Welcome to this tutorial on Dilutions and pH measurement.
In this tutorial, we will learn to measure: Change in pH for solutions of acid and base on dilution.
Variation in pH by common-ion effect for weak acids and bases.
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Here, I am using: Mac OS version 10.10.5
ChemCollective Vlabs version 2.1.0
Java version 8.
For this demonstration, let us dilute - 0.1 molar Hydrochloric acid to 0.01 molar Hydrochloric acid.
0.1 molar sodium hydroxide to 0.01 molar sodium hydroxide.
For this experiment, 2 ml of 0.1 M Hydrochloric acid or sodium hydroxide is diluted to 20 ml with distilled water.
I have opened a “Virtual Chemistry labs” application window.
Stockroom explorer opens with solutions of various acids and bases. Double-click on Strong-acids cabinet.
A list of acidic solutions with various strengths opens.
Double-click to choose 0.1 Molar Hydrocloric acid. Click on the flask, observe the pH on the pH Meter.
Note that pH Meter reads 1. Click on the radio button near Strong-acids cabinet, to close the list.
Next, from Stockroom explorer, double-click on Strong-bases cabinet. A list of basic solutions with various strengths opens.
Double-click to choose point one molar Sodium Hydroxide.
Click on the flask, observe the pH on the pH Meter. Note that pH Meter reads 13.
Click on the radio button near Strong-bases cabinet to close the list.
Double click on Distilled water icon from Stockroom explorer.
Let us now add the required apparatus to the workbench.
Click on glassware icon. Select Graduated Cylinders.
From the sub-menu, select 10 mL Graduated Cylinder to add to the workbench.
For this experiment, we need two Graduated cylinders. Right-click on the Graduated Cylinder.
From the context menu, select copy. Place the cursor at a different place.
Right-click and choose Paste option from the menu.
Click on glassware icon. Select Erlenmeyers.
From the sub-menu, select 250 mL Erlenmeyer Flask. We require two flasks. Right-click to copy and paste.
Label the Graduated Cylinders as A and B. Right-click on the Graduated Cylinder.
From the Context-menu, select Rename option.
A text box opens. Type A in the text box and click on Ok button.
Similarly, label the other Graduated Cylinder as B.
Similarly, label Erlenmeyer Flasks as A and B.
Place each set of apparatus at a different position on the workbench. Use set A for acids and set B for bases.
To perform this experiment, we will dilute 2 ml of 0.1 Molar Hydrocloric acid with 18 ml of distilled water.
Drag 0.1 molar Hydrochloric acid flask over to the Graduated Cylinder A.
In the Transfer amount input bar, type 2. Click on Pour.
Move the flask aside.
Move the Distilled water tank over Flask A. In the Transfer amount input bar, type 18.
Click on Pour. Move the Distilled water aside.
Place the Graduated Cylinder A over Flask A.
Change the transfer mode to Realistic transfer. Click on Pour button gradually.
Move the Graduated Cylinder A aside.
Click on Flask A, observe the pH on the pH meter. pH meter reads 2.
Similarly, let us dilute 0.1 Molar Sodium hydroxide and measure pH.
Measure 2 mL of 0.1 Molar Sodium hydroxide in Graduated Cylinder B.
Drag 0.1 molar Sodium Hydroxide flask over to Graduated Cylinder B.
Using Tools menu, change the transfer mode to Precise transfer.
In the transfer amount input bar, type 2. Click on Pour.
Move the flask aside.
Move the Distilled water tank over Flask B.
In the Transfer amount input bar, type 18. Click on Pour.
Move the Distilled water aside.
Transfer 2 ml of sodium hydroxide from Graduated Cylinder B to Flask B. Place the Graduated Cylinder B over Flask B.
Change the transfer mode to realistic transfer. Click on Pour button gradually.
Move the Graduated Cylinder B aside.
Click on Flask B, observe the pH on the pH meter. pH meter reads 12.
