ChemCollective-Virtual-Labs/C4/Determination-of-pKa-of-Acetic-acid/English
| Visual Cue | Narration |
| Slide Number 1
Title Slide |
Welcome to the spoken tutorial on Determination of pKa of Acetic Acid using ChemCollective Vlabs. |
| Slide Number 2
Learning Objectives |
In this tutorial, we will,
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| Slide Number 3
Learning Objectives |
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| Slide Number 4
System Requirement |
This tutorial is recorded using,
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| Slide Number 5
Pre-requisites |
To follow this tutorial you should be familiar with ChemCollective Vlabs interface.
For the prerequisite tutorials please visit this website. |
| Slide Number 6
Dissociation of a Weak Acid Ka= [H+][A-]/ [HA] Pka= log Ka pH = pKa+ log {[A-] / [HA]}
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Let us begin with the dissociation of a weak acid.
The dissociation of a weak acid is mathematically described by the Henderson-Hasselbalch equation. It relates pH and pKa to the equilibrium concentrations of acid and its conjugate base. At the half-equivalence point, the concentration of acid and its conjugate base are equal.
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| Point to the Default lab setup window. | Here I have opened the Vlabs interface in the Default Lab Setup. |
| Point to the workbench1.
Point to the Stockroom Explorer and chemicals. |
Workbench 1 opens by default.
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| Cursor on the workbench 1.
Titrate 0.1 molar Acetic acid with 0.1 molar Sodium hydroxide. Estimate the equivalence point for this titration |
Let us titrate 0.1 molar acetic acid with 0.1 molar sodium hydroxide.
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| Cursor on the workbench 1. | First, let us place all the required chemicals and glassware on the workbench. |
| From the Stockroom Explorer double-click to select Distilled water. | From the Stockroom Explorer double-click to select Distilled water. |
| Double-click on the Strong-bases cabinet.
From the cabinet, double-click to select 0.1 M NaOH. Click on the radio button to close. |
From the Strong-bases cabinet select 0.1 molar sodium hydroxide.
Close the Strong-bases cabinet. |
| Double-click on the Weak-acids cabinet.
From the cabinet double-click to select 1 M CH3COOH. Click on the radio button to close. |
Open the Weak-acids cabinet.
The Weak acids cabinet has acetic acid and dichloroacetic acid. We will choose 1 molar acetic acid. Close the Weak-acids cabinet. |
| Cursor on the workbench. | For the titration, we need 0.1 molar acetic acid.
So we will dilute 1 molar acetic acid by 10 times. |
| Click on the Glassware menu and select 250 mL Erlenmeyer Flask. | From the glassware menu select 250 ml Erlenmeyer flask. |
| Right-click on the Flask.
Select Rename from the context menu. Type 0.1 M Acetic acid >> click on OK button. |
Let us rename the flask as 0.1 M Acetic acid using the context menu. |
| Point to Erlenmeyer flask.
Drag the distilled water tank over to the Erlenmeyer flask. In the Transfer amount input bar type 90 >> Click on the Pour button. |
Let us add 90 mL of water to the Erlenmeyer flask.
Drag the distilled water tank over the Erlenmeyer flask. In the Transfer amount input bar type 90 and then click on Pour button. |
| Drag the distilled water tank to the top left corner. | Drag the distilled water tank back to its place. |
| Point to 1 molar acetic acid. | Now let us add 10 mL of 1 molar acetic acid to the Erlenmeyer flask.
For this, we will pipet out 10 mL of 1 molar acetic acid. |
| Click on Glassware menu >> Select Pipets >> From the submenu select 10 mL pipet | From the glassware menu, select 10 mL pipet. |
| Place the pipet in 1 molar acetic acid flask.
In the Transfer amount input bar type 10 >> Click on Withdraw button. |
Place the pipet in 1 molar acetic acid flask and withdraw 10 mL. |
| Place the filled pipet in the Erlenmeyer flask.
In the Transfer amount input bar type 10 >> Click on the Pour button. |
Then place the filled pipet in the Erlenmeyer flask.
Type 10 in the input bar and click on Pour. |
| Drag the 1 M acetic acid flask and pipet aside. | Drag and place the 1 molar acetic acid flask and pipet aside. |
| Click on the pipet to select >> press Delete key on the keyboard.
