Biochem Ch. 2 Test Questions

5.60.

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: 5.60. 8.90. 9.13. 9.32. The pH cannot be determined from this information.

7.4.

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: 6.5. 6.8. 7.2. 7.4. 7.5.

6.0.

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: The pH cannot be determined from this information. 5.4. 5.6. 6.0. 6.2.

T

Almost every biological process is pH-dependent.

T

Amphipathic molecules contain regions that are polar (charged) and regions that are nonpolar (uncharged).

the rates of the forward and reverse reactions are equal

At equilibrium, __________. the rates of the forward and reverse reactions are equal the rate constants of the forward and reverse reactions are equal all chemical reactions have ceased the value of the equilibrium constant is 1 the limiting reagent has been consumed

F

Cells maintain a specific and constant cytosolic pH near pH 5.

F (polar dissolves polar)

Compounds that dissolve easily in water are hydrophilic including nonpolar solvents like chloroform and benzene.

polar, nonpolar

Compounds that dissolve in water are ___ such as glycerol; whereas compounds that do not dissolve in water are ___ such as wax.

sodium acetate formed precipitates because it is less soluble than acetic acid.

Consider an acetate buffer, initially at the same pH as its pKa (4.76). When sodium hydroxide (NaOH) is mixed with this buffer, the: pH remains constant. pH rises more than if an equal amount of NaOH is added to an acetate buffer initially at pH 6.76. pH rises more than if an equal amount of NaOH is added to unbuffered water at pH 4.76. ratio of acetic acid to sodium acetate in the buffer falls. sodium acetate formed precipitates because it is less soluble than acetic acid.

F

Hydrophobic bonds have a strength of ~10 kcal/mol.

three-dimensional folding of a polypeptide chain.

Hydrophobic interactions make important energetic contributions to: binding of a hormone to its receptor protein. enzyme-substrate interactions. membrane structure. three-dimensional folding of a polypeptide chain. All of the above are true.

osmotic lysis

In their natural environment, cells generally contain higher concentrations of biomolecules than their natural surroundings so osmotic pressure tends to drive water into cells. If not counterbalanced, the inward movement of water would lead to bursting of the cells by [x].

DNA, Nylon, cellulose

Name three types of biomolecules where noncovalent bonds play a significant role in the 3D structure of the molecule: [x], [y], and [z]

water molecule across a membrane.

Osmosis is movement of a: charged solute molecule (ion) across a membrane. gas molecule across a membrane. nonpolar solute molecule across a membrane. polar solute molecule across a membrane. water molecule across a membrane.

H2PO4-

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: H3PO4. H2PO4-. HPO42-. PO43-. none of the above.

lipid

Select the amphipathic molecule: Phenylalanine Glycine Aspartate Lipid

F

T/F: A hydrogen bond is the only noncovalent bond that is on the order of covalent bonds in terms of strength.

T

T/F: Hydrogen bonds are strongest when the bonded molecules are oriented at an angle (i.e. when the acceptor atom is at an angle with the covalent bond between the donor atom and H).

T

T/F: The C-H bond is very weakly polar and thus generally do not participate in hydrogen bonding.

F

T/F: The bond dissociation energy of a hydrogen bond is around ~23 kJ/mol.

T

T/F: The dissociation energy of a hydrogen bond is about 350 kJ/mol making it about as strong as a covalent O-H bond.

relates the pH of a solution to the pKa and the concentrations of acid and conjugate base.

The Henderson-Hasselbalch equation: allows the graphic determination of the molecular weight of a weak acid from its pH alone. does not explain the behavior of di- or tri-basic weak acids. employs the same value for pKa for all weak acids. is equally useful with solutions of acetic acid and of hydrochloric acid. relates the pH of a solution to the pKa and the concentrations of acid and conjugate base.

0.1 M NaOH (highest concentration of strong base)

The aqueous solution with the highest pH is: 1 M HCl. 1 M NH3 (pKa = 9.25). 0.5 M NaHCO3 (pKa = 3.77). 0.1 M NaOH. 0.001 M NaOH.

0.1 M HCl (highest concentration of strong acid)

The aqueous solution with the lowest pH is: 0.01 M HCl. 0.1 M acetic acid (pKa = 4.86). 0.1 M formic acid (pKa = 3.75). 0.1 M HCl. 10-12 M NaOH.

hydrophobic

The force that hold nonpolar regions of molecules together are called [x] interactions.

one million times lower [H+] than the gastric juice. (log scale; 6 away is equal to six zeroes)

The pH of a sample of blood is 7.4, while gastric juice is pH 1.4. The blood sample has: 0.189 times the [H+] as the gastric juice. 5.29 times lower [H+] than the gastric juice. 6 times lower [H+] than the gastric juice. 6000 times lower [H+] than the gastric juice. one million times lower [H+] than the gastric juice.

