Biochemistry Test Bank (All 23 chapters with complete, Questions and Answers with complete solutions

1. The aqueous solution with the LOWEST pH is: A) 0.01 M HCl. B) 0.1 M acetic acid (pKa = 4.86). C) 0.1 M formic acid (pKa = 3.75). D) 0.1 M HCl. E) 10–12 M NaOH. 2. The aqueous solution with the HIGHEST pH is: A) 1 M HCl. B) 1 M NH3 (pKa = 9.25). C) 0.5 M NaHCO3 (pKa = 3.77). D) 0.1 M NaOH. E) 0.001 M NaOH. 4 3. Phosphoric acid is tribasic, with pKa's of 2.14, 6.86, and 12.4. The ionic form that predominates at pH 3.2 is: A) H3PO4. B) H2PO–. C) HPO 2–. D) PO 3–. E) None of the answers is correct. 4. Which statement about buffers is TRUE? A) A buffer composed of a weak acid of pKa = 5 is stronger at pH 4 than at pH 6. B) At pH values lower than the pKa, the salt concentration is higher than that of the acid. C) The pH of a buffered solution remains constant no matter how much acid or base is added to the solution. D) The strongest buffers are those composed of strong acids and strong bases. E) When pH = pKa, the weak acid and salt concentrations in a buffer are equal. 5. A compound has a pKa of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30 mL of 1.0 M hydrochloric acid. The resulting solution is pH: A) 6.5. B) 6.8. C) 7.2. D) 7.4. E) 7.5. 6. The Henderson-Hasselbalch equation: A) allows the graphic determination of the molecular weight of a weak acid from its pH alone. B) does not explain the behavior of di- or tri-basic weak acids. C) employs the same value for pKa for all weak acids. D) is equally useful with solutions of acetic acid and of hydrochloric acid. E) relates the pH of a solution to the pKa and the concentrations of acid and conjugate base. 5 7. Consider an acetate buffer, initially at the same pH as its pKa (4.76). When sodium hydroxide (NaOH) is mixed with this buffer, the: A) pH remains constant. B) pH rises more than if an equal amount of NaOH is added to an acetate buffer initially at pH 6.76. C) pH rises more than if an equal amount of NaOH is added to unbuffered water at pH 4.76. D) ratio of acetic acid to sodium acetate in the buffer falls. E) sodium acetate formed precipitates because it is less soluble than acetic acid. 8. A compound is known to have a free amino group with a pKa of 8.8, and one other ionizable group with a pKa between 5 and 7. To 100 mL of a 0.2 M solution of this compound at pH 8.2 was added 40 mL of a solution of 0.2 M hydrochloric acid. The pH changed to 6.2. The pKa of the second ionizable group is: A) The pH cannot be determined from this information. B) 5.4. C) 5.6. D) 6.0. E) 6.2. 9. Three buffers are made by combining a 1 M solution of acetic acid with a 1 M solution of sodium acetate in the ratios shown below. 1 M acetic acid 1 M sodium acetate Buffer 1: 10 mL 90 mL Buffer 2: 50 mL 50 mL Buffer 3: 90 mL 10 mL Which statement is TRUE of the resulting buffers? A) pH of buffer 1 pH of buffer 2 pH of buffer 3 B) pH of buffer 1 = pH of buffer 2 = pH of buffer 3 C) pH of buffer 1 pH of buffer 2 pH of buffer 3 D) The problem cannot be solved without knowing the value of pKa. E) None of the statements is true. 6 10. A 1.0 M solution of a compound with 2 ionizable groups (pKa's = 6.2 and 9.5; 100 mL total) has a pH of 6.8. If a biochemist adds 60 mL of 1.0 M HCl to this solution, the solution will change to pH: A) 5.60. B) 8.90. C) 9.13. D) 9.32. E) The pH cannot be determined from this information. 11. You want to maintain pH = 7.0 for an enzyme-catalyzed reaction that will produce hydrogen ions along with the desired product. At equal concentrations, which weak acid, if any, will serve as the better buffer for the reaction: acid A, with pKa = 6.5, or acid B, with pKa = 7.5? A) acid A B) Water is as good as either of the acids available. C) acid B D) Both are equally effective. 12. In which reaction does water NOT participate as a reactant (rather than as a product)? A) conversion of an acid anhydride to two acids B) conversion of an ester to an acid and an alcohol C) conversion of ATP to ADP D) photosynthesis E) production of gaseous carbon dioxide from bicarbonate 13. Which property of water does NOT contribute to the fitness of the aqueous environment for living organisms? A) cohesion of liquid water due to hydrogen bonding B) high heat of vaporization C) high specific heat D) the density of water being greater than the density of ice E) the very low molecular weight of water 14. The ionization of water is due to the strongly electronegative element: A) hydrogen donating an electron to oxygen. B) oxygen donating an electron to hydrogen. C) hydrogen receiving an electron from oxygen. D) oxygen receiving an electron from hydrogen. E) None of the answers is correct. 15. The dipole moment in a water molecule Which statement about hydrogen bonds is NOT true? A) Hydrogen bonds account for the anomalously high boiling point of water. B) In liquid water, the average water molecule forms hydrogen bonds with three to four other water molecules. C) Individual hydrogen bonds are much weaker than covalent bonds. D) Individual hydrogen bonds in liquid water exist for many seconds and sometimes for minutes. E) The strength of a hydrogen bond depends on the linearity of the three atoms involved in the bond. 16. A TRUE statement about hydrophobic interactions is that they: A) are the driving force in the formation of micelles of amphipathic compounds in water. B) do not contribute to the structure of water-soluble proteins. C) have bonding energies of approximately 20–40 Kjoule per mole. D) involve the ability of water to denature proteins. E) primarily involve the effect of polar solutes on the entropy of aqueous systems.