Biochemistry I Chapter 2 Problems

The pK of acetic acid is 4.76. What is the effective buffering range? pH = 3.76-5.76 pH = 4.66-4.86 pH = 4.76 pH = 1-7

pH = 3.76-5.76

A ________ is a solution that resists pH changes.

buffer

Which of the following statements about the buffering system in blood in not true? The buffering capacity of blood depends primarily on two equilibria: The first, CO2 + H2O → H2CO3 and the second, H2CO3 → H+ + HCO3-, where the overall pK for these two sequential reactions is 5.35. Blood is buffered so that it maintains a pH of 7.4. Alkalosis is a medical condition where the pH of blood rises above normal. Hyperventilation accelerates the loss of CO2 and causes respiratory alkalosis, which can be ameliorated by breathing in an atmosphere enriched in CO2. The most significant buffering compound in human blood is bicarbonate (HCO3-).

*The buffering capacity of blood depends primarily on two equilibria: The first, CO2 + H2O → H2CO3 and the second, H2CO3 → H+ + HCO3-, where the overall pK for these two sequential reactions is 5.35.

Why are the ionic mobilities of H+ and OH- in an aqueous environment much greater than that of other ions? They have unit charge. The hydrogen ion can hop from one water molecule to another. They are smaller than most other ions. They have less mass than other ions.

*The hydrogen ion can hop from one water molecule to another.

Which of the following statements about water is FALSE? A network of electrostatic interactions exists between the molecules in water. The hydrophobic effect is driven largely by the entropy of the non-polar substance. Water will act as a hydrogen bond acceptor with hydroxyl groups. Water carries a dipole because of the geometry of the two oxygen-hydrogen bonds. Substances with polar functional groups are generally soluble in water.

*The hydrophobic effect is driven largely by the entropy of the non-polar substance.

Which of the following statements about water is false? Water has a high melting point relative to its molecular mass. The hydrophobic effect results when solutes are unable to break hydrogen bonds between water molecules. Nonpolar substances decrease the entropy of water molecules. Water molecules make hydrogen bonds readily with each other and with other polar substances. Water is a cohesive substance.

*The hydrophobic effect results when solutes are unable to break hydrogen bonds between water molecules.

Which statement about the pK is not true? The pK is equal to the pH at the maximum slope of the titration curve. The pK is equal to the pH at the midpoint of the titration. The pK is equal to the pH when [A-] = [HA]. The pK is equal to the pH at the maximal buffering capacity.

*The pK is equal to the pH at the maximum slope of the titration curve.

Consider a dialysis procedure in which an aqueous solution is separated from pure water by a membrane that is permeable to both water and solutes. Which statement is true? Dialysis cannot separate molecules based upon their relative sizes. The random movement of molecules, called diffusion, proceeds only until the concentration is the same on each side of the dialysis membrane. The tendency of solutes to diffuse from a region of high concentration to a region of lower concentration is thermodynamically favored. The membrane can be fine-tuned to selectively allow large molecules instead of smaller molecules to diffuse through it.

*The tendency of solutes to diffuse from a region of high concentration to a region of lower concentration is thermodynamically favored.

Which statement best explains why nonpolar substances have low solubility in water? They are much denser than water. They have strong mutual attractions. They cannot form hydrogen bonds to water. They are much larger than water.

*They cannot form hydrogen bonds to water.

Acetic acid (pK = 4.76) is an effective buffer against hydroxide at pH 3.76. True False

*True

Amphiphiles or amphipathic molecules are molecules that have both polar and nonpolar segments. True False

*True

Hydrogen bonds are an example of weak non-covalent interactions, which are common in biological molecules. False True

*True

Hydrophobic or nonpolar substances are insoluble in water. False True

*True

In general, a hydrogen bond can be represented as D-H•••A, where D-H is a weakly acidic "donor" group such as O-H, N-H, or sometimes S-H, and A is a weakly basic "acceptor" atom such as O, N, or occasionally S. False True

*True

Ionic or electrostatic interactions occur between two groups of opposite charge. False True

*True

Molecules in liquid water are each hydrogen bonded to four nearest neighbors, as they are in ice. These hydrogen bonds are distorted, however, so the networks of linked molecules are irregular and varied whereas they are ordered in ice, where each water molecule is tetrahedrally surrounded by four nearest neighbors to which it is hydrogen bonded. False True

