Biochem Ch.2

The pH of blood is affected by A) the reaction of CO2 with H2O to form carbonic acid. B) the ionization of aqueous carbonic acid to hydronium ions and the bicarbonate anions. C) the decrease of the blood pH due to the production of hydronium ions. D) the excretion of bicarbonate and ammonium ions from the kidneys. E) all of the above

E) all of the above

_____ is exceptionally soluble in water due to the formation of hydrogen bonds.

Ethanol

To make a phosphate buffer at pH 6.82 starting with one liter of 10 mM phosphoric acid (pKs are 2.15, 6.82, and 12.38), you could add

You can't make a buffer by adding HCl or KOH.

The pH of coffee is 5.6. The pH of grapefruit juice is 2.6. This means that the proton concentration in coffee is

a thousand times lower than in grapefruit juice.

Amphiphilic molecules

have both hydrophilic and hydrophobic groups.

Osmotic pressure is a function of

solute concentration.

In a hydrogen bond between a water molecule and another biomolecule

the hydrogen bond will typically form between a hydrogen atom of the water molecule and either a nitrogen, sulfur, or oxygen atom of the other molecule.

Molecules such as methanol and ethanol are very soluble in water because

they contain O─H groups that can form multiple H-bonds with water.

To make an acetate buffer at pH 4.76 starting with 500 mL of 0.1 M sodium acetate (pK acetic acid = 4.76), you could add

0.025 mol of HCl.

The pK1 of citric acid is 3.09. What is the citric acid : monosodium citrate ratio in a 1.0 M citric acid solution with a pH of 2.09?

10:1

Kw, the ionization constant of water, is _____ at _____.

10^-14; 25°C

You mix 999 mL pure water and 1 mL of 2.0 M NaOH. Calculate the pH of the resulting solution. (assume the total volume is 1.0 L).

11.3

Covalent C─C and C─H bonds have bond strengths that are approximately ____ times higher than those of H-bonds.

20

Methanol can act both as a H-bond donor and as a H-bond acceptor. What is the maximal number of H-bonds a single molecule of methanol can form with surrounding water molecules.

3

A solution is made by mixing 1.0 mL of 1.0 M acetic acid (pK = 4.76, Ka = 1.74 x 10-5 ) with one 999 mL of pure water. Calculate the pH of the resulting solution (assume the total volume is 1.0 L).

3.9

Urea is a water-soluble product of nitrogen metabolism. How many hydrogen bonds can one urea molecule donate to surrounding water molecules?

4

A solution is made by mixing 0.05 mL of 1.0 M HCl with 999.95 mL of pure water. Calculate the pH of the resulting solution (assume the total volume is 1.0 L).

4.3

What is the approximate pKa of a weak acid HA if a solution 0.1 M HA and 0.3 M A- has a pH of 6.5?

6.0

The pKs of succinic acid are 4.21 and 5.64. How many grams of monosodium succinate (FW = 140 g/mol) and disodium succinate (FW = 162 g/mol) must be added to 1 L of water to produce a solution with a pH 5.28 and a total solute concentration of 100 mM? (Assume the total volume remains 1 liter, answer in grams monosodium succinate, grams disodium succinate, respectively.)

9.7 g, 4.9 g

Intracellular fluids and the fluids surrounding cells in multicellular organisms are full of dissolved substances, including nucleotides, amino acids, proteins and ions. The total concentration of substances determines the colligative properties of a fluid. Osmosis is one of several such colligative properties. a. Give the definition of osmosis. b. Eukaryotic cells are aqueous solutions surrounded by semipermeable membranes. Consequently, incubation of a cell in a solution of lower osmotic pressure would cause the cell to swell up and burst. Discuss two solutions that have developed during evolution to solve this problem.

: a. Osmosis is the movement of solvent (water) through a semipermeable membrane from a region of high concentration to a region of low concentration (with concentration referring to the concentration of water). b. Plant cells are surrounded by a rigid cell wall that prevents the cell from expanding and therefore water from flowing into the cell. Animals surround their cells with a solution that has the same osmotic pressure as is found inside their cells. As a consequence there is no flow of water into or out of these cells.

