ChemCollective-Virtual-Labs/C4/Determination-of-pKa-of-Acetic-acid/English-timed

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Time Narration
00:01 Welcome to the spoken tutorial on Determination of pKa of Acetic Acid using ChemCollective Vlabs.
00:09 In this tutorial, we will,

Titrate 0.1 molar acetic acid with 0.1 molar sodium hydroxide.

00:18 Estimate the equivalence point for the titration using an indicator
00:23 Find the equivalence point by the pH titration method.
00:28 Draw the titration curve to determine the pH at the half-equivalence point.
00:34 This tutorial is recorded using,

Ubuntu Linux OS version 18.04

00:41 ChemCollective Vlabs version 2.1.0
00:46 Java version 11.0.8
00:51 To follow this tutorial you should be familiar with ChemCollective Vlabs interface.
00:57 For the prerequisite tutorials please visit this website.
01:01 Let us begin with the dissociation of a weak acid.
01:05 The dissociation of a weak acid is mathematically described by the Henderson-Hasselbalch equation.
01:12 It relates pH and pKa to the equilibrium concentrations of acid and its conjugate base.
01:20 At the half-equivalence point, the concentration of acid and its conjugate base are equal.
01:27 Hence pH at this point is equivalent to pKa.
01:32 Here I have opened the Vlabs interface in the Default Lab Setup.
01:37 Workbench 1 opens by default.
01:41 Stockroom Explorer on the left contains required chemicals.
01:46 Let us titrate 0.1 molar acetic acid with 0.1 molar sodium hydroxide.
01:53 We will estimate the equivalence point for this titration.
01:57 First, let us place all the required chemicals and glassware on the workbench.
02:03 From the Stockroom Explorer double-click to select Distilled water.
02:08 From the Strong-bases cabinet select 0.1 molar sodium hydroxide.
02:14 Close the Strong-bases cabinet.
02:17 Open the Weak-acids cabinet.
02:21 The Weak acids cabinet has acetic acid and dichloroacetic acid.
02:26 We will choose 1 molar acetic acid.
02:29 Close the Weak-acids cabinet.
02:33 For the titration, we need 0.1 molar acetic acid.
02:38 So we will dilute 1 molar acetic acid by 10 times.
02:43 From the glassware menu select 250 ml Erlenmeyer flask.
02:49 Let us rename the flask as 0.1 M Acetic acid using the context menu.
02:57 Let us add 90 mL of water to the Erlenmeyer flask.
03:02 Drag the distilled water tank over the Erlenmeyer flask.
03:07 In the Transfer amount input bar type 90 and then click on Pour button.
03:14 Drag the distilled water tank back to its place.
03:18 Now let us add 10 mL of 1 molar acetic acid to the Erlenmeyer flask.
03:25 For this, we will pipet out 10 mL of 1 molar acetic acid.
03:31 From the glassware menu, select 10 mL pipet.
03:36 Place the pipet in 1 molar acetic acid flask and withdraw 10 mL.
03:44 Then place the filled pipet in the Erlenmeyer flask.
03:48 Type 10 in the input bar and click on Pour.
03:52 Drag and place the 1 molar acetic acid flask and pipet aside.
03:58 Let us delete the used pipet, Distilled water and 1M Acetic acid flask from the workbench.
04:06 Let us arrange the apparatus.
04:09 Select 0.1 M Acetic acid flask if not already selected.
04:15 Note the concentrations of: acetic acid, acetate ion and hydronium ions from the Solution Info panel.
04:24 Using these values we can calculate the Ka and pKa values of acetic acid.
04:31 Acetic acid is a weak acid.
04:34 In water, acetic acid dissociates into acetate ions and protons.
04:40 All these ions will be in equilibrium with each other.
04:45 Substitute the concentrations of acetic acid, acetate ion and hydronium ions in the equation.
04:54 The values of Ka and pKa of acetic acid are shown here.
05:00 The calculated value of pKa for acetic acid is 4.757.
05:07 Back to the workbench.
05:10 Let’s estimate the amount of sodium hydroxide needed to neutralize 20 ml of 0.1 molar acetic acid.
05:19 For this titration, we will use Phenolphthalein as an indicator.
05:24 Select Phenolphthalein from the Indicators cabinet.
05:28 Close the cabinet.
05:30 In the Solution Info panel, check the radio button for Spectrometer.
05:35 Phenolphthalein has an absorption around 465 to 610 nano metres.
05:43 This will allow us to accurately detect the endpoint.
05:48 From the glassware menu let us retrieve a 250 mL Erlenmeyer flask.
05:54 In the same manner, retrieve 50 mL buret and 10 mL pipet.
06:02 Rename the Erlenmeyer flask as Flask A
06:07 Here we will use precise transfer mode to transfer the chemicals.
06:12 Let’s pipet out 20 mL of 0.1 molar acetic acid into Flask A.
06:19 Since we are using a 10 mL pipet , we will pipet out twice.
06:24 Place the 10 mL pipet on the 0.1 molar Acetic acid flask.
06:30 In the Transfer amount input bar type 10 and click on Withdraw.
06:37 Place the pipet on Flask A, type 10 and click on Pour.
06:43 Similarly, we will withdraw 10 more mL of 0.1 molar acetic acid.
06:50 Then pour it into Flask A.
06:54 Let us delete the used pipet from the workbench.
06:58 Let’s add 0.2 mL of phenolphthalein to the flask using precise transfer.
07:06 In the Solution Info panel click on Aqueous radio button.
07:11 Then click on Flask A.
07:14 Let’s fill the buret with 50 mL of 0.1 molar sodium hydroxide.
07:20 Place the 0.1 molar sodium hydroxide flask on the buret.
