Procedure 1: The prepared iodine ( I 2 ) solution on the Materials shelf with a
ID: 1045782 • Letter: P
Question
Procedure 1:
The prepared iodine (I2) solution on the Materials shelf with a stated concentration of 0.015 M is standardized (confirming the concentration) by performing the following titration:
1. Take a clean volumetric flask from the Containers shelf and place it on the workbench.
2. Add 0.100 g ascorbic acid (C6H8O6)from the Materials shelf to the volumetric flask.
3. Fill the volumetric flask with water. (This is done by adding the water normally and checking the "Fill to the Mark" option.) This is the most precise way of making a 100 mL solution. Record the amount of ascorbic acid (C6H8O6) used and the total volume prepared in your Lab Notes.
4. Take a 150 Erlenmeyer flask from the Containers shelf and place it on the workbench.
5. Pour 20.00 mL of the ascorbic acid (C6H8O6) solution into the Erlenmeyer flask.
6. Add 1 mL of the 2% starch indicator solution from the Materials shelf to the Erlenmeyer flask.
7. Rough Titration:
(a) Take a burette from the Containers shelf and place it on the workbench. Fill the burette with 50.00 mL of 0.015 M iodine (I2) solution. Record the initialburette reading.
Place the Erlenmeyer flask on the lower half of theburette — this will connect them.
(b) Perform a rough titration, adding larger increments of the standard iodine (I2) solution from the burette by pressing and holding the black knob at the bottom of the burette. Each time you addthe standard iodine (I2) solution, check the volume remaining in the burette. Iodine (I2) is reduced by the ascorbic acid (C6H8O6) to form iodide (I-). At the endpoint of the titration, the solution turns suddenly bright blue.
(c) Record in your notes both the last volume where the solution was colorless and the first volume where the solution was bright blue. This gives you the range within which to do the fine titration.
(d) Clear your station by placing the containers in the recycling bin.
8. Fine Titration:
(a) Set up the titration as before: (i) An Erlenmeyer flask filled with 20.00 mL of the ascorbic acid (C6H8O6) solution and 1 mL of the 2% starch solution. (ii) A burette filled with 50.00 mL of the standard iodine (I2) solution. Record the initialburette reading. (iii) Place the Erlenmeyer flask on the lower half of the burette.
(b) Click and hold the black knob of the burette to quickly add enough standard iodine (I2) solution to just get into the range of the coarse titration but still have the solution in the flask appear colorless. This isnear, but not yet at, the titration's endpoint.
(c) Add standard iodine (I2) solution in small incremements, down to one drop at a time, until the addition of just one more drop causes the solution in the flask to turn bright blue. Record the final burettereading in your lab notes.
(d) Clear your station.
e) Make sure to save your Lab Notes.
Procedure 2:
1. You will now determine the ascorbic acid (C6H8O6) concentration in commercial orange juice, first from a freshly opened container and then from a container that was opened one week ago.
2. Prepare a sample of fresh orange juice for titration — take a clean 150 mL Erlenmeyer flask from the container shelf and add 1 mL of 2% starch indicator solution to the flask. Now add 40.00 mL of the fresh orange juice from the Materials shelf to the same flask. The programing for this lab requires that you place the starch indicator into the flask first, and then add the orange juice sample. In a real lab the order of this addition is not important.
3. Rough Titration:
(a) Take a burette from the Containers shelf and place it on the workbench. Fill the burette with 50.00 mL iodine (I2) solution. Record the initial burettereading.
Place the Erlenmeyer flask with the fresh OJ sample on the lower half of the burette - this will connect them.
(b) Perform a rough titration, adding larger increments of the standard iodine (I2) solution from the burette by pressing and holding the black knob at the bottom of the burette. Each time you addthe standard iodine (I2) solution, check the volume remaining in the burette. Iodine (I2) is reduced by the ascorbic acid (C6H8O6) to form iodide (I-). At the endpoint of the titration, the solution turns suddenly bright blue.
(c) Record both the last volume where the solution was colorless and the first volume where the solution was bright blue. This gives you the range within which to do the fine titration.
(d) Clear your station by placing the containers in the recycling bin.
4. Fine Titration:
(a) Set up the titration as before: (i) An Erlenmeyer flask containing 1 mL of the 2% starch solution and 40.00 mL of the fresh orange juice solution – in that order. (ii) A burette filled with 50.00 mL of the standard iodine (I2) solution. Record the initialburette reading. (iii) Place the Erlenmeyer flask on the lower half of the burette - this will connect them.
(b) Click and hold the black knob of the burette to quickly add enough standard iodine (I2) 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 endpoint.
