View source for ChemCollective-Virtual-Labs/C4/Determination-of-pKa-of-Acetic-acid/English


{| border=1
|| '''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, 
* 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.
|-
|| '''Slide Number 3'''

'''System Requirement'''
|| This tutorial is recorded using,

* '''Ubuntu Linux''' OS version 18.04
* '''ChemCollective Vlabs''' version 2.1.0
* Java version 11.0.8

|-
|| '''Slide Number 4'''

'''Pre-requisites'''

'''https://spoken-tutorial.org.'''
|| To follow this tutorial you should be familiar with '''ChemCollective Vlabs''' interface.

For the prerequisite tutorials please visit this website.
|-
|| '''Slide Number 5'''

'''Dissociation of a Weak Acid'''

Ka= [H<sup>+</sup>][A<sup>-</sup>]/ [HA] 

Pka= log Ka
pH = pKa+ log {[A<sup>-</sup>] / [HA]}


pH =pKa
|| 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''' pK<sub>a'''</sub> 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'''.
|-
|| 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.


'''Stockroom Explorer ''' on the left contains required chemicals.
|-
|| 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.


We will estimate the equivalence point for this titration.
|-
|| 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 CH<sub>3</sub>COOH.'''

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''' pipe'''t, '''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<sup>+</sup> ion value.

Point to CH<sub>3</sub>COO<sup>-</sup> ion value. 

Point to CH<sub>3</sub>COOH 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 '''K<sub>a</sub> '''and''' pKa''' values of acetic acid.
|-
|| '''Slide Number 6 '''

'''Calculations'''

CH<sub>3</sub>COOH + H<sub>2</sub>O -> CH<sub>3</sub>COO<sup>-</sup> + H<sub>3</sub>O<sup>+</sup>


[H<sub>3</sub>O<sup>+</sup>] =1.314x 10<sup>-3</sup> 

[CH<sub>3</sub>COO<sup>-</sup> ]= 1.314x 10<sup>-3</sup>

[CH<sub>3</sub>COOH]= 9.869x10<sup>-2</sup>


Ka<sup> </sup><nowiki>=</nowiki><sup> </sup>[CH<sub>3</sub>COO<sup>-</sup> ][H<sub>3</sub>O / [CH<sub>3</sub>COOH]


|| 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 7'''

'''Calculations'''


Ka = (1.314x 10<sup>-3</sup> X 1.314x 10<sup>-3</sup> / 9.869x10<sup>-2)</sup>

Ka =1.75 X 10<sup>-5</sup>


pka =-logka = -(1.75 X 10<sup>-5</sup>)

pka = 4.757
||  The values of '''K<sub>a''' </sub>and '''pKa '''of acetic acid are shown here.


The calculated value of '''pKa ''' for acetic acid is 4.757.
|-
|| Estimate the amount of sodium hydroxide needed to neutralize 20 ml of 0.1 molar acetic acid.
|| Back to the '''workbench.'''


Let’s estimate the amount of sodium hydroxide needed to neutralize 20 ml of 0.1 molar acetic acid.
|-
|| 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'''. 


'''Phenolphthalein''' has an absorption around 465 to 610 '''nano metres'''. 

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.


Place the 0.1 molar sodium hydroxide flask on the buret.

|-
|| 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 ''' 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.


Point to the ''' PH Meter '''after each addition.
|| Then continue to add sodium hydroxide, in increments of 0.2 mL up to 18 '''mL'''.


Note the change in '''pH''' values
|-
|| (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.


Point to the''' PH Meter '''after each addition.
|| 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 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.

|-
|| Point to Grace software.
|| 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.
|-
|| '''Slide Number 8'''

'''Summary '''

|| 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.

|-
|| '''Slide Number 9'''

'''Assignment '''
|| As an Assignment,

# Titrate 0.1 M sodium hydroxide against 0.1 M dichloroacetic acid 
# Estimate the '''pH''' at the endpoint. 
# Find '''Ka''' and''' pKa''' values.
# Draw the graph to find the half-equivalence and equivalence points

|-
|| '''Slide Number 10'''

'''About Spoken Tutorial project'''
|| The video at the following link summarizes the Spoken Tutorial project.

Please download and watch it.
|-
|| '''Slide Number 11'''

'''Spoken Tutorial workshops'''
|| The spoken tutorial project team, 

Conducts workshops and gives certificates. 

For more details, please write to us
|-
|| '''Slide Number 12'''

'''Forum for specific questions:'''

'''   '''
|| Please post your timed queries in this forum.

|-
|| '''Slide Number 13'''

'''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.
|-
|}