A graduate student at SDSU wants to measure the activity of a particular enzyme at pH 4.0. To buffer her reaction, she will use a buffer system based on one of the acids listed below, which acid is most appropriate for the experiment? A) Formic acid (Ka 1.78 × 10−4) B) MES (Ka 8.13 × 10−7) C) PIPES (Ka 1.74 × 10−7) D) Tris (Ka 8.32 × 10−9) E) Piperidine (Ka 7.58 × 10−12)

A) Formic acid (Ka 1.78 × 10^−4)

Which of the following statements about water is incorrect? A) It is a small, polar molecule with a low dielectric constant. B) It has a marked dipole moment. C) It is largely hydrogen bonded, although any single H-bond exists only for a very short period of time (~10-12 s ). D) Acid-base reactions are very fast due to the mobility of hydronium ions in water which is a consequence of the ability of individual protons to "jump" from one water molecule to another. E) It has a bent geometry with each O─H bond approximately 0.958 Å long and with an O─H bond energy of approximately 460 kJ/mol.

A) It is a small, polar molecule with a low dielectric constant.

Which of the following statements about hydrophobic interactions is not true? A) They are caused by hydrophobic molecules interacting strongly with each other. B) They are the driving force for micelle formation. C) When nonpolar molecules come in contact with water, a highly-ordered shell of water molecules forms at the interface between the nonpolar molecules and water. A hydrophobic interaction is caused by the desire of water molecules to regain the entropy lost during this organization around the nonpolar substance by excluding the substance from interaction with water molecules. D) They are entropy driven. E) They are the main driving force for protein folding into three dimensional structures.

A) They are caused by hydrophobic molecules interacting strongly with each other.

Rank the following interactions in order of increasing strength (start with the weakest interaction). A) ionic interactions, hydrogen bonds, London dispersion forces, covalent bonds B) London dispersion forces, hydrogen bonds, ionic interactions, covalent bonds C) London dispersion forces, ionic interactions, hydrogen bonds, covalent bonds D) covalent bonds, London dispersion forces, ionic interactions, hydrogen bonds E) hydrogen bonds, London dispersion forces, ionic interactions, covalent bonds

B) London dispersion forces, hydrogen bonds, ionic interactions, covalent bonds

Which of the following statements about water is not true? A) It has a high dielectric constant. B) It dissolves salts and polar substances. C) It can form two hydrogen bonds per water molecule. D) It packs in a hexagonal (honeycomb) shaped lattice when the temperature falls below 0°C. E) In the liquid state it is only 15% less hydrogen bonded than in the solid state at 0°C.

C) It can form two hydrogen bonds per water molecule.

Which of the following statements about water is not true? A) The electron-rich oxygen atom of one water molecule can interact with the electron-poor proton on another water molecule to form a hydrogen bond. B) Liquid water is only 15% less hydrogen bonded than ice. C) Water is a nonpolar molecule that with a bent molecular geometry. D) Water can form highly ordered, cage-like, structures around nonpolar molecules. E) Water is a key player in the energetics of hydrophobic interactions.

C) Water is a nonpolar molecule that with a bent molecular geometry.

You mix equal volumes of 0.05 M NaH2PO4 and 0.05 M Na2HPO4 (pK's for phosphoric acid are 2.15, 6.82 and 12.38). Which of the following best describes the resulting solution? A) pH 2.15 and poorly buffered B) pH 2.15 and well buffered C) pH 6.82 and well buffered D) pH 12.38 and well buffered E) pH 6.82 and poorly buffered

C) pH 6.82 and well buffered

Which of the following statements about water is incorrect? A) Water is an excellent solvent for polar molecules. B) Pure water has a concentration of approximately 55.5 M. C) Cations are solvated by shells of water molecules oriented with their hydrogen atoms pointed toward the ions. D) Nonpolar molecules do not dissolve in water, but form a separate phase. E) Amphiphilic detergents often form micelles with the polar groups on the outside exposed to water (solvent) and the nonpolar groups sequestered in the interior.

Cations are solvated by shells of water molecules oriented with their hydrogen atoms pointed toward the ions.

Ice A) is a crystal of water molecules packed in an open structure stabilized by hydrogen bonds. B) is less dense than liquid water. C) contains 17% more hydrogen bonds then water. D) All of the statements above are true. E) None of the statements above are true.

D) All of the statements above are true.

