**Consult your instructor before starting Part D, to see if he/she wants you to follow the normal or OPTIONAL procedure. Part D. Determining the Value of K a for an Unknown Acid by Titration (Normal procedure) 1. Obtain a 50-mL buret from the stockroom. A buret stand should be available in the laboratory room. Rinse your buret, small funnel, and four 150-mL beakers several times using deionized water. 2. Select one of the 150-mL beakers and label it “NaOH”. Rinse this beaker once more with about 5 mL of 0.2-M NaOH. Then use it to collect about 75 mL of the 0.2-M NaOH solution (available in the reagent fume hood). 3. Rinse the 50-mL buret and funnel once with about 5 mL of 0.2-M NaOH solution. Clamp the buret to the buret stand making sure that it is vertical. Insert your funnel into the top of the buret. 4. Fill the buret with the 0.2-M NaOH solution from your beaker to just above the 0.00-mL mark. Remove the funnel. Using a waste beaker allow the NaOH solution to flow from the buret tip dropwise until the bottom of the meniscus of the NaOH solution in the buret is exactly at the 0.00-mL mark when read at eye level. If you miss this mark, add some additional 0.2-M NaOH from your beaker and try again. pH Measurement and its Applications Page 8 of 19
Chemistry 12 Santa Monica College 5. Obtain a vial containing your unknown solid acid from your instructor and record the letter and number of this unknown acid on your data sheet. 6. Weighing by difference measure between 1.5 and 2.0 grams of the unknown acid into one of the clean rinsed 150-mL beakers. Record this mass on your data sheet. Label this beaker, “A – ”. 7. Using your large graduated cylinder, measure out exactly 100.0 mL of deionized water. Combine this with the unknown solid acid sample in your 150-mL beaker. Stir your solution to completely dissolve the solid acid. 8. Using your large graduated cylinder, measure out 50.0 mL of your unknown acid solution and transfer this to a second 150-mL beaker. Label this second beaker “HA” and set it aside for now. 9. Add 2 drops of phenolphthalein indicator to the remaining 50.0-mL of unknown acid solution in the beaker labeled “A – ”. 10. Titrate the solution in the beaker labeled “A - “ until it reaches the phenolphthalein end point. The end point is near when the pink color from the phenolphthalein indicator begins to persist in solution longer before vanishing. When you notice these changes slow down your addition rate to just 2 to 3 drops per addition. When you feel you are nearing the endpoint, slow down your addition rate to just 1 drop per addition. When the pink color from the phenolphthalein indicator persists for at least 2 minutes you have reached the endpoint of your titration. 11. We now need to equalize the volumes in the two beakers labeled “HA” and “A – ”. Using your large graduated cylinder measure a volume of deionized water equal to the total amount of the 0.2-M NaOH you added during your titration and add this volume of deionized water to the contents of the beaker labeled, “HA”. Swirl gently to mix.
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