*True

Solutions in which [H+] = [OH-] are said to be neutral. True False

*True

Solutions in which [H+] > 10-7 are said to be acidic. False True

*True

The Henderson-Hasselbalch equation, given by pH = pK + log ([A-]/[HA]), is very useful for calculating the pH of solutions of weak acids. True False

*True

The buffering capacity of a weak acid is greatest when pH = pKa. True False

*True

The hydrogen atoms in water occupy two positions at corners of a nearly perfect tetrahedron with oxygen at the center. True False

*True

The hydrophobic effect is due largely to entropy changes in the solvent water. False True

*True

The pK values for phosphoric acid are 2.15, 6.82, and 12.38. At pH 12.38, [HPO42-] = [PO43-]. True False

*True

The pK values for phosphoric acid are 2.15, 6.82, and 12.38. At pH 6.82, [H₂PO₄⁻] = [HPO₄²⁻]. False True

*True

Water solvates both positive and negative ions. False True

*True

A hydronium ion (H3O+) has the potential to be a hydrogen bond acceptor. False. True.

*True.

Which of the following correctly lists bonding interactions in terms of their increasing strength: Van der Waals interactions<hydrogen bonding<ionic interactions<covalent bonds Hydrogen bonds<ionic interactions<covalent interactions<Van der Waals interactions Covalent bonds<hydrogen bonds<ionic interactions<Van der Waals interactions Van der Waals interactions<ionic interactions<hydrogen bonding<covalent bonds

*Van der Waals interactions<hydrogen bonding<ionic interactions<covalent bonds

Which statement is NOT a reason for the centrality of water in biochemistry? The medium for most biochemical reactions is water. Water is oxidized to oxygen during photosynthesis. Water in oceans covers most of the surface of our planet. Water and its ionic components participate in many biochemical reactions.

*Water in oceans covers most of the surface of our planet.

Which of the following statements is not accurate when considering water and biological molecules? Polar substances are soluble because of their interaction with water. The hydrogen bonding pattern of water is disrupted by the presence of hydrophobic molecules. Water is more likely to act as a hydrogen bond donor than a hydrogen bond acceptor.

*Water is more likely to act as a hydrogen bond donor than a hydrogen bond acceptor.

Is the molecule shown below soluble in water? No. Yes.

*Yes.

The pK of acetic acid is 4.76. Which statement is true at pH 5.0? [A-] > [HA] [A-] >> [HA] [A-] = [HA] [A-] < [HA]

*[A-] > [HA]

At the first horizontal inflection point in the titration curve of phosphoric acid, which statement is true? [H3PO4] > [H2PO4-] [H2PO4-] < [HPO42-] [H3PO4] = [H2PO4-] [H2PO4-] = [HPO42-]

*[H3PO4] = [H2PO4-]

The pK of ammonia is 9.25. Which statement is true at pH 7.0? [NH4+] = [NH3] [NH4+] < [NH3] Ammonia is an effective buffer at this pH. [NH4+] > [NH3]

*[NH4+] > [NH3]

According to the Brönsted-Lowry definition, a ________ is a substance that can accept a proton.

*base

The ________ ion is a hydrogen ion associated with one or more molecules of water.

*hydronium

The tendency of water to minimize its contacts with hydrophobic molecules is termed the ________.

*hydrophobic effect

In a palmitic acid micelle, H3C-(CH2)11-COO- would be [inside the micelle with the carboxylate directed toward the water or completely inside the micelle or inside the micelle with the nonpolar hydrocarbon chain directed toward the water or completely outside the micelle?]. H3C-(CH2)11-CH3 would be [inside the micelle with the carboxylate directed toward the water or completely inside the micelle or completely outside the micelle or inside the micelle with the nonpolar hydrocarbon chain directed toward the water?].

*inside the micelle with the carboxylate directed toward the water *completely inside the micelle

The negative of the log[H+] is defined as ________.

*pH

Which best describes the buffering range of phosphoric acid? pH = 2.15, 6.82, 12.38 pH = 2.15-12.38 pH = 1.15-3.15, 5.82-7.82, 11.38-13.38 pH = 1.15-13.38

*pH = 1.15-3.15, 5.82-7.82, 11.38-13.38

The pK of ammonia is 9.25. What is the effective buffering range? pH = 9.15-9.35 pH = 7-13 pH = 8.25-10.25 pH = 9.25

*pH = 8.25-10.25

Substances that can undergo more than one ionization are known as ________ acids.

