BIOCHEMISTRY 3521
Spring 1996 - 2/4/97
Study Set #1 -- pH, pKa, and Henderson-Hasselbalch
Material Provided in part by R. Morse (ILSTU, Chem 242)

1. Define the concept of pH in your own words.

2. Define the Henderson-Hasselback equation in your own words.

3. Define pKa in your own words. What does "p" mean? Write out in exponential notation (i.e 4.35 X 10-4 the concentration of hydrogen ion for a solution at pH 8.3.

4. Fill in the following table given a buffer with a pKa of 5.3:

5. Using the information from your table, what quantitative relationship exists between the ratio of salt to acid relative to the difference between the pH and the pKa ?

6a). If one liter of a 25 mM buffer is at its pKa, how many millimoles of hydrogen ion can can you add and only allow the pH to change by one pH unit (hint: will the pH go up or down when you add hydrogen ion)?

b). If one liter of a 25 mM buffer is at its pKa, how many millimoles of hydroxyl ion can can you add and only allow the pH to change by one pH unit (hint: will the pH go up or down when you add hydroxyl ion)? (Double hint: what is the ratio of salt to acid at the pKa, and what is the ratio of salt to acid at one pH unit away from the pKa?)

7. Nine grams of buffer (pKa 7.2, molecular weight = 180 g/mole, acid form) and 0.5 gram of NaOH (molecular weight = 40 g/mole) are placed in 100 ml of water. What is the pH of the final solution? Show your work! Repeat your calculations for 1000 ml of water and compare your two calculations. Explain similarities or differences.

8. You are titrating a buffer with a pKa of 8. You have been given the buffer in the acid form (i.e. white powder as HA).

a) What is the pH of a 0.1M solution of the buffer in water before anything else is added? b) Would you use acid or base to titrate the resulting solution? Why? c) If this buffer was a carboxylic acid, over what pH ranges would it have a charge of approximately 0, -1/2, and -1? Explain why.

9. Carefully (!) draw the titration curve from pH 5 to pH 9.for a substance with a pKa of 7. Label the axes.

10. Draw a titration curve from pH 1 to pH 13 for a compound with a single amine (pKa 8.5) and carboxylic acid (pKa 4.5). Show in an additional table where this compound would have a NET charge of +1, 0, and -1.

            Charge on carboxylic acid   Charge on amine    Total charge on molecule
    pH           (at this pH)               (at this pH)             (at this pH)