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

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.

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