*polyprotic

Occasionally, a C-H group can form a hydrogen bond. This is more likely if the C is next to an N because the [protonated or deprotonated?] and therefore ________ charged N would promote the equality of [charge or separation of charge?] in the adjacent C-H bond so that the carbon would have a partial ________ charge and the H would have a partial ________ charge. This would make the H more likely to be donated to a H-bond [donator or acceptor?] group.

*protonated *positively *separation of charge *negative *positive *acceptor

From the pK's in table 2-4, considering all other factors equal, which weak acid/conjugate base pair will give the best buffer for a pH of 4.3? succinate-/ succinate2- acetic acid/acetate- oxalic acid/oxalate- succinic acid/succinate-

*succinic acid/succinate-

A plot of pH versus the [OH-] added to a solution of a weak acid is called a ________.

*titration curve

Noncovalent associations between neutral molecules are due to forces known generally as ________ forces.

*van der Waals

Acids that only partially ionize are called ________ acids.

*weak

Which property of liquid water is also true for solid water? The density of liquid water, 1.0 g•mL-1. Liquid water consists of a rapidly fluctuating, three-dimensional network of hydrogen-bonded molecules. Liquid water has an extensive hydrogen bond network. Liquid water tends to form hydrogen bonded rings of three to seven molecules.

*Liquid water has an extensive hydrogen bond network.

Which of the properties listed is NOT true of liquid water? Liquid water is only 15% less hydrogen bonded than ice at 0o C. Liquid water is less dense than ice. Liquid water tends to form hydrogen bonded rings of three to seven molecules. Liquid water consists of a rapidly fluctuating, three-dimensional network of hydrogen-bonded molecules.

*Liquid water is less dense than ice.

Which description best characterizes the length of a hydrogen bond between water molecules? About the same length as a covalent O-H bond Longer than a covalent O-H bond, but shorter than the calculated van der Waals distance About the same length as a calculated O-H van der Waals distance Shorter than a covalent O-H bond

*Longer than a covalent O-H bond, but shorter than the calculated van der Waals distance

________ are spheroidal globules of up to several thousand amphiphilic molecules, in which the hydrophobic groups are buried and the polar groups form the surface.

*Micelles

Which of the following statements is not accurate when considering water and biological molecules? Water can act as a hydrogen bond donor. Water can act as a hydrogen bond acceptor. Polar substances are soluble because of their interaction with water. Non-polar substances associate with water to maximize their exposed surface area.

*Non-polar substances associate with water to maximize their exposed surface area.

Which of the following types of bonding interactions is NOT classified as non-covalent? Hydrogen bonding. Electrostatic interactions. Van der Waals. Dipole-Dipole interactions. None of the above.

*None of the above.

What would the final salt concentration be if instead of placing the dialysis bag in 4.0 L of distilled water for 12 h, you placed the bag in 1.0 L of distilled water for 6.0 h and then in another 1.0 L of fresh distilled water for another 6.0 h? It would be ________ mM. Use formula: (initial amount of NaCl)/(total volume)

*0.012 **1st step of concentration: (0.005 L)(0.5 M)/(1.005 L) = 0.0025 M **2nd step of concentration: (0.005 L)(0.0025 M)/(1.005 L) = 0.0000124 M = 0.012 M

You have a 5.0 ml sample of a protein in 0.5M NaCl. You place the protein/salt sample inside dialysis tubing and place the bag in a large beaker of distilled water. Your goal is to remove as much NaCl from the sample as possible. Since you know that the final salt concentration will be the same inside the tubing as out, you can calculate the final concentration of the salt. If you decide to place the dialysis bag in 4.0 L of distilled water for 12 h the final concentration of the salt would be ________ mM. Use formula: (initial amount of NaCl)/(total volume)

*0.62 **(0.005 L)(0.5 M)/(4.005 L) = 0.000624 M = 0.624 mM

Match each of the following 1-5: Hydrogen bond → ? London dispersion forces → ? Covalent bond → ? Dipole-dipole interaction → ? Ionic interaction → ? 1. ~20 kJ/mol 2. ~10 kJ/mol 3. ~400 kJ/mol 4. ~80 kJ/mol 5. ~0.4kJ/mol

*1 *5 *3 *2 *4

What is the bond angle of the three atoms in water? 109.5°, the tetrahedral angle 104.5° 180° 90°

*104.5°

What is the concentration of hydrogen ions at pH 3? 0.3 M 10⁻³ M 10³ M 3 M

*10⁻³ M

What is the concentration of hydroxide ions at pH 10? 10⁻¹⁰ M 10⁻⁴ M 10⁴ M 10¹⁰ M

*10⁻⁴ M

What is the pH of 1 M NaOH? 13 15 1 14

*14

A red blood cell has an internal salt concentration of ~150mM. The cell is placed in a beaker of 500mM salt. Assuming that the cell membrane is permeable to water but not to ions, water will move from the ________ salt concentration to the ________ salt concentration. If the membrane were permeable to ions, the solutes would diffuse [out of or into?] the cell.