Drag and place the flask on the top of the workbench. |
Let us delete the used pipet, Distilled water and 1M Acetic acid flask from the workbench.
Let us arrange the apparatus. |
| Select 0.1 M Acetic acid flask.
Point to the values in the Solution Info panel. Point to the H+ ion value. Point to CH3COO- ion value. Point to CH3COOH value. |
Select 0.1 M Acetic acid flask if not already selected.
Note the concentrations of: acetic acid, acetate ion and hydronium ions from the Solution Info panel. Using these values we can calculate the Ka and pKa values of acetic acid. |
| Slide Number 7
Calculations CH3COOH + H2O -> CH3COO- + H3O+
[CH3COO- ]= 1.314x 10-3 [CH3COOH]= 9.869x10-2
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Acetic acid is a weak acid.
In water, acetic acid dissociates into acetate ions and protons. All these ions will be in equilibrium with each other. Substitute the concentrations of acetic acid, acetate ion and hydronium ions in the equation. |
| Slide Number 8
Calculations
Ka =1.75 X 10-5
pka = 4.757 |
The values of Ka and pKa of acetic acid are shown here.
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| Estimate the amount of sodium hydroxide needed to neutralize 20 ml of 0.1 molar acetic acid. | Back to the workbench.
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| Double-click on the Indicators cabinet >> double-click to select Phenolphthalein.
Click on the radio button to close. |
For this titration, we will use Phenolphthalein as an indicator.
Select Phenolphthalein from the Indicators cabinet. Close the cabinet. |
| Point to the Solution Info Panel.
Point to the absorbance. |
In the Solution Info panel, check the radio button for Spectrometer.
This will allow us to accurately detect the endpoint. |
| Click on the Glassware menu >> select 250 mL Erlenmeyer Flask
Click on the Glassware menu>> Select 50 mL Buret Click on the Glassware menu >> Pipets >> Select 10 mL pipet. |
From the glassware menu let us retrieve a 250 mL Erlenmeyer flask.
In the same manner, retrieve 50 mL buret and 10 mL pipet. |
| Right-click on the Flask.
Select Rename from the context menu. Type Flask A >> click on the OK button. |
Rename the Erlenmeyer flask as Flask A |
| Tools >> Transfer Bar >> Precise Transfer. | Here we will use precise transfer mode to transfer the chemicals. |
| Point to 0.1 M acetic acid and 250 mL Erlenmeyer flask.
Place the Pipet in the 0.1 M Acetic acid flask. |
Let’s pipet out 20 mL of 0.1 molar acetic acid into Flask A.
Since we are using a 10 mL pipet , we will pipet out twice. Place the 10 mL pipet on the 0.1 molar Acetic acid flask. |
| Type 10 in the transfer amount input bar.
Click on Withdraw. |
In the Transfer amount input bar type 10 and click on Withdraw. |
| Place the pipet on 250 mL Erlenmeyer flask.
Type 10 in the transfer amount input bar. Click on Pour. |
Place the pipet on Flask A, type 10 and click on Pour. |
| Place the Pipet in the 0.1 M Acetic acid flask.
Type 10 in the transfer amount input bar. Click on Withdraw. |
Similarly, we will withdraw 10 more mL of 0.1 molar acetic acid. |
| Place the pipet on 250 mL Erlenmeyer flask.
Type 10 in the transfer amount input bar. Click on Pour. |
Then pour it into Flask A. |
| Click on the pipet to select >> press Delete key on the keyboard. | Let us delete the used pipet from the workbench. |
| Place the phenolphthalein bottle on the flask
In the Transfer amount input bar type 0.2 and click on Pour. |
Let’s add 0.2 mL of phenolphthalein to the flask using precise transfer. |
| In the Solution Info panel click on Aqueous radio button.
Click on Flask A and check the solution info panel. |
In the Solution Info panel click on Aqueous radio button.
Then click on Flask A. |
| Point to 0.1 molar sodium hydroxide.
Place the 0.1 molar sodium hydroxide flask on the buret. |
Let’s fill the buret with 50 mL of 0.1 molar sodium hydroxide.