13 (take -log)

The pH of a solution of 0.1 M NaOH is: 0.1. 1.0. 12.8. 13. 14.

1

The pH of a solution of 1 M HCl is: 0. 0.1. 1. 10. -1.

.11 to .21 nm

The typical van der Waals radius is what size? .11 to .21 nm 1 to 3 nm 1 to 3 pm 1 to 3 um

0.25

The value of Keq for the following reaction is 0.25: SO2 (g) + NO2 (g) SO3 (g) + NO (g) The value of Keq at the same temperature for the reaction below is __________. 2SO2 (g) + 2NO2 (g) 2SO3 (g) + 2NO (g)

pH of buffer 1 > pH of buffer 2 > pH of buffer 3

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 of these statements is true of the resulting buffers? pH of buffer 1 < pH of buffer 2 < pH of buffer 3 pH of buffer 1 = pH of buffer 2 = pH of buffer 3 pH of buffer 1 > pH of buffer 2 > pH of buffer 3 The problem cannot be solved without knowing the value of pKa. None of the above

pH optimum

What is the pH at which catalytic activity of an enzyme performs optimally (i.e performance of the enzymes declines dramatically on either side of the pH curve)?

1-3 kJ/mol

What is the strength of van der Waals forces? 4 to 17 kJ/mole 40-80 kJ/mol 1-3 kJ/mol 100 - 200 kJ/mol

T

When two dipoles weakly attract each other, the weak interactions are called van der Waals interactions

Hydrogen bonds form readily in aqueous solutions.

Which of the following is true about the properties of aqueous solutions? A pH change from 5.0 to 6.0 reflects an increase in the hydroxide ion concentration ([OH-]) of 20%. A pH change from 8.0 to 6.0 reflects a decrease in the proton concentration ([H+]) by a factor of 100. Charged molecules are generally insoluble in water. Hydrogen bonds form readily in aqueous solutions. The pH can be calculated by adding 7 to the value of the pOH.

The very low molecular weight of water

Which of the following properties of water does not contribute to the fitness of the aqueous environment for living organisms? Cohesion of liquid water due to hydrogen bonding High heat of vaporization High specific heat The density of water is greater than the density of ice The very low molecular weight of water

When pH = pKa, the weak acid and salt concentrations in a buffer are equal.

Which of the following statements about buffers is true? A buffer composed of a weak acid of pKa = 5 is stronger at pH 4 than at pH 6. At pH values lower than the pKa, the salt concentration is higher than that of the acid. The pH of a buffered solution remains constant no matter how much acid or base is added to the solution. The strongest buffers are those composed of strong acids and strong bases. When pH = pKa, the weak acid and salt concentrations in a buffer are equal.

When pH = pKa, the weak acid and salt concentrations in a buffer are equal.

Which of the following statements about buffers is true? CHECK A buffer composed of a weak acid of pKa = 5 is stronger at pH 4 than at pH 6. At pH values lower than the pKa, the salt concentration is higher than that of the acid. The pH of a buffered solution remains constant no matter how much acid or base is added to the solution. The strongest buffers are those composed of strong acids and strong bases. When pH = pKa, the weak acid and salt concentrations in a buffer are equal.

hydrophilic

Which of the following term describes polar molecules: hydrophobic electrostatic hydrophilic amphipathic

Individual hydrogen bonds in liquid water exist for many seconds and sometimes for minutes.

Which of these statements about hydrogen bonds is not true? Hydrogen bonds account for the anomalously high boiling point of water. In liquid water, the average water molecule forms hydrogen bonds with three to four other water molecules. Individual hydrogen bonds are much weaker than covalent bonds. Individual hydrogen bonds in liquid water exist for many seconds and sometimes for minutes. The strength of a hydrogen bond depends on the linearity of the three atoms involved in the bond.

varying the initial concentrations of reactants

Which one of the following will change the value of an equilibrium constant? changing the volume of the reaction vessel changing temperature adding a catalyst to the reaction adding other substances that do not react with any of the species involved in the equilibrium varying the initial concentrations of reactants

hypertonic

Which term describes when the osmolarity of the inside of a cell or membrane is higher than the outside. hypotonic isotonic hypertonic osmotic

Acid A

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? Acid A Water is as good as either of the acids available. Acid B Both are equally effective.

micelles

[x] are stable structures composed of amphipathic compounds in water where the hydrophobic groups are sequestered from the water.