*150mM *500mM *into

What is the pH of a solution made by mixing equal volumes of 1 M sodium acetate and 1 M acetic acid? (The pK of acetic acid is 4.76.) 3.76 5.76 1 4.76

*4.76

What is the pH of a solution made by mixing equal volumes of 1 M sodium acetate and 0.1 M acetic acid? (The pK of acetic acid is 4.76.) 5.76 4.86 4.66 3.76

*5.76

Estimate the volume of a solution of 1.4 M NaOH that must be added to adjust the pH from 4.0 to 9.0 in 98 ml of a 94 mM solution of phosphoric acid. ________ ml

*6.58 **At pH 4.0, essentially all the phosphoric acid is in the H₂PO4⁻ form, and at pH 9.0, essentially all is in the HPO4²⁻ form. Therefore, the concentration of OH- required is equivalent to the concentration of the acid. **For 98 ml of a 94 mM solution of phosphoric acid: (0.094 mol·L⁻¹ phosphoric acid)(0.098 L) = 0.0092 mol **(0.0092 mol)(1 L/ 1.4 mol NaOH) = 6.58 mL

Which is the weakest interaction? A hydrogen bond between two water molecules. A van der Waals interaction between two methyl groups. A covalent bond between the O and H atoms of water. An ionic interaction between a carboxylic acid and an amine.

*A van der Waals interaction between two methyl groups.

Which of the following statements accurately describes hydrogen bonds in water at room temperature? ▢ A. The hydrogen bonds are a constantly changing network. ▢ B. There is a network of interactions where each water molecule is hydrogen-bonded to 3 other water molecules. ▢ C. There is an organized and regular network of interactions between water molecules.

*A. The hydrogen bonds are a constantly changing network.

Which of the following statements is not true? Acetic acid (pK = 4.76) is an effective buffer against acid at pH 5.76. Acetic acid (pK = 4.76) is an effective buffer against acid at pH 3.76. Ammonium hydroxide (pK = 9.25) is an effective buffer against base at pH 9.75. Ammonium hydroxide (pK = 9.25) is an effective buffer against base at pH 8.25.

*Acetic acid (pK = 4.76) is an effective buffer against acid at pH 3.76.

To what class of biological molecule does the following belong? Monosaccharide. Amino acid. Lipid. Carbohydrate.

*Amino acid.

Would ammonia or piperidine be a better buffer at pH 9? [Piperidine or Ammonia?]

*Ammonia

Which of the following statements is not true? Ammonium hydroxide (pK = 9.25) is an effective buffer against acid or hydroxide at pH 9.25. Ammonium hydroxide (pK = 9.25) is an effective buffer against acid at pH 10.25. Ammonium hydroxide (pK = 9.25) is an effective buffer against acid at pH 8.25. Ammonium hydroxide (pK = 9.25) is not an effective buffer against acid or hydroxide at pH 7.75.

*Ammonium hydroxide (pK = 9.25) is an effective buffer against acid at pH 8.25.

In acid dissociation, why is the dissociation constant Ka defined as K[H2O]? Because the concentration of water is essentially a constant. Because the concentration of water can be neglected. Because the concentration of water does not influence the reaction. Because the concentration of water is always 1 M.

*Because the concentration of water is essentially a constant.

Which of the following functional groups could NOT act as a hydrogen bond donor? Both aldehyde and ester. An amino group. An aldehyde. A hydroxyl group. An ester.

*Both aldehyde and ester.

Rank the following bond/interaction distances correctly in order from shortest to longest: Van der Waals interactions, covalent bond, hydrogen bond. Covalent bond, hydrogen bond, van der Waals interactions. Van der Waals interactions, hydrogen bond, covalent bond. Hydrogen bond, van der Waals interactions, covalent bond. Covalent bond, van der Waals interactions, hydrogen bond.

*Covalent bond, hydrogen bond, van der Waals interactions.