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| Type 50 in the Transfer amount input bar and click on the Pour button. | Type 50 in the Transfer amount input bar and click on Pour. |
| Place the buret on the flask containing acetic acid and indicator. | Now let’s start the titration.
Place the buret on the flask containing acetic acid and indicator. |
| Point to the buret containing sodium hydroxide. | Initially let’s add sodium hydroxide in the increments of 0.5 mL. |
| In the Transfer amount input bar type 0.5.
(Flask A volume 32.2 mL) Click on Pour continuously to add 0.5 mL increments of sodium hydroxide. |
In the Transfer amount input bar type 0.5.
Click on Pour button continuously to add 0.5 mL increments of sodium hydroxide. Now we have added 12 mL of sodium hydroxide. |
| Click on the buret.
Point to Solution info panel. Solution Info panel shows 38 mL. (50 -12 = 38 ) |
Click on the buret.
Solution Info panel shows 38 mL. |
| Type 0.1 in the transfer amount Input bar.
Click on Pour. |
Let’s now add sodium hydroxide in 0.1 mL increments. |
| Click on the Spectrometer radio button in the right panel. | Let’s click on the Spectrometer radio button in the right panel. |
| Click on the Pour button continuously to add sodium hydroxide. | Let us continue to add sodium hydroxide in 0.1 mL increments. |
| Cursor on the Absorbance values in the right panel.
Point to the pink colour in the flask. |
Observe the spectrometer in the right panel to note the neutralization point.
The neutralization point is also called the endpoint or equivalence point. You will see some absorbance values in the Spectrometer at the endpoint. You will also see a faint pink colour in flask A. This indicates the endpoint of the titration. |
| (Flask A volume 40.2 mL)
Click on the buret and see the volume of NaOH in the Solution Info Panel. |
Click on the buret and note the final volume of sodium hydroxide added. |
| Solution Info panel shows 30 mL.
(50- 20= 30 mL) |
Solution Info panel shows 30 mL.
The volume of sodium hydroxide rundown is 20 mL. This is the amount of sodium hydroxide required to neutralize 20 mL of acetic acid. |
| Perform a pH titration to determine the pH at the half-equivalence point. | Let’s perform a pH titration to determine the pH at the half-equivalence point. |
| Record the pH changes on every addition of sodium hydroxide to acetic acid. | For this, let's record the pH changes on every addition of sodium hydroxide to acetic acid. |
| Click on the buret to select >> press the Delete key on the keyboard.
Click on the Erlenmeyer flask to select >> press Delete key on the keyboard. |
Let us delete the used buret and Erlenmeyer flask from the workbench. |
| Click on the Glassware menu >> select 250 mL Erlenmeyer Flask
Click on the Glassware menu>> Select 50 mL Buret. Click on the Glassware menu>> Pipets >> Select 10 mL pipet. |
From the glassware menu let us retrieve a 250 mL Erlenmeyer flask.
In the same manner, retrieve 50 mL buret and 10 mL pipet. |
| Right-click on the Flask.
Select Rename from the context menu. Type Flask B >> click on the OK button. |
Let’s rename the Erlenmeyer flask as Flask B. |
| Click on 0.1 sodium hydroxide flask to select >> press the Delete key on the keyboard. | Let us also delete the used 0.1 molar sodium hydroxide flask. |
| Double-click on the Strong-bases cabinet.
From the cabinet, double-click to select 0.1 M NaOH. Click on the radio button to close. |
Let us again retrieve 0.1 molar sodium hydroxide from the Strong-bases cabinet.
Close the cabinet. |
| Click on the flask containing 0.1 molar acetic acid and note the pH on the pH Meter.
Point to the value on the PH Meter. |
Click on the flask containing 0.1 molar acetic acid and note the pH.
The pH Meter shows the value as 2.88. |
| Point to the 250 mL Erlenmeyer flask.
Place the Pipet in the 0.1 M Acetic acid flask. Type 10 in the transfer amount input bar. Click on Withdraw. |
Let us transfer 20 mL of 0.1 molar acetic acid to Flask B.
First, let us withdraw 10 mL using the pipet. |
| Place the pipet on 250 mL Erlenmeyer flask.
