Preparation of Reagents
1. Starch indicator will be provided
2. Solid potassium iodide will be available
3. 0.3 M H2SO4 will be available
4. ~0.04 M Sodium thiosulfate solution will be provided.
You should be able to complete the experiment with 250 mL of this solution.
5. Preparation of 0.01M KIO3 Solution:
a. Accurately weigh approximately 0.535 g of solid reagent and record the mass to
4 decimal places.
b. Deliver the KIO3 to a 250 mL volumetric flask and add 100 mL of DI water. Swirl to dissolve, then dilute to volume.
c. Compute the molarity of the solution. (FW = 214.00)
Standardizing the Thiosulfate Solution
1. Collect 250 mL of the thiosulfate solution. You must use the same solution for the entire experiment.
2. Pipet 25.00 mL of the KIO3 solution into each of 3 Erlenmeyer flasks. Using the calculated KIO3 solution concentration, calculate the volume of titrant (thiosulfate) required assuming that the thiosulfate concentration is 0.04 M. This gives the approximate endpoint.
3. Add 1 g of KI and 20 mL of 0.3 M sulfuric acid solution to each flask.
4. Titrate the triiodide with the thiosulfate solution until the brown solution becomes pale yellow. Then add 2 mL of the starch indicator solution and titrate until the violet color of the starch‐iodine complex just disappears. This is the endpoint.
5. Repeat this procedure for a total of three precise titrations.
Analyzing the Vitamin C
1. Weigh a sufficient number of vitamin tablets so that approximately 500 mg of ascorbic acid is obtained (normally one tablet – your TA may tell you how many tablets to use). Grind the tablets with a mortar and pestle.
2. Record the weight of the resulting powder that is actually analyzed. Then transfer the known mass of powder to a 250 mL volumetric flask.
3. Add 100 mL of 0.3 M sulfuric acid. Swirl the flask for about 10 minutes, and then let it stand for several minutes. Swirl again, and then dilute to the mark with 0.3 M sulfuric acid. Because of the fillers and binders used in vitamin tablets, your solution may be cloudy – this is normal.
4. Deliver 25.00 mL of the vitamin C solution to an Erlenmeyer flask.
5. Add 1 g of solid KI and 25.00 mL of standardized KIO3 to the flask. Titrate the remaining triiodide with the standardized thiosulfate solution as above, taking care to add the starch solution just before the end point.
6. Repeat this titration twice for a total of three precise determinations.
7. Calculate the average mass of vitamin C in each tablet and the uncertainty in the determination, as indicated on the report sheet.
Mass of KIO3: 0.5254
Molarity of KIO3 solution by mass: 0.002455
Average volume of titrant: 31.06 +/_ 1.002
Molarity of thiosulfate solution: ? +/_ ?
Mass of the Vitamin C tablet(s):0.6166
Mass of Vitamin C powder in volumetric flask: 0.599
Total tablet mass (if different): 0.6166
Average volume of titrant used: = 21.03 +/- 0.7637
Moles of ascorbic acid titrated:= ? +/_ ?
Grams of ascorbic acid titrated: =? +/_ ?
Mass of ascorbic acid in pill: =? +/_ ?
Mass percent of ascorbic acid in pill =? +/_ ?
the one with question mark is the one that I have problem trying to solve it
Volume of titrant = 31.06
Volume of KIO3 present in the sample = 25
IO3- + 5I- + 6H+ -----> 3I2 + 3H2O
3I2 + 6S2O32- -------> 6I- + 3S4O6
Moles of KIO3 titrated = 25 x 0.002455 = 0.061375
Moles of I2 produced = 3 x 0.061375 = 0.184125
Moles of Thiosulphate required to titrate = 2 x 0.184125 = 0.36825
Molarity of Thiosulphate solution = 0.36825 / (31.06+25) = 0.006569 M
Moles of ascorbic acid titrated = Moles of I3- produced = 3 x 0.061375 = 0.184125
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