20. Using your large graduated cylinder measure out 25-mL of the solution from the beaker labeled “HA” and transfer this volume to your fourth clean rinsed 150 -mL beaker. Label this beaker, “50 - 50 buffer mixture.” 21. Now measure out 25- mL of the solution from the beaker labeled “A – ” and combine this with the solution in your beaker labeled “50 - 50 buffer mixture”. Swirl gently to mix. Using your pH meter, measure the pH of this solution and record the value on your data sheet.
Chemistry 12 Santa Monica College pH Measurement and its Applications Page 11 of 17 22. The pH of the solution in your beaker labeled, “50 -50 b uffer mixture,” is also the p K a of your unknown acid. Do you know why? You will need to tell your instructor this value for Part E. 23. Save the remaining solutions in the beakers labeled, “HA” and “A – ” and the beaker containing the remaining 0.2-M NaOH solution for the next part of this experiment. Part E. Buffer Solutions Report the p K a value you determined for your unknown acid in Part D to your instructor who will assign you the pH value of the buffer solution you will prepare in this part of the experiment. Now using the remaining solutions in the beakers labeled “HA” and “A – ”, prepa re a buffer solution that will maintain the pH assigned to you by your instructor (see background section). It is suggested you use only a portion of each of these two solutions in case your first attempt does not succeed. Use your pH meter to confirm the pH of your buffer solution. Your measured pH value should be within 0.2 pH units of your assigned value. Record your measured value on your data sheet and obtain your instructor’s initials confirming your success. Now we will test the buffer solution you prepared against changes in pH. Rinse and fill another 150-mL beaker with a volume of deionized water equal to that of your buffer solution. Using your pH meter measure the pH of the deionized water. Do not be alarmed if this pH is less than neutral. It should be between 5.2 and 7.0. Do you know why? What species from the atmosphere dissolves in water that is left standing? Add 5 drops of the remaining 0.2 M NaOH solution to both the beaker containing your buffer solution and that containing the deionized water. Measure the pH of each of these solutions following this addition and determine the change in pH of each. Record these values on your data sheet. Clean up. Discard all chemicals in the proper chemical waste container. Clean and then return all borrowed equipment to the stockroom.