In: Chemistry
Part I - Prepare and standardize a solution of Sodium Thiosulfate 1. Prepare 250 mL of a solution which is approximately 0.025 M Na2S2O3• 5H20. Because sodium thiosulfate is a hydrate, it cannot be used as a primary standard. It must be standardized in order to determine its exact concentration. (Note for Su’17: this solution has already been prepared for you). 2. Pipet 10.0 mL of 0.01 M KIO3 into a 150 mL Erlenmeyer flask. Add about 20 mL of distilled water to the flask. 3. Dissolve about 2 g of solid KI to the KIO3. The solution should turn a dark orange-brown. Add about 20 drops of 2M HCl and swirl to mix completely. 4. Calculate the approximate amount of Na2S2O3 that will be required to reach the endpoint, assuming that the Molarity of the Na2S2O3 will be approximately 0.025 M. 5. Quickly add all but about 2 or 3 mL of the volume you calculated above to the flask. 6. Add about 40 drops of a 2% starch solution. The solution should turn dark blue. 7. Continue adding Na2S2O3 dropwise until the blue color just disappears. Record the volume of Na2S2O3 in your lab notebook. 8. Repeat one more time. Calculate the concentration of the Na2S2O3 from each trial and average your results. If they differ greatly, a third trial may be necessary.
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