07:25 Type 50 in the Transfer amount input bar and click on Pour.
07:31 Now let’s start the titration.
07:35 Place the buret on the flask containing acetic acid and indicator.
07:40 Initially let’s add sodium hydroxide in the increments of 0.5 mL.
07:47 In the Transfer amount input bar type 0.5.
07:52 Click on Pour button continuously to add 0.5 mL increments of sodium hydroxide.
08:00 Now we have added 12 mL of sodium hydroxide.
08:05 Click on the buret.
08:07 Solution Info panel shows 38 mL.
08:11 Let’s now add sodium hydroxide in 0.1 mL increments.
08:17 Let’s click on the Spectrometer radio button in the right panel.
08:22 Let us continue to add sodium hydroxide in 0.1 mL increments.
08:28 Observe the spectrometer in the right panel to note the neutralization point.
08:34 The neutralization point is also called the endpoint or equivalence point.
08:40 You will see some absorbance values in the Spectrometer at the endpoint.
08:45 You will also see a faint pink colour in flask A.
08:50 This indicates the endpoint of the titration.
08:54 Click on the buret and note the final volume of sodium hydroxide added.
08:59 Solution Info panel shows 30 mL.
09:03 The volume of sodium hydroxide rundown is 20 mL.
09:08 This is the amount of sodium hydroxide required to neutralize 20 mL of acetic acid.
09:15 Let’s perform a pH titration to determine the pH at the half-equivalence point.
09:22 For this, let's record the pH changes on every addition of sodium hydroxide to acetic acid.
09:29 Let us delete the used buret and Erlenmeyer flask from the workbench.
09:35 From the glassware menu let us retrieve a 250 mL Erlenmeyer flask.
09:41 In the same manner, retrieve 50 mL buret and 10 mL pipet.
09:49 Let’s rename the Erlenmeyer flask as Flask B.
09:54 Let us also delete the used 0.1 molar sodium hydroxide flask.
10:00 Let us again retrieve 0.1 molar sodium hydroxide from the Strong-bases cabinet.
10:07 Close the cabinet.
10:09 Click on the flask containing 0.1 molar acetic acid and note the pH.
10:15 The pH Meter shows the value as 2.88.
10:20 Let us transfer 20 mL of 0.1 molar acetic acid to Flask B.
10:27 First, let us withdraw 10 mL using the pipet.
10:31 Then pour it into Flask B.
10:35 Similarly, we will withdraw and pour 10 more mL of 0.1 molar acetic acid.
10:47 Let’s add 0.2 mL of phenolphthalein to Flask B using precise transfer.
10:55 Fill the buret with 50 mL of 0.1 molar sodium hydroxide.
11:03 Place the buret on Flask B.
11:05 Initially let us add sodium hydroxide in 0.5 mL increments to acetic acid.
11:14 Note the change in pH after every addition.
11:18 We continue to add sodium hydroxide, till 9 mL in the increments of 0.5 mL.
11:26 Note the change in pH after every addition.
11:30 At 9 mL we will change the transfer amount to 0.2 mL.
11:35 Then continue to add sodium hydroxide, in increments of 0.2 mL up to 18 mL.
11:43 Note the change in pH values
11:46 At 18 mL let's change the transfer amount to 0.1 mL.
11:52 Continue the titration till the endpoint is detected.
11:56 Again note the changes in pH values
12:00 At exactly 20 mL we see the equivalence point with a pH change.
12:06 Now again change the transfer amount to 0.2 mL.
12:11 Continue to add sodium hydroxide till 22 mL.
12:15 Here we see a very rapid rise in the pH values.
12:19 Note the pH changes again after each addition.
12:23 Note the final volume of sodium hydroxide in the buret.
12:28 Here I have noted the values and tabulated them.
12:32 The volume of sodium hydroxide added and the corresponding change in pH.
12:41 Now let’s plot a graph.
12:43 This is for the volume of sodium hydroxide added versus the change in pH.
12:49 Here is the plot.
12:51 It shows an equivalence point at pH 7.
12:55 At this point there is a sharp increase in pH.
12:59 The volume of NaOH at this point is 19.9 mL.
13:05 At exactly half volume is the half-equivalence point.
13:10 The pH at this point is 4.75.
13:14 According to the Henderson-Hasselbalch equation, at this point pH is equal to pKa.
13:21 Hence the value of pKa of acetic acid is 4.75.
13:26 Here we have used Grace software to plot the graph.
13:30 You can use any software that is convenient for you to plot the graph.
13:35 Let us summarize.
13:37 In this tutorial we have,

Titrated 0.1 Molar acetic acid with 0.1 Molar sodium hydroxide.

13:46 Estimated the equivalence point for the titration using an indicator
13:51 Found the equivalence point by the pH titration method.
13:56 Drawn the titration curve to determine the pH at the half-equivalence point.
14:02 As an Assignment,

Titrate 0.1 molar sodium hydroxide against 0.1 molar dichloroacetic acid

14:11 Estimate the pH at the endpoint.
14:14 Find Ka and pKa values.
14:17 Draw the graph to find the half-equivalence and equivalence points
14:23 The video at the following link summarizes the Spoken Tutorial project.

Please download and watch it.

14:29 The spoken tutorial project team, Conducts workshops and gives certificates.

For more details, please write to us

14:37 Please post your timed queries in this forum.
14:41 Spoken Tutorial Project is funded by the Ministry of Education, Government of India.
14:46 This tutorial is contributed by Madhuri Ganapathi and Snehalatha Kaliappan from IIT-Bombay.

Thank you for watching.

Contributors and Content Editors

PoojaMoolya