Complete the following questions.
1.
a. For each of the following compounds, identify the acid form and its conjugate base. Consider only functional groups with pKa's in the range 1-12. (Note: Only one form is shown. You will have to draw the other form as well as identify it.)
b. For each of the compounds in your answer to question #1 part a, indicate (by circling the appropriate structure) which species will predominate (i.e. be present in the highest concentration) at pH 8. You will need a table of approximate pKa values for various functional groups to answer this question.
2. The structure of a small peptide is shown below. You want to make an aqueous solution of this peptide at the highest concentration possible. Would you choose a dilute acid (say 50 mM HCl or a dilute base (again, say 50 mM NaOH) or a buffer near neutral pH? Why? (Hint: Think about what factors govern water solubility and how pH will affect one of them.)
3. The figure shown below illustrates the binding of substrates (homoserine, on the left and ATP on the right) to an enzyme. Each of the dotted lines indicates a non-covalent interaction between the substrate and the enzyme. For each of these interactions identify its type (charge-charge, hydrogen bond or van der Walls); for hydrogen bonds also indicate which participant is the donor and the acceptor.
4. Draw a molecular level view for each of the indicated substances in water.
5. Calculate the pH and percent ionization for 0.500 M Potassium bicarbonate solution. See Table 2.4 in your text for the appropriate Ka value.
ANSWER (pH):
Answer (% ionization):
6.
a. Prepared lactate buffer has significant concentrations of both lactic acid and its conjugate base lactate present. Determine the relative concentrations (ratio) of these two species in a solution having a pH of 4.20. Would this buffer respond better (buffer more effectively) to added acid or added base? Briefly explain using proper chemistry terminology.
Ratio:
Explain:
b. Calculate the relative percentages of lactic acid and lactate in buffer at pH of 4.20.
Answer:
c. How would you prepare approximately 2.00 L (+/- 0.100 L) of ~0.400 M lactic acid/lactate buffer at pH 4.20 by using a solution of 0.250 M lactate and adding concentrated hydrochloric acid (12.0 M)? Calculate the volumes of each solution that are required to prepare the ~2.00 L of buffer. What is the ‘actual' lactate concentration (total combined concentration of lactic acid and lactate species present in solution) of the prepared buffer you prepared? HINT: You have the relative percentages of the WA/CB required calculated in 5b - use them (and 0.250 moles of ‘lactate species' per L) to calculate the moles of WA and CB that need to be present (per liter is easiest - scale up later to give 2.00 L) to give pH=4.20. From these data, you can determine the moles of strong acid that must be added to convert CB to WA in the appropriate amounts. An ICE table would be very helpful here!
Answer:
7. Acetic acid has a pKa of 4.8. How many milliliters of 0.2 M acetic acid and 0.2 M sodium acetate are required to prepare 1 liter of 0.1 M buffer solution having a pH of 4.8?
Answer:
8. Which of the following solutions is not an effective buffer? Why? (Hint: The concentration of the various solutes is irrelevant for our purposes.)
a) 25 mM HEPES, pH 7.9 b) 0.1 M borate, pH 9.1 c) 0.05 M citrate, pH 4.5
d) 0.15 M NaCl, pH 6.8 e) 200 mM Tris, pH 7.4