In this section, you will learn how to use Virtual Lab software to show hydrofluoric acid titrated with the strong base NaOH. The software allows solution preparation using volumetric glassware, realistic solution transfers where the mouse button simulates the release of a pipet bulb, and both pH and temperature readings. Various indicators can also be added for visual endpoint determination. Again, Virtual Lab Simulator is a free software to download at . You can also use the web-based version that comes with this unit; just click on the first tab labeled "Virtual Lab Simulator" located on the lower "Workbench" panel.

1. Spend a few moments investigating the contents of the pulldown menus, the stockroom, and the glassware and tools icons to the left of the Workbench.


Figure 1: the screen interface of the Virtual Lab Simulator program

2. Conduct this virtual titration experiement:
Substances: 1M HF flask, Distilled H2O, the Phenolphthalien, 0.1M NaOH flask
Equipment: a 100mL volumetric flask, a 10mL pipet, a 250mL Erlenmeyer flask, a 25mL pipet, a Disposable Pipet, a 50mL Buret

STEP1: measuring the acid HF
a. Expand the Weakacids folder in the Stockroom (left window). Double click on the 1M HF flask to select it. <A flask of 1M HF should appear on the Workbench>. Use your mouse and click/drag the flask to the lower part of the Workbench.
b. On the side of the Workbench (middle) window, click on the equipment icon Retrieve a 100mL volumetric flask and place it next to the acid flask.

c. In a similar fashion, retrieve a 10mL pipet and place it into the acid flask. <You should see a zoomin of the fill line>.

d. Select the pipet and measure 10mL of the 1M HF acid; to do that, select the pipet with the mouse, then choose Tools > Transfer Bar > Realistic Transfer from the top pull down menu.

Below the Workbench window you should see Withdraw and Pour buttons. These buttons simulate the actions of a pipet bulb. Use your mouse to Click/hold down the Withdraw button and notice the animated fillbar to the left. Also notice what happens to the pipet.

e. Drag the pipet to the volumetric flask and insert it. Click/hold down the Pour button until the pipet is empty. Remove the pipet from the flask.

The series of figures below illustrate the actions we just described Figure 2: actions of STEP1.

STEP2: adding distilled water to dilute the acid

a. Double click the Distilled H2O container in the Stockroom.

b. Drag the container on top of the volumetric flask.

c. Choose Tools > Transfer Bar > Precise Transfer from the top pull down menu.

d. Below the workbench, type in 90.00 in the Transfer amount window and click Pour. <Your volumetric flask should be filled to the fill line>. Drag the water container away. (Of course, there are better, more realistic ways to do this last step!).
Figure 3: actions of STEP2

Exercise: What is the molarity of the HF solution you have prepared? Answer: ____________ M

e. Retrieve a 250mL Erlenmeyer flask and place it at the center bottom of the Workbench. Use a 25mL pipet and the Realistic Transfer to place 25mL of diluted acid solution into the empty Erlenmeyer.
Figure 4: actions of STEP2

STEP3: adding the indicator Phenolphthalien

. Expand the Indicators folder in the Stockroom window. Double click on the Phenolphthalien bottle. <A bottle of Phenolphthalein should appear on the Workbench>.

b. Retrieve a Disposable Pipet. Using Precise Transfer, place 0.3mL in the pipet and transfer it to the Erlenmeyer flask. Remove the pipet from the flask.
Figure 5: actions of STEP3

c. Click on the Erlenmeyer flask. <In the Solution Info area, you should see a histogram and table showing the molarity of all species present>.

Figure 6: Focus on the Solution Info window

Do the values make sense? ____________

STEP4: adding the base NaOH

a. Retrieve a 50mL Buret.

b. Expand the Strongbases folder in the Stockroom. Double click on the 0.1M NaOH flask. Drag the flask on top of the buret.

c. Transfer 50mL of base into the buret using Precise Transfer.

d. Drag the 0.1M NaOH flask away, and place the burette into the Erlenmeyer.

e. To perform the Pour action from the burette to the flask, change first the method of transfer of the burette; select the burette then choose Tools > Transfer Bar > Realistic Transfer.

