Please answer the following questions and explain, Thank you. (Biochemistry) Pra
ID: 1076034 • Letter: P
Question
Please answer the following questions and explain, Thank you. (Biochemistry)
Practice Problems: Acids, Bases and Buffers
“He had just started a new job at San Francisco Biotech, Inc. His first experiment was to assay a “cloned” human morphine receptor protein using an elegant ligand-receptor binding method. The company wanted him to purify the receptor protein and use it to screen for new analgesic drugs. They planned to make millions. He knew he was in the big time now. But wait, the procedure calls for 10 mM HEPES buffer, pH 7.80. It says it’s important that the concentration and pH be exact. OMG, what is HEPES anyway and how do I get both pH and concentration exact? It’s probably too trivial to ask about. This is embarrassing. Didn’t we learn something about buffers in Chem or Biochem? Too bad I crammed for that section. Let’s ask that Biochem student…..
Calculate the Kb values for the following weak bases:
a. Formate
b. Tris base
c. Dihydrogen phosphate
2. The Ka value for a weak acid is 7.24 x 10-4. What is the pKa for this acid?
3. Calculate the pH of buffer solutions having the following compositions:
0.06 M Tris base / 0.04 M Tris-H+Cl-
0.02 M formic acid / 0.02 M sodium formate
0.10 M MES-H+ / 0.08 M sodium MES
0.08 M NaH2PO4 / 0.20 M K2HPO4
4. What are the ratios of conjugate base to conjugate acid for each of the following buffers?
a HEPES solution of pH 8.47
a sodium phosphate solution of pH 2.15
a sodium phosphate solution of pH 8.00
5. Which of the following chemical systems will act as buffers?
0.10 M acetic acid
sodium phosphate, pH 4.5
0.03 M lactic acid / 0.03 M sodium lactate
0.02 M HEPES, pH 9.55
0.2 M MOPS, pH 7.50
0.05 M Tris base
0.20 M sodium PIPES / 0.1 M disodium PIPES
6. What is the concentration of the anionic form of HEPES in a 0.10 M HEPES buffer having a pH of 6.80?
7. What is the concentration of phosphoric acid (H3PO4) in a mixture of phosphoric acid and NaH2PO4 having a pH of 2.70 and a total phosphate concentration of 0.50 M?
8. Describe the preparation of one liter of 0.20 M MES buffer, pH 5.80, starting with
crystalline MES in the zwitterion form and sodium MES in the anionic form.
(MW of MES = 195; MW of Sodium MES = 217).
9. Describe the preparation of one liter of 0.20 M MES buffer, pH 5.80, starting with crystalline MES in the zwitterionic form and NaOH. Give the number of moles of MES and NaOH you would use and describe how you would go about mixing them with water to make the buffer.
10. Explain how you could make the buffer described in number 9 above if you had forgotten how to use the Henderson Hasselbalch equation.
11. Describe the preparation of one liter of 1 M MOPS buffer, pH 7.15, using the two crystalline forms of MOPS conjugate acid and conjugate base. (MW of MOPS = 209; MW of Sodium MOPS = 231).
12. How many moles of HCl needs to be added to 0.20 mole of Tris base to make one liter of a Tris buffer with a pH of 7.60?
13. Give an example of a conjugate acid / conjugate base pair that could be used to make a buffer having each of the following pH values:
a) 6.0 b) 2.6 c) 8.4 d) 9.4 e) 12.7 f) 6.9
14. How many moles of NaOH should be mixed with 1 mole of NaH2PO4 to prepare one liter of a 1 M sodium phosphate buffer with a pH of 6.90?
15. Draw the titration curve for PIPES starting with the conjugate acid form. Which form (or forms) of PIPES will predominate at
a) pH 3 b) pH 6.8 c) pH 12
16. Using the titration curve for H3PO4, give the form (or forms) of phosphate that would predominate at
a) pH 1 b) pH 2.2 c) pH 10 d) pH 12.5
17. Would a Tris buffer having a pH of 7.60 do a better job of buffering against strong acid (HCl) or strong base (NaOH)? (Hint: When in doubt draw the titration curve).
18. identify and draw the conjugate acid and conjugate base structures of Tris,
MES, HEPES, PIPES and MOPS.
Explanation / Answer
1) The Ka values for the conjugate weak acids are listed in tables and we need to use the Ka values to derive the Kb values.
Ka and Kb are related as
Ka*Kb = Kw where Kw is the ionic product of water and is equal to 1.0*1014
====> Kb = Kw/Ka
a) Formate:
The Ka of formic acid is 1.77*10-4; therefore, Kb for formate anion = Kw/Ka = (1.0*10-14)/(1.77*10-4) = 5.65*10-11 (ans)
b) Tris base:
The pKa of Tris HCl is 8.1; therefore, Ka = antilog (-Pka) = antilog (-8.1) = 7.94*10-9.
The Kb of Tris base is given as Kb = Kw/Ka = (1.0*10-14)/(7.94*10-9) = 1.26*10-6 (ans).
c) Dihydrogen phosphate:
The Ka of phosphoric acid is 7.52*10-3; the Kb of the conjugate base, dihydrogen phosphate is given as Kb = Kw/Ka = (1.0*10-14)/(7.52*10-3) = 1.33*10-12 (ans).
2) The Ka of a weak acid is given as 7.24*10-4.
We define the pKa of the weak acid as pKa = -log (Ka); therefore, pKa = -log (7.24*10-4) = 3.14 (ans).