Acetic acid (pK = 4.76) is an effective buffer against hydroxide at pH 5.76. False True

*False

Colligative properties are physical properties of solutions that depend upon the nature of the substances dissolved. False True

*False

For convenience, acid dissociation constants are transformed to pK's, which are equal to log K. True False

*False

Hydrogen-bonded atoms are separated by van der Waals distances. False True

*False

London dispersion forces are long range, but weak forces. False True

*False

Osmosis is the movement of the solute across a semipermeable membrane from a region of high concentration to a region of low concentration. True False

*False

Solutions in which [OH-] < 10-7 are said to be basic whereas solutions in which [OH-] > 10-7 are said to be acidic. False True

*False

The hydrophobic effect is the movement of solvent across a semipermeable membrane from a region of high solvent concentration to a region of lower solvent concentration. True False

*False

The pH of a 1 L buffer solution containing 0.05 M formic acid (pK = 3.75) and 0.05 M sodium formate after the addition of 1mL of 5 M NaOH is pH 3.79. True False

*False

The pK values for phosphoric acid are 2.15, 6.82, and 12.38. At pH 12.38, [H₂PO₄⁻] = [HPO₄²⁻]. True False

*False

The pK values for phosphoric acid are 2.15, 6.82, and 12.38. At pH 2.15, [H3PO4] = [PO43-]. False True

*False

The properties of water are typical for molecules of its size. True False

*False

The value of Kw is 10¹⁴ M² at 25°C. True False

*False

Water is a neutral molecule that ionizes to a great extent. True False

*False

Water is an effective solvent for living organisms because of its inert behavior. False True

*False

Water is considered a polar molecule because the oxygen atom is much larger than the hydrogens. False True

*False

Weak acids have dissociation constants that are greater than one. False True

*False

The properties of water are typical for molecules of its size. False. True.

*False.

Which statement about the Bronsted-Lowry formulation of acid-base chemistry is false? An acid is a substance that can donate a proton. H3O+ is the conjugate base of the acid H2O. A- is the conjugate base of the acid HA. A base is a substance that can accept a proton.

*H3O+ is the conjugate base of the acid H2O.

Would HEPES or Tris be a better buffer at pH 7.5? [Tris or HEPES?]

*HEPES

What is the ionic species of phosphoric acid that predominates at pH 11? ________ What is the ionic species of phosphoric acid that predominates at pH 8? ________

*HPO₄²⁻ *HPO₄²⁻ **In both cases the same species predominates (monohydrogen phosphate).

An amphiphilic molecule: Has both polar and non-polar groups. Has polar groups. Has non-polar groups. Has neither polar nor non-polar groups.

*Has both polar and non-polar groups.

Which statement is NOT an important aspect of the hydrophobic effect? The entropy of water increases when hydrophobic molecules are excluded from water. Hydrophobic molecules have a strong affinity to interact with each other instead of water. The ∆H is near zero or positive when hydrophobic molecules are excluded from water. The larger the surface area of a hydrophobic molecule, the greater the loss of entropy by interacting solvent water.

*Hydrophobic molecules have a strong affinity to interact with each other instead of water.

Which statement best explains why water can solvate both negative and positive ions? It is very small. It can ionize. It can hydrogen bond. It is dipolar.

*It is dipolar.

Which of the following words makes the statement regarding this reaction true? R-COOH ↔ R-COO⁻ + H+ In a solution with low pH the equilibrium of this reaction will be to the ____________ and the molecule will be ___________. Right; Positive Left; Positive Left; Neutral Right; Neutral

*Left; Neutral

Consider a solution separated from pure water by a semipermeable membrane that permits the passage of water, but not solutes. Which statement is false? Osmotic pressure could be eliminated if enough solute were added to the region of pure water. Osmosis is the movement of solvent from a region of high solventconcentration to a solution of lower solvent concentration. Osmotic pressure is the pressure that must be applied to the solution to prevent the inflow of water. Osmotic pressure depends upon the size of the solute molecules.

*Osmotic pressure depends upon the size of the solute molecules.

Consider the transfer of benzene from water to pure benzene. Which statement is false? The ∆H of transfer is about zero. The ∆G of transfer is negative. T∆S is positive. Process is less favorable at higher temperature.

*Process is less favorable at higher temperature.

________ acids, such as HCl, transfer all of their protons to H2O.

*Strong

Would phosphoric acid or succinic acid be a better buffer at pH 5? [Phosphoric acid or Succinic acid?]

*Succinic acid