Type 10 in the transfer amount input bar. Click on Pour. |
Then pour it into Flask B. |
| Place the Pipet in the 0.1 M Acetic acid flask.
Type 10 in the transfer amount input bar. Click on Withdraw. Place the pipet on 250 mL Erlenmeyer flask. Type 10 in the transfer amount input bar. Click on Pour. |
Similarly, we will withdraw and pour 10 more mL of 0.1 molar acetic acid. |
| Place the phenolphthalein bottle on the flask
In the Transfer amount input bar type 0.2 and click on Pour. |
Let’s add 0.2 mL of phenolphthalein to Flask B using precise transfer. |
| Place 0.1 molar Sodium hydroxide flask on the buret.
In the transfer amount input bar type 50 >> click on Pour. |
Fill the buret with 50 mL of 0.1 molar sodium hydroxide. |
| Place the buret on the Erlenmeyer flask. | Place the buret on Flask B. |
| Type 0.5 in the transfer amount input bar >> click on the Pour button. | Initially let us add sodium hydroxide in 0.5 mL increments to acetic acid.
Note the change in pH after every addition. |
| Click on the Pour button continuously.
Point to the PH Meter after each addition. |
We continue to add sodium hydroxide, till 9 mL in the increments of 0.5 mL.
Note the change in pH after every addition. |
| (Flask B volume 29.2 mL)
Point to the Solution info panel. Solution info panel shows 41 mL. (50−9 = 41) |
At 9 mL we will change the transfer amount to 0.2 mL. |
| Type 0.2 in the transfer amount input bar >> click on the Pour button continuously.
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Then continue to add sodium hydroxide, in increments of 0.2 mL up to 18 mL.
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| (Flask B volume 38.2 mL)
Point to the Solution info panel. Solution info panel shows 32 mL. (50−18= 32) |
At 18 mL let's change the transfer amount to 0.1 mL. |
| Type 0.1 in the transfer amount input bar >> click on the Pour button continuously.
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Continue the titration till the endpoint is detected.
Again note the changes in pH values |
| (Flask B volume 40.2 mL)
Point to the flask with faint pink colour. Point to the PH Meter. |
At exactly 20 mL we see the equivalence point with a pH change. |
| (Flask B volume 42.2 mL)
Type 0.2 in the transfer amount input bar >> click on the Pour button continuously. (50-22= 28 mL) |
Now again change the transfer amount to 0.2 mL.
Continue to add sodium hydroxide till 22 mL. |
| Point to the PH Meter.
Click on the buret to note the final volume of sodium hydroxide. |
Here we see a very rapid rise in the pH values.
Note the pH changes again after each addition. Note the final volume of sodium hydroxide in the buret. |
| Show the table containing the Volume of NaOH and pH change. | Here I have noted the values and tabulated them.
The volume of sodium hydroxide added and the corresponding change in pH. |
| Point to the volume of sodium hydroxide.
Point to pH values |
Now let’s plot a graph.
This is for the volume of sodium hydroxide added versus the change in pH. |
| Show the plot.
Point to the plot. Point to the plot. Point to the value 4.75. |
Here is the plot.
It shows an equivalence point at pH 7. At this point there is a sharp increase in pH.
The pH at this point is 4.75. According to the Henderson-Hasselbalch equation, at this point pH is equal to pKa. Hence the value of pKa of acetic acid is 4.75. |
| Point to Grace software. | Here we have used Grace software to plot the graph.
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| Let us summarize. | |
| Slide Number 9
Summary |
In this tutorial we have,
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| Slide Number 10
Summary |
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| Slide Number 11
Assignment |
As an Assignment,
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| Slide Number 12
About Spoken Tutorial project |
The video at the following link summarizes the Spoken Tutorial project.
Please download and watch it. |
| Slide Number 13
Spoken Tutorial workshops |
The spoken tutorial project team,
Conducts workshops and gives certificates. For more details, please write to us |
| Slide Number 14
Forum for specific questions:
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Please post your timed queries in this forum. |
| Slide Number 15
Acknowledgement |
Spoken Tutorial Project is funded by the Ministry of Education, Government of India. |
| This tutorial is contributed by Madhuri Ganapathi and Snehalatha Kaliappan from IIT-Bombay.
Thank you for watching. |