(c) Add standard iodine (I2) 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 turn bright blue. Record the final burettereading in your lab notes.
(d) Place the Erlenmeyer flask in the recycling bin.
(e) When standardizing a solution, you should repeat the titration so that your results are within a few drops of each other. The results are then averaged to determine the precise molarity.
5. Repeat steps 2-4 with old orange juice in place of fresh orange juice.
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1. Calculate the concentration of the standard ascorbic acid (Vit C) solution:
Table 1
a
Record the mass of ascorbic acid used - this is the mass you weighed-out!
b
Convert the mass of ascorbic acid from line a to mg - refer back to unit conversions if you are having trouble here (g to mg conversion)!
c
Record the total volume of the standard ascorbic acid solution in the volumetric flask after adding water (mL)
d
Standard ascorbic acid solution concentration (b/c)
Units will be = mg/mL
2. Record and calculate the quantitative realtionship between the standard ascobic acid solution and the I2 titrant:
Table 2
a
What volume of standard ascorbic acid solution was used for the titration? (in mL)
b
Use the standard ascorbic acid solution concentration from line d, Table 1 and the volume of this solution used in the titration to calculate the mass of ascorbic acid in the sample that was titrated (in mg)
c
What volume of I2 solution was added to color change? (endpoint in mL)
d
Determine the conversion factor that relates mg of ascorbic acid (vit C) to mL of I2 solution by dividing (b/ c).
Conversion factor has units = mg Vit C/mL I2
Procedure 2
1. For the fine titration of the fresh orange juice, record and calculate the following:
Table 3
a
What volume of fresh orange juice was used for the titration? (in mL)
b
What volume of I2 solution was added to color change? (endpoint in mL)
c
Use the conversion factor from line d of Table 2 and the volume of I2 solution required to reach the endpoint in this titration to calculate the mass of vitamin C in the fresh orange juice sample (in mg)
d
Calculate the concentration of vitamin C in the fresh orange juice sample based upon the volume of orange juice used in the titration
Units will be = mg/mL orange juice
2. For the fine titration of the week-old orange juice, record and calculate the following:
Table 4
a
What volume of week-old orange juice used for the titration? (in mL)
b
What volume of I2 solution was added to color change? (endpoint in mL)
c
Use the conversion factor from line d of Table 2 and the volume of I2 solution required to reach the endpoint in this titration to calculate the mass of vitamin C in the week-old orange juice sample (in mg)
d
Calculate the concentration of vitamin C in the week-old orange juice sample based upon the volume of orange juice used in the titration
Units will be = mg/mL orange juice
3. The minimum daily requirement for vitamin C is 60 mg per day. What percentage of this requirement is in one cup (200 mL) of fresh orange juice?
Show all calculations and pay attention to significant figures.
4. The minimum daily requirement for vitamin C is 60 mg per day. What percentage of this requirement is in one cup (200 mL) of week-old orange juice?
Show all calculations and pay attention to significant figures.
5. What happens to the ascorbic acid in orange juice over time? (hint: oxygen makes up 20% of our air.)
a
Record the mass of ascorbic acid used - this is the mass you weighed-out!
b
Convert the mass of ascorbic acid from line a to mg - refer back to unit conversions if you are having trouble here (g to mg conversion)!
c
Record the total volume of the standard ascorbic acid solution in the volumetric flask after adding water (mL)
d
Standard ascorbic acid solution concentration (b/c)
Units will be = mg/mL
Explanation / Answer
1.
(a) Record the mass of ascorbic acid used - this is the mass you weighed-out : 0.1 g
(b) Convert the mass of ascorbic acid from line a to mg - refer back to unit conversions if you are having trouble here (g to mg conversion): 0.1 *1000 mg = 100 mg
(c) Record the total volume of the standard ascorbic acid solution in the volumetric flask after adding water (mL) : 100 mL
(d) Standard ascorbic acid solution concentration (b/c): 100 mg/ 100 mL = 1 mg/ mL
2.
(a) What volume of standard ascorbic acid solution was used for the titration? (in mL) : 20 mL.
(b) Use the standard ascorbic acid solution concentration from line d, Table 1 and the volume of this solution used in the titration to calculate the mass of ascorbic acid in the sample that was titrated (in mg): 20 mL* 1mg / ml = 20 mg.
(c) What volume of I2 solution was added to color change? (endpoint in mL): 7.6 mL
(d) Determine the conversion factor that relates mg of ascorbic acid (vit C) to mL of I2 solution by dividing (b/ c): 20 /7.6 mg/ mL = 2.63 mg/ mL.
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