Figure 7: Actions of STEP4

f. Record the initial burette reading (to two decimal places) and the pH meter reading in the space below

Initial Burette Reading[labeled 1 in figure below]: _____________

Initial pH Reading [labeled 2 in figure below: ____________

Figure 8: Actions of STEP4

g. Clicking on the Pour button simulates opening the burette stopcock. Slowly add the base to the Erlenmeyer and record both volume and pH in the table on the next page. Try to get as many data points as possible near the equivalence point (pH ~5.5 to 9.5).

h. Stop pouring when the liquid [labeled 3 in the figure above] changes color to pale pink.

Buret Reading	pH Reading

Table 1: Tabulate your titration virtual experiment data here

STEP5: observations

What happened to the solution color at the end point?
What happened to the relative amounts of Phenolphthalein and its deprotonated anion?
What happened to the relative amounts of the other species involved?
Do the changes in these values make sense?

3. Quantitative analysis of the experiment data
In this part we need to tabulate the data in a spreadsheet, create a chart with it that represents the titration curve of the experiment we just conducted, and finally we need to locate the end point.

STEP1. Open Calc and retrieve the spreadsheet TITRATION that you created in the previous step. It should look like this:

Figure 9: The TITRATION spreadsheet

STEP2 Entering the experiment data into the spreadsheet
Type your initial burette reading into cell D4
Type the entire set of burette readings in column A, starting on cell A7
Type the entire set of pH readings in column F, starting on cell F7
Type 0 on cell B7
Type "=A8-$D$4" on cell B8 and hit <Enter>. This formula calculates the exact amount of NaOH added to the solution; it will correct all of the buret readings based on the initial reading.
Copy the formula of cell B8 to the rest of the cells in Column B
In cell C8, type "=(B8+B7)/2". This will provide the averaged NaOH readings. Copy the formula of cell C8 to the rest of the cells in Column C.
In Cell D8, type "=(G8-G7)/( B8-B7)". This calculates the first derivative, (¢GGpH/¢GGmL). Copy the formula of cell D8 to the rest of the cells in Column D
In Cell E9 type "=(D9-D8)/( C9-C8)". This calculates the second derivative ¢GG[¢GGpH/¢GGmL]/¢GGmL.
Copy the formula of cell E9 to the rest of the cells of Column E.
Your spreadsheet should now resemble that shown on the next page. Remember to save the file at this point before continuing with the rest of the exercise.
Figure 10: TITRATION spreadsheet with all the data entered

STEP 3: Ploting the graph of the titration curve
a. Generate the titration curve. To do that, use the chart wizard and select a x-y scattered plot with columns F (pH values) and B (corrected burette readings); such that the pH values are the y-axis and base values are the x-axis.

Figure 11: Titration Curve from the experiment data shown previously [columns F and B]

b. In a similar fashion generate a x-y scattered plot with columns D (1st der values) and C (avg NaOH values); such that the column D are the y-axis and those of column C are the x-axis

c. Generate a x-y scattered plot with columns E (2nd der values) and B (buret readings); such that the values of column E are the y-axis and those of column B are the x-axis

STEP 4: Locating the end point
The end point is where the second derivative is equal to zero (see Figure 5).

Figure 12: the data from the virtual titration experiment. Highlighted in grey is the end point.

Looking at the data highlighted in Figure 5; what can you say about the end point of the virtual titration experiment?

Additional Ideas
¡P Titrate a different weak acid and compare the results. Is the initial pH of a different weak acid the same as that for HF?
¡P Titrate a mixture of weak acids and see if the end points are distinguishable.
¡P Titrate two different strong acids and compare the results of these with that of the weak acid titration data. Is the initial pH of the two different strong acid solutions (assuming equal molarity) the same? Is it the same as that for a weak acid? Explain.
¡P Titrate phosphoric acid (H3PO4), a triprotic acid. Can you see all three end points?