Procedures

Experiment 1: Standardize the Iodine Solution

Part 1: Prepare the Materials

Take a 100 mL volumetric flask from the Containers shelf and place it on the workbench.

Take ascorbic acid from the Materials shelf and add 0.100 g to the volumetric flask.

Take water from the Materials shelf and add 30 mL to the flask to dissolve the ascorbic acid. Complete the solution by filling the volumetric flask with water to the 100 mL mark by checking the "Fill to the Mark" box. This is the most precise way of making a 100 mL solution. Record the amount of ascorbic acid used and the total volume prepared in your Lab Notes. Remember to press Save Notes.

Take an Erlenmeyer flask from the Containers shelf and place it on the workbench.

Add 20.0 mL of the ascorbic acid solution to the Erlenmeyer flask.

Take starch indicator solution from the Materials shelf and add 1.0 mL to the Erlenmeyer flask.

Part 2: Perform a Coarse Titration

Take a burette from the Containers shelf and place it on the workbench.

Take 0.015 M iodine solution from the Materials shelf and add 50.00 mL to the burette. Record the initial burette reading in your Lab Notes. Pass the mouse over the burette and a gray tool tip will briefly display the total and dispensed volumes. You may want to use the + and - buttons in the lower right of the screen for a closer view.

Move the Erlenmeyer flask onto the base of the burette.

Perform a coarse titration, adding large increments (~ 2 mL) of the iodine solution from the burette by pressing and holding the black stopcock at the bottom of the burette for a few seconds. Pause after each dispensation. Record the volume dispensed in your Lab Notes.

Check if the end point is passed. When the reaction reaches the end point, the solution changes color.

Record both the last total and dispensed volume where the solution was colorless and the first total and dispensed volume where the solution changed color in your Lab Notes. This gives you the range within which to do the fine titration.

Clear your station by dragging the Erlenmeyer flask to the recycling bin beneath the workbench.

Part 3: Perform a Fine Titration

Set up the titration as before:

Take an Erlenmeyer flask from the Containers shelf and place it on the workbench.

Add 20.0 mL of the ascorbic acid solution to the Erlenmeyer flask.

Take starch indicator solution from the Materials shelf and add 1.0 mL to the Erlenmeyer flask.

Take 0.015 M iodine solution from the Materials shelf and add enough to the burette so that the volume in the burette is 50.00 mL. Record the initial burette reading in your Lab Notes.

Move the Erlenmeyer flask onto the base of the burette.

Click and hold the stopcock of the burette to quickly add enough iodine solution to just get into the range of the coarse titration but still have the solution in the flask appear colorless. This is near, but not yet at, the titration's end point.

Add titrant in small increments, down to one drop at a time, until the addition of just one more drop causes the solution in the flask to change color. Record the final total and dispensed volume reading in your Lab Notes.

Clear your station by dragging the Erlenmeyer flask to the recycling bin beneath the workbench.

Repeat the fine titration. Add more iodine solution to the burette as needed. If the result is very different from your first fine titration, perform an additional fine titration to determine which volume is not accurate.

Experiment 2: Determine the Concentration of Ascorbic Acid in Orange Juice

Part 1: Prepare a Sample of Fresh Orange Juice

Take an Erlenmeyer flask and place it on the workbench.

Take OJ – fresh from the Materials shelf and add 40.0 mL to the Erlenmeyer flask.

Take starch indicator solution from the Materials shelf and add 1.0 mL to the Erlenmeyer flask.

Part 2: Perform a Coarse Titration

Take 0.015 M iodine solution from the Materials shelf and add enough to the burette so that the volume in the burette is 50.00 mL. Record the initial burette reading in your Lab Notes.

Move the Erlenmeyer flask onto the base of the burette.

Perform a coarse titration, adding large increments of the iodine solution from the burette by pressing and holding the black stopcock at the bottom of the burette. Pause after each dispensation. Record the total and dispensed volumes in your Lab Notes.

Check if the end point is passed. When the reaction reaches the end point, the solution changes color. Please note that the color will not be the same as in experiment 1. Due to the orange color of the orange juice, the color at the end point will be dark orange-red. If your solution turns dark purple, you have added too much titrant.

Record both the last total and dispensed volumes where the solution was orange and the first total and dispensed volumes where the solution was dark orange-red in your Lab Notes. This gives you the range within which to do the fine titration.

Clear your station by dragging the Erlenmeyer flask to the recycling bin beneath the workbench.

Part 3: Perform a Fine Titration

Set up the titration as before:

Take an Erlenmeyer flask from the Containers shelf and place it on the workbench.

Take OJ - fresh from the Materials shelf and add 40.0 mL to the Erlenmeyer flask.

Take starch indicator solution from the Materials shelf and add 1.0 mL to the Erlenmeyer flask.

Take 0.015 M iodine solution from the Materials shelf and refill the burette. Record the initial burette reading in your Lab Notes.

Move the Erlenmeyer flask onto the base of the burette.

Click and hold the stopcock of the burette to quickly add enough standard iodine solution to just get into the range of the coarse titration, but still have the solution in the flask appear orange. This is near, but not yet at, the titration's end point.

