Minutes in beakers this is to remove any remaining

This preview shows page 33 - 36 out of 77 pages.

minutes in beakers. This is to remove any remaining carbon dioxide in the water. Do not worry even if you have less than 900 mL of water after boiling, because you are not preparing the exact
0.1 M of NaOH solution. Whatever concentration of NaOH solution you prepared, you will find out the exact concentration of it using accurately weighed KHP. c. When boiling is completed, let the water cool for 10 minutes, then transfer into 1 L glass bottle and cap on it. d. Let the water cool to room temperature. You may cool the glass bottle under the running water to save time. Then add ~ 100 mL of 1 N NaOH (volumetric standard) to the cooled deionized water in 1-L glass bottle. NaOH solution, especially the saturated one, is used to make a dilute solution of NaOH-not solid NaOH pellets. This is because solid NaOH always contains some Na 2 CO 3 . We do not want Na 2 CO 3 in our NaOH titrant solution because the equivalent molecular weight of Na 2 CO 3 is different from that of NaOH, i.e., if a sample contains equal weight of NaOH and Na 2 CO 3 then the amount of HCl needed to neutralize NaOH will be different from the amount of HCl used to neutralize Na 2 CO 3 . Na 2 CO 3 is essentially insoluble in a saturated solution of NaOH (ca. 17 M), so that the saturated liquid contains very little Na 2 CO 3 . d. After adding the NaOH, shake the solution thoroughly and allow it to cool to room temperature. The glass bottle should be capped when not in use to minimize the absorption of CO 2 . e. Seal the glass bottle with parafilm, label, and store in your drawer until next week’s lab period. Part 2 (work individually) 1. Dried KHP a. Bring one clean and dry weighing bottle to obtain ~ 3 g of dried KHP from TA, and keep the weighing bottle in the dessicator. After placing a dried, hot sample into a desiccator, be sure to vent it periodically by removing the lid for a few seconds. If this is not done, the lid may “pop off” spontaneously due to pressure build-up. b. Close the weighing bottle with lid, and seal it with parafilm if necessary. 2. Standardization of NaOH a. Check out a clean 50 mL buret and mount the buret in a buret stand. b. Rinse the buret with deionized water, and then rinse with 5-10 mL portions of the NaOH solution which you prepared last week. c. Fill the buret with NaOH solution, and read the initial volume.
d. Using a weighing paper or dish, accurately weigh out about 0.7 g portion of dried analytical grade potassium biphthalate [KHP, M.W. 204.24 g/mol, KO 2 C(C 6 H 4 )CO 2 H], and quantitatively transfer into 250 mL Erlenmeyer flasks. Add about 25 mL of deionized water and 3 drops of phenolphthalein. e. Titrate upon dropwise addition with manual swirling, until the color changes to a faint pink (endpoint) . The pink color should persist for at least 15 seconds after swirling, or carefully add one additional drop. f. Repeat “c”, “d” and “e” for two more trials. g. You must repeat the titration until you have three trials with a precision of 0.2 % or less with respect to the mean. See below for more information.

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture