve. 

(3) Titration of Ammonia with HCl
Aqueous ammonia (35ml) was added into a 50ml beaker. 20ml of it was then pipetted into a 250 Erlenmeyer flask, followed by setting up a titration system with 0.100M HCl in the buret. As in the case of titration of an acetic acid with sodium hydroxide, a pH meter was used to monitor the pH of the solution as HCl was being added. HCl was added in an interval of 1ml until the pH of the solution was 9. When the solution pH reached 9.5, HCl was added in an increment of 0.2ml until the equivalence was passed. After this, HCl was added in an interval of 1ml until there was minimal change in the solution pH. The volume of HCl used and pH for each addition of HCl was recorded and used to draw the titration curve. 

Calculations:
Part 1: Titration of acetic acid with sodium hydroxide
Graphs: 



(1)Using your titration curve, determine the volume of NaOH required to reach the equivalence point. Identify this volume and the equivalence point on your graph.
Answer: 9.3ml. 

(2) Using your titration curve, determine the volume of NaOH required to reach the half-equivalence point.
Answer: 4.7ml. 

(3) Calculate the concentration (M) of the acetic acid solution.
Answer: 0.0235M.

(4) Calculate the concentration of original acetic acid.
Answer: 0.094M.

(5)Determine pKa and Ka of the acetic solution.
pKa=4.3
Ka=5.012 X 10-5

Part 2: Titration of ammonia with HCl
(1) Using your titration curve, determine the volume of HCl required to reach the equivalence point. Identify this volume and the equivalence point on your graph.
Answer: 16.4ml.

(2) Calculate the concentration (M) of the aqueous ammonia solution.
Answer: 0.0571M.

(3) Using your titration curve, determine the volume of HCl required to reach the half-equivalence point.
Answer: 8.2ml.

(4) Determine the pKa and Ka for the ammonia ion.
Answer: pKa=4.43, and 3.715 X 10-5.

(5) Determine pKb and Kb for ammonia.
Answer: pKb=9.57, and 2.692 X 10-10.

(1) Write a brief paragraph your two titration curves. You should discuss the similarities and the differences between the components of the solutions at each of the following points. You should write equations and use your data to support your answers.
(a) Between the initial pH and equivalence point
(b) At the equivalence point 

(2) Compare the similarities or differences of your curves along x-axis and y-axis. 

(3) The accepted values for the pKa and Ka of acetic acid are 4.74 and 1.8 X 10-5 respectively. How do your experimentally determined values compare to accepted values? Percent error is a good way to make this comparison. 

(4) The accepted values for pKb and Kb are of ammonia are 4.75 and 1.76 X 10-5 respectively. How do your experimentally determined values compare to accepted values? Percent error is a good way to make this comparison. 

(5) Which method of monitoring a titration (using a pH meter or using indicator) do you consider to be more useful? Explain. 

Comments

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