Add iodine solution in small increments, down to one drop at a time, until the addition of just one more drop causes the solution in the flask to change color. Record the final total and dispensed volume in your Lab Notes.

Clean your station by dragging the Erlenmeyer flask to the recycling bin beneath the workbench.

Repeat the fine titration. Add more iodine solution to the burette as needed. If the result is very different from your first fine titration, perform an additional fine titration to determine which volume is not accurate.

Repeat Experiment 2 using old orange juice (OJ – week old) instead of fresh orange juice.

Clear your workstation by dragging all the containers to the recycling bin beneath the workbench. Remember to press Save Notes.

Results: Exp 1 -

Pt. 2: coarse titration

before end point vol of burette: 43.25 mL; dispensed: 6.75 mL; vol of flask: 27.77 mL end point vol of burette: 42.21 mL; dispensed: 7.79 mL; vol of flask: 28.81 mL

Pt. 3: fine titration end point vol of burette: 42.42 mL; dispensed: 7.58 mL; vol of flask: 28.60 mL Exp: 2

Pt. 2: coarse titration end point vol of burette 43.28 mL; dispensed: 6.72 mL; vol of flask: 47.74 mL

Pt. 3: fine titration end point vol of burette 46.57 mL; dispensed: 3.43 mL; vol of flask: 44.44 mL old OJ Fine Titration end point vol of burette 46.69 mL; dispensed: 3.31 mL; vol of flask 44.32 mL

Questions

1. How many moles of ascorbic acid were reacted with iodine in each titration? The molar mass of ascorbic acid is 176.12 g/mol. Choose the closest answer.

A. 2.28 × 10-4 moles

B. 1.14 × 10-2 moles

C. 1.14 × 10-4 moles

D. 5.68 × 10-3 moles

2. Based on the number of moles of ascorbic acid and iodine reacted during the titration, what is the stoichiometry of the reaction?

A. 2 I2 : 1 C6H8O6

B.1 I2 : 2 C6H8O6

C.1 I2 : 1 C6H8O6

D.5 I2 : 1 C6H8O6

3. Based on the observed stoichiometric relationship, which of the following balanced equations represents the reaction between the ascorbic acid and iodine?

A. C6H8O6 + I2 → C6H6O6 + 2I- + 2H+

B. 2C6H8O6 + I2 → 2C6H6O6 + 2I- + 4H+

C. C6H8O6 + I2 → C6H6O6 + I- + H+

D. C6H8O6 + 2I2 → C6H6O6 + 4I- + 2H+

4. How many moles of iodine were needed to reach the end point during one of your fine titrations of the fresh orange juice? Choose the closest answer.

A. 1.0 × 10-3 moles

B. 1.0 × 10-4 moles

C. 1.0 × 10-1 moles

D. 1.5 × 10-4 moles

5. How many moles of iodine were needed to reach the end point during one of your fine titrations of the week old orange juice? Choose the closest answer.

A. 4.1 × 10-5 moles

B. 1.0 × 10-4 moles

C. 3.5 × 10-4 moles

D. 4.1 × 10-1 moles

6. Based on the results of your fine titration, how many moles of ascorbic acid are in 40 mL of fresh orange juice? Choose the closest answer.

A. 0.40200 moles

B. 0.00040 moles

C. 0.10050 moles

D. 0.00010 moles

7. Based on the results of your fine titration, how many moles of ascorbic acid are in 40 mL of week old orange juice? Choose the closest answer.

A. 1.5 × 10-5 moles

B. 2.7 × 10-5 moles

C. 4.1 × 10-2 moles

D. 4.1 × 10-5 moles

8. Based on the results of your fine titration, how many mg of ascorbic acid are in 40 mL of fresh orange juice? The molar mass of ascorbic acid is 176.12 g/mol. Choose the closest answer.

A. 10 mg

B. 60 mg

C. 18 mg

D. 37 mg

9. What is the recommended amount of vitamin C per day for an adult?

A. 10 - 20 mg

B. 20 - 30 mg

C. 90 mg - 2 g

D. 60 - 70 g

10. Which of the following conclusions are consistent with the data you obtained from your orange juice titrations?

A. Vitamin C is highly unstable, degrading over time.

B. Vitamin C is highly unstable after one week.

C. Vitamin C is highly stable over time.
D. Vitamin C is highly stable for one week.

11. Sulfur dioxide reacts with dichromate ions under acidic conditions to form chromium (III) and sulfate. What is the correct balanced equation for this redox reaction?

A. Cr2O72-(aq) + SO2(aq) + H+(aq) → 2Cr3+(aq) + SO42-(aq) + 7H2O(l)

B. Cr2O72-(aq) + 3SO2(aq) + 2H+(aq) → Cr3+(aq) + 3SO42-(aq) + H2O(l)

C. Cr2O72-(aq) + 3SO2(aq) + 2H+(aq) → 2Cr3+(aq) + 3SO42-(aq) + H2O(l)

D. Cr2O72-(aq) + SO2(aq) + H+(aq) → Cr3+(aq) + SO42-(aq) + H2O(l)