Here is a table which shows results of our experiment.
pH of 0.1 Molar Hydrocholric acid is 1. When diluted, acidity decreases. After dilution, pH was found to be 2.
Similarly, pH of 0.1 Molar Sodium Hydroxide is 13. When diluted, alkalinity decreases.
Hence, after dilution, pH was found to be 12. Since pH is a logarithmic scale, for every 10 times dilution pH changes by 1 unit.
Back to the workbench. For the next experiment, load a new workbench using File menu.
Click on Workbench 2.
Let us study the variation in pH by common ion effect.
We will demonstrate this, using weak acids and weak bases.
For this demonstration , we will use two examples- 1. Ethanoic acid and its conjugate base, sodium ethanoate. 2. Ammonia and its conjugate acid, Ammonium Chloride.
Back to workbench. Place the required chemicals from the Stockroom explorer on the workbench.
From Stockroom explorer, double-click on Weak-acids cabinet.
A list of weak acid solutions with various strengths opens. Double-click to choose 1 molar Ethanoic acid. Close the list.
Again from Stockroom explorer, double-click on Weak-bases cabinet. A list of weak bases with various strengths opens.
Double-click to choose 1 molar Ammonia solution. Close the list.
Double-click on Conjugate-acids cabinet.
Double-click on 1 molar Ammonium Chloride solution. Close the list.
Double-click on Conjugate-bases cabinet. Double-click on 1 molar Sodium Ethanoate solution from the list. Close the list.
Next, click on new glassware icon. Select 250 ml Beaker.
We need 4 Beakers for this experiment. Right-click on the Beaker. Using copy and paste option make 3 copies of the Beaker.
Label the beakers as A, B, C and D.
Place the Beakers A and B to one side of the workbench.
Place the Beakers C and D on other side of the workbench.
Place Ethanoic acid and its conjugate base, Sodium Ethanoate on one side.
Place Ammonia and its conjugate acid Ammonium Chloride on the other side.
Transfer 25 ml of Ethanoic acid in beakers A and B using Precise transfer mode. Type 25 in the transfer amount input bar.
Click on pour. Similarly, transfer 25 ml of Ethanoic acid to beaker B.
Move the flask aside.
Transfer 25 ml of Ammonia solution into beakers C and D.
Move the flask aside.
To demonstrate the effect of common ion- Transfer 25 ml of Sodium ethanoate into beaker B.
Place 1 molar Sodium ethanoate flask over beaker B.
Type 25 in the transfer amount input bar. Click on pour. Keep the flask aside.
To check the pH of 1M Ethanoic acid present in beaker A, click on beaker A. pH meter shows pH value of 2.38.
Click on beaker B. pH meter shows value of 4.76 .
Now transfer 25 ml of Ammonium Chloride into beaker D. Place 1 Molar Ammonium Chloride flask over beaker D.
Type 25 in the transfer amount input bar. Click on pour. Keep the flask aside.
To check the pH of 1molar Ammonia present in beaker C, click on beaker C.
pH meter shows pH value of 11.62.
Click on beaker D. pH meter shows pH value of 9.25. Now, let us tabulate the results.
Due to the presence of common ion that is ethanoate ion, ionization of ethanoic acid decreases. Hence pH value increases.
Similarly, due to the presence of ammonium ion, ionization of Ammonia decreases. Hence pH value decreases.
Let us summarize. In this tutorial, we have learnt to measure- Change in pH for solutions of 0.1 Molar Hydrochloric acid and 0.1 Molar Sodium Hydroxide on dilution.
Variation in pH by common-ion effect for ethanoic acid and ammonia solutions.
As an assignment: Measure change in pH for- 1 Molar and 0.01 Molar Sulphuric acid.
1 Molar and 0.01 Molar Sodium hydroxide.
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This tutorial is contributed by Snehalatha Kaliappan and Madhuri Ganapathi from IIT-Bombay. Thank you for joining.