Welcome to the spoken tutorial on Determination of pKa of Acetic Acid using ChemCollective Vlabs.
In this tutorial, we will, Titrate 0.1 molar acetic acid with 0.1 molar sodium hydroxide.
Estimate the equivalence point for the titration using an indicator
Find the equivalence point by the pH titration method.
Draw the titration curve to determine the pH at the half-equivalence point.
This tutorial is recorded using, Ubuntu Linux OS version 18.04
ChemCollective Vlabs version 2.1.0
Java version 11.0.8
To follow this tutorial you should be familiar with ChemCollective Vlabs interface.
For the prerequisite tutorials please visit this website.
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.
Hence pH at this point is equivalent to pKa.
Here I have opened the Vlabs interface in the Default Lab Setup.
Workbench 1 opens by default.
Stockroom Explorer on the left contains required chemicals.
Let us titrate 0.1 molar acetic acid with 0.1 molar sodium hydroxide.
We will estimate the equivalence point for this titration.
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 Strong-bases cabinet select 0.1 molar sodium hydroxide.
Close the Strong-bases cabinet.
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.
For the titration, we need 0.1 molar acetic acid.
So we will dilute 1 molar acetic acid by 10 times.
From the glassware menu select 250 ml Erlenmeyer flask.
Let us rename the flask as 0.1 M Acetic acid using the context menu.
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 back to its place.
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.
From the glassware menu, select 10 mL pipet.
Place the pipet in 1 molar acetic acid flask and withdraw 10 mL.
Then place the filled pipet in the Erlenmeyer flask.
Type 10 in the input bar and click on Pour.
Drag and place the 1 molar acetic acid flask and pipet aside.
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 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.
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.
The values of Ka and pKa of acetic acid are shown here.
The calculated value of pKa for acetic acid is 4.757.
Back to the workbench.
Let’s estimate the amount of sodium hydroxide needed to neutralize 20 ml of 0.1 molar acetic acid.
For this titration, we will use Phenolphthalein as an indicator.
Select Phenolphthalein from the Indicators cabinet.
Close the cabinet.
In the Solution Info panel, check the radio button for Spectrometer.
Phenolphthalein has an absorption around 465 to 610 nano metres.
This will allow us to accurately detect the endpoint.
From the glassware menu let us retrieve a 250 mL Erlenmeyer flask.
In the same manner, retrieve 50 mL buret and 10 mL pipet.
Rename the Erlenmeyer flask as Flask A
Here we will use precise transfer mode to transfer the chemicals.
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.
In the Transfer amount input bar type 10 and click on Withdraw.
Place the pipet on Flask A, type 10 and click on Pour.
Similarly, we will withdraw 10 more mL of 0.1 molar acetic acid.
Then pour it into Flask A.
Let us delete the used pipet from the workbench.
Let’s add 0.2 mL of phenolphthalein to the flask using precise transfer.
In the Solution Info panel click on Aqueous radio button.
Then click on Flask A.
Let’s fill the buret with 50 mL of 0.1 molar sodium hydroxide.
Place the 0.1 molar sodium hydroxide flask on the buret.
Type 50 in the Transfer amount input bar and click on Pour.
Now let’s start the titration.
Place the buret on the flask containing acetic acid and indicator.
Initially let’s add sodium hydroxide in the increments of 0.5 mL.
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.
Solution Info panel shows 38 mL.
Let’s now add sodium hydroxide in 0.1 mL increments.
Let’s click on the Spectrometer radio button in the right panel.
Let us continue to add sodium hydroxide in 0.1 mL increments.
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.
Click on the buret and note the final volume of sodium hydroxide added.
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.
Let’s perform a pH titration to determine the pH at the half-equivalence point.
For this, let's record the pH changes on every addition of sodium hydroxide to acetic acid.
Let us delete the used buret and Erlenmeyer flask from the workbench.
From the glassware menu let us retrieve a 250 mL Erlenmeyer flask.
In the same manner, retrieve 50 mL buret and 10 mL pipet.
Let’s rename the Erlenmeyer flask as Flask B.
Let us also delete the used 0.1 molar sodium hydroxide flask.
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.
The pH Meter shows the value as 2.88.
Let us transfer 20 mL of 0.1 molar acetic acid to Flask B.
First, let us withdraw 10 mL using the pipet.
Then pour it into Flask B.
Similarly, we will withdraw and pour 10 more mL of 0.1 molar acetic acid.
Let’s add 0.2 mL of phenolphthalein to Flask B using precise transfer.
Fill the buret with 50 mL of 0.1 molar sodium hydroxide.
Place the buret on Flask B.
Initially let us add sodium hydroxide in 0.5 mL increments to acetic acid.
Note the change in pH after every 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.
At 9 mL we will change the transfer amount to 0.2 mL.
Then continue to add sodium hydroxide, in increments of 0.2 mL up to 18 mL.
Note the change in pH values
At 18 mL let's change the transfer amount to 0.1 mL.
Continue the titration till the endpoint is detected.
Again note the changes in pH values
At exactly 20 mL we see the equivalence point with a pH change.
Now again change the transfer amount to 0.2 mL.
Continue to add sodium hydroxide till 22 mL.
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.
Here I have noted the values and tabulated them.
The volume of sodium hydroxide added and the corresponding change in pH.
Now let’s plot a graph.
This is for the volume of sodium hydroxide added versus the change in pH.
Here is the plot.
It shows an equivalence point at pH 7.
At this point there is a sharp increase in pH.
The volume of NaOH at this point is 19.9 mL.
At exactly half volume is the half-equivalence point.
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.
Here we have used Grace software to plot the graph.
You can use any software that is convenient for you to plot the graph.
Let us summarize.
In this tutorial we have, Titrated 0.1 Molar acetic acid with 0.1 Molar sodium hydroxide.
Estimated the equivalence point for the titration using an indicator
Found the equivalence point by the pH titration method.
Drawn the titration curve to determine the pH at the half-equivalence point.
As an Assignment, Titrate 0.1 molar sodium hydroxide against 0.1 molar dichloroacetic acid
Estimate the pH at the endpoint.
Find Ka and pKa values.
Draw the graph to find the half-equivalence and equivalence points
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This tutorial is contributed by Madhuri Ganapathi and Snehalatha Kaliappan from IIT-Bombay. Thank you for watching.