At the endpoint, the solution should be a very, very faint pink and the color change should occur from just one last drop of NaOH. 12. Record the final volume of NaOH in your lab notebook. The difference between the final and initial volume represents the total volume of NaOH added. For this first trial, the volume is an estimate of where the endpoint occurs. Save the titrated sample for comparison to the next trial. 13. Repeat steps 8-12. If the total volume of NaOH is less than 20 mL, then you do not need to refill your buret. Calculate the molarity of NaOH for this second trial. 14. Refill your buret and check for air bubbles in the tip before proceeding. Repeat steps 8-12. 15. You will have a total of three trials, one estimate (#1), and two data trials (#2 and #3). The molarity of NaOH calculated from trials 2 and 3 should agree closely. If the difference is greater than 1%, you should do a fourth trial. Use the molarities from your two best trials to calculate the average molarity of the NaOH solution. Part B: Determining the Concentration of Acetic Acid in a Vinegar Solution 16. Obtain about 25 mL of vinegar solution in a clean, dry, labeled beaker. Record the mass percent of acetic acid in the vinegar solution which appears on the label of the bottle. 17. Using a pipet pump and a 5 mL pipet, pull a small volume of vinegar solution into the pipet and roll it around to rinse it. Discard the rinse into a waste beaker. 18. Draw more vinegar solution into the pipet, but this time go a little above the ring in the neck of the pipet. Do not draw any solution into the pipet pump. Wipe the tip of the pipet with a clean tissue (Kimwipe). 19. Holding the tip against the inside edge of the beaker, slowly release the solution, using the pump, until the bottom of the meniscus coincides with the ring. 20. Slowly remove the pipet from the beaker and place it at a 45 o angle over the neck of a clean Erlenmeyer flask. Press the release lever of the pump (do not use the wheel) so as to allow the solution to drain into the flask. Then carefully move the pipet into the flask so as to allow the tip of the pipet to touch the glass surface for about 10 seconds. Withdraw the pipet from the flask. You have now dispensed 5.00 mL of vinegar solution into the flask. 21. Add approx 20 mL of water, and 3 drops of phenolphthalein to the vinegar solution. 22. Repeat steps 10-12 until you have a total of two consistent data trials for the vinegar. 23. Use the information from your best two trials to determine the average molar concentration of the vinegar. From the molar concentration you should be able to calculate a mass percent concentration of acetic acid in the vinegar. 24. Unused NaOH (still in volumetric flask or in buret) should be recycled in the NaOH recycle container. NO WASTE IS TO BE DISPOSED OF IN THIS CONTAINER. 25. All waste solutions should be disposed of in the waste carboy.
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- Fall '16
- John B Michel
- Mole, Sodium hydroxide, Erlenmeyer flask, Buret, KHP