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| From:  Abalo3 (Original Message) | Sent: 1/13/2008 3:58 AM |
Hi Steve,
I had difficulties with this lab until the teacher sent me the lab results to finish up it. I am totally out of touch and need some suggestions.Your inputs will be greatly appreciated.
Thank you.
Procedure 1:
Standard versus iodine stock solution:
16.00 mL was too much
13.00 mL was too much
12.40 mL just changed color
12.35 mL just changed color
12.35 mL just changed color
LAB:
The prepared iodine solution on the Chemicals shelf with a stated (anticipated) concentration of 0.0015M is standardized (to determine the current actual concentration) by performing the following titration:
1. Prepare a solution of pure ascorbic acid of known strength.
1b. Weight out and add exactly 0.05 g ascorbic acid to the volumetric flask.
1c. Fill the volumetric flask with water. (This is the most precise way of making a 100 mL solution.
2. Do a rough (approximate) titration of the iodine stock solution. (I do not understand when they ask about doinig a rough approximate????????????)
2a. Take a 150 Erlenmeyer flask from the Glassware shelf and place it on the workbench.
2b. Pour 10.0 mL of the ascorbic acid solution into the flask.
2c. Add 10.0 mL of distilled water to the flask to dilute the ascorbic acid solution added.
2d. Add 1.0 mL of the starch indicator to the flask.
2f. Fill the burette with 50.00 mL of the iodine stock solution (with nominal strength of 0.0015M.)
2g. Drag the flask and drop it on the lower half of the burette (the "base of the stand".)
2h. Open the Properties window and move the window to a convenient location on the computer screen. Click back on the burette, and then click the pushpin in the corner of the Properties window to lock it on the burette.
2i. Titrate the ascorbic acid sample by adding iodine until the solution in the flask turns dark blue. (As you learned in the Titration Tutorial, you should do this first rough titration by adding the iodine solution 1-2 mL at a time in order to quickly find the range in which the endpoint is reached.)
2j. Refill the burette with additional iodine solution to bring it back to the 50.00 mL mark. Be careful not to overfill the burette, since it would make a mess on a real lab bench.
3. Do two or more "fine" (or precise) titrations of other samples of the standard ascorbic acid solution. Strive to cause the starch indicator change color when adding only one more drop (the "last 0.05 mL") of iodine solution.
4. Calculate the current molarity (today's "standardized value") of the iodine solution using the average of your best titration results of the ascorbic acid solution.
Questions
1. Calculate the molarity of the freshly-prepared ascorbic acid standard (known strength) solution:
(a) Mass of ascorbic acid used:
Do I use 0.05 g of ascorbic acid use in the first flask????
(b) Moles of ascorbic acid (MW=176.1 g/mol):
(c) Total volume of standard solution prepared (mL):
(d) Ascorbic acid concentration in standard solution (mol/L):
2. For each titration, record and calculate the following. (Be sure to show your data and calculations clearly).
(a) Volume of iodine solution added (mL):
(b) Concentration of the iodine solution, using the formula M1*V1 = M2*V2 (where V1 and V2 are the volumes of the two concentrated chemical solutions added to the flask):
3. From your two or more best "fine" titrations, calculate the average iodine concentration, thus determining today's standard value for the iodine solution.
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