Which of the following is the best explanation for the hydrophobic effect? A) It is caused by an affinity of hydrophobic groups for each other. B) It is caused by the affinity of water for hydrophobic groups. C) It is an entropic effect, caused by the desire of water molecules to increase their entropy by forming highly ordered structures around the hydrophobic groups. D) It is an entropic effect, caused by the desire of water molecules to increase their entropy by excluding hydrophobic groups, which they must otherwise surround with highly ordered structures. E) It is an entropic effect caused by the desire of hydrophobic groups to increase their entropy by associating with other hydrophobic groups.

D) It is an entropic effect, caused by the desire of water molecules to increase their entropy by excluding hydrophobic groups, which they must otherwise surround with highly ordered structures.

The capacity of a buffer to resist changes in pH upon addition of protons or hydroxide ions depends on A) the pKa of the weak acid in the buffer. B) the pH of the buffer. C) the total concentration of the weak acid and its conjugate base in the buffer. D) all of the above E) none of the above

D) all of the above

Phosphate (pK1 = 2.15, pK2 = 6.82, and pK3 = 12.38) will be mostly in the HPO42- form at pH 7.2. At pH 5.82 it is mostly in the ______ form.

H2PO4-

A phosphate buffer solution at pH = pK1 = 2.15 would have equal amounts of phosphate in the ______ form and the H2PO4- form.

H3PO4

Phosphoric acid is a polyprotic acid, with pK values of 2.14, 6.86, and 12.38. Which ionic form predominates at pH 9.3?

HPO4^2−

The hydrophobic effect is an important driving force for protein folding and for the assembly of molecules into cellular structures. a. Give the definition of the hydrophobic effect b. What are amphiphilic (amphipathic) molecules? c. Which cellular structures are composed of many amphipathic molecules that are driven together under the influence of the hydrophobic effect?

a. It is defined as the tendency of water to minimize contact with hydrophobic substances. b. Molecules with both polar and nonpolar regions c. Cellular membranes meet these criteria.

Biological processes can be best understood in the context of water. a. What effect does water have on the noncovalent interactions between either charged or polar groups/molecules? b. Why is this effect important with respect to biochemical processes?

a. Water reduces the strength of those interactions. b. Water reduces the strength of polar and ionic interactions. This makes those interactions reversible and life is based on reversible interaction between biomolecules.

A solution containing a weak acid (pK = 7.5) and its conjugate base at pH of 8.5 has a good capacity to buffer the addition of ______.

acid

Fatty acids such as palmitate and oleate are usually characterized as

amphiphilic

Hydrogen bonds within liquid water

are dipole-dipole attractions.

Weak acids

are only partially ionized in an aqueous solution.

The pH of a 0.1 M solution of sodium acetate would be

basic, because of the acetate ion reacts with water to form acetic acid and OH-.

Sheets composed of two layers of amphipathic molecules arranged with the hydrophilic groups on the surface and the hydrophobic groups buried in the center that form in water are called

bilayer membranes.

Physical properties that depend on the amounts of various species, rather than the identities of those species, are called

colligative properties.

Octane molecules dispersed in water tend to aggregate because that allows water molecules to be more ______.

disordered

The pH at the midpoint in the titration of an acid with a base is

equal to the pK of the corresponding acid.

The polarity of the O-H bond is caused by the ______ of oxygen relative to that of hydrogen.

higher electronegativity

For the ______ represented by D-H....A, the donor D is weakly acidic and the acceptor A is weakly basic.

hydrogen bond

The strongest noncovalent interactions are

ionic interactions.

Globules of up to several thousand amphiphilic molecules arranged with the hydrophilic groups on the surface and the hydrophobic groups buried in the center that form in water are called

micelles.

The insolubility of nonpolar molecules in water is due to the large ______, which is the result of water molecules forming an ordered network surrounding nonpolar molecules.

negative entropy

A strong acid is completely ionized in water, whereas a weak acid is ______.

only partially ionized

In the energetics of transferring hydrocarbons from water to nonpolar solvents, the factor TS is commonly

positive

Hydrophobic interactions between nonpolar molecules

require the presence of surrounding water molecules.

Translational and ______ thermal motion causes liquid water molecules to reorient approximately every 10-12 seconds.

rotational

The 104.5° bond angle in the water molecule is the result of the ______ of electron orbitals around oxygen.

tetrahedral arrangement

The boiling point of water is 264°C higher than the boiling point of methane because

water molecules are connected to each other by H-bonds.