Questions 2 – Flashcards
Unlock all answers in this set
Unlock answersquestion
Which of these statements 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.
answer
D
question
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.
answer
A
question
Hydrophobic interactions make important energetic contributions to: A) binding of a hormone to its receptor protein. B) enzyme-substrate interactions. C) membrane structure. D) three-dimensional folding of a polypeptide chain. E) all of the above are true.
answer
E
question
Dissolved solutes alter some physical (colligative) properties of the solvent water because they change the: A) concentration of the water. B) hydrogen bonding of the water. C) ionic bonding of the water. D) pH of the water. E) temperature of the water.
answer
A
question
Osmosis is movement of a: A) charged solute molecule (ion) across a membrane. B) gas molecule across a membrane. C) nonpolar solute molecule across a membrane. D) polar solute molecule across a membrane. E) water molecule across a membrane.
answer
E
question
A hydronium ion: A) has the structure H3O+. B) is a hydrated hydrogen ion. C) is a hydrated proton. D) is the usual form of one of the dissociation products of water in solution. E) all of the above are true.
answer
E
question
The pH of a solution of 1 M HCl is: A) 0 B) 0.1 C) 1 D) 10 E) -1
answer
A
question
The pH of a solution of 0.1 M NaOH is: A) 0.1 B) 1.0 C) 12.8 D) 13 E) 14
answer
D
question
Which of the following is true about the properties of aqueous solutions? A) A pH change from 5.0 to 6.0 reflects an increase in the hydroxide ion concentration ([OH-]) of 20%. B) A pH change from 8.0 to 6.0 reflects a decrease in the proton concentration ([H+]) by a factor of 100. C) Charged molecules are generally insoluble in water. D) Hydrogen bonds form readily in aqueous solutions. E) The pH can be calculated by adding 7 to the value of the pOH.
answer
D
question
The pH of a sample of blood is 7.4, while gastric juice is pH 1.4. The blood sample has: A) 0.189 times the [H+] as the gastric juice. B) 5.29 times lower [H+] than the gastric juice. C) 6 times lower [H+] than the gastric juice. D) 6,000 times lower [H+] than the gastric juice. E) a million times lower [H+] than the gastric juice.
answer
E
question
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.
answer
D
question
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.
answer
D
question
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) H2PO4-. C) HPO42-. D) PO43-. E) none of the above.
answer
B
question
Which of the following statements 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.
answer
E
question
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
answer
D
question
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.
answer
E
question
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.
answer
D
question
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
answer
C
question
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? A) pH of buffer 1 < pH of buffer 2 pH of buffer 2 > pH of buffer 3 D) The problem cannot be solved without knowing the value of pKa. E) None of the above.
answer
C
question
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.
answer
A
question
In which reaction below 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.
answer
E
question
Which of the following properties 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 is greater than the density of ice. E) The very low molecular weight of water.
answer
E
question
Name and briefly define four types of noncovalent interactions that occur between biological molecules.
answer
Hydrogen bonds: weak electrostatic attractions between one electronegative atom (such as oxygen or nitrogen) and a hydrogen atom covalently linked to a second electronegative atom; (2) electrostatic interactions: relatively weak charge-charge interactions (attractions of opposite charges, repulsions of like charges) between two ionized groups; (3) hydrophobic interactions: the forces that tend to bring two hydrophobic groups together, reducing the total area of the two groups that is exposed to surrounding molecules of the polar solvent (water); (4) van der Waals interactions: weak interactions between the electric dipoles that two close-spaced atoms induce in each other.
question
Explain the fact that ethanol (CH3CH2OH) is more soluble in water than is ethane (CH3CH3).
answer
Ethanol can form hydrogen bonds with water molecules, but ethane cannot. When ethanol dissolves, the decrease in the system's entropy that results from formation of ordered arrays of water around the CH3CH2- group is partly compensated by the favorable interactions (hydrogen bonds) of the hydroxyl group of ethanol with water molecules. Ethane cannot form such hydrogen bonds.
question
Explain the fact that triethylammonium chloride ((CH3CH2)3N•HCl) is more soluble in water than is triethylamine ((CH3CH2)3N).
answer
The positive charge on the N atom in triethylammonium chloride is more polar than the uncharged N atom in triethylamine. This increased polarity leads to stronger interactions with water, leading to increased solubility.
question
Explain with an appropriate diagram why amphipathic molecules tend to form micelles in water. What force drives micelle formation?
answer
Micelle formation minimizes the area of the hydrophobic part of amphipathic molecules that contacts the polar solvent, water. Hydrophobic interactions between hydrophobic moieties are the driving force for micelle formation. When amphipathic molecules form micelles in water, the entropy decrease due to the formation of ordered arrays of water molecules around the hydrophobic moieties is minimized. (See Fig. 2-7, p. 48.)
question
(a) Briefly define "isotonic," "hypotonic," and "hypertonic" solutions. (b) Describe what happens when a cell is placed in each of these types of solutions.
answer
a) An isotonic solution has the same osmolarity as the solution to which it is being compared. A hypotonic solution has a lower osmolarity than the solution to which it is being compared. A hypertonic solution has a higher osmolarity than the solution to which it is being compared. (b) Higher osmolarity results in osmotic pressure, which generally leads to movement of water across a membrane. In an isotonic solution, in which the osmolarity of the solution is the same as the cell cytoplasm, there will be no net water movement. In a hypotonic solution, water will move into the cell, causing the cell to swell and possibly burst. In a hypertonic solution, water will move out of the cell and it will shrink.
question
For each of the pairs below, circle the conjugate base. RCOOH RCOO- RNH2 RNH3+ H2PO4- H3PO4 H2CO3 HCO3-
answer
RCOO-, RNH2, H2PO4-, HCO3-
question
Phosphoric acid (H3PO4) has three dissociable protons, with the pKa's shown below. Which form of phosphoric acid predominates in a solution at pH 4? Explain your answer. Acid pKa H3PO4 2.14 H2PO4- 6.86 HPO42- 12.4
answer
At pH 4, the first dissociable proton (pKa = 2.14) has been titrated completely, and the second (pKa = 6.86) has just started to be titrated. The dominant form at pH 4 is therefore H2PO4-, the form with one dissociated proton (see Fig. 2-15).
question
Define pKa for a weak acid in the following two ways: (1) in relation to its acid dissociation constant, Ka, and (2) by reference to a titration curve for the weak acid.
answer
(1) pKa = -log Ka; (2) See Fig. 2-17, p. 59; pKa is the value of pH at the inflection point in a plot of pH vs. extent of titration of the weak acid. At the pKa, the concentration of ionized acid equals the concentration of un-ionized acid.
question
Give the general Henderson-Hasselbalch equation and sketch the plot it describes (pH against amount of NaOH added to a weak acid). On your curve label the pKa for the weak acid, and indicate the region in which the buffering capacity of the system is greatest.
answer
The inflection point, which occurs when the weak acid has been exactly one half titrated with NaOH, occurs at a pH equal to the pKa of the weak acid. The region of greatest buffering capacity (where the titration curve is flattest) occurs at pH values of pKa ±1. (See Fig. 2-17, p. 59.)
question
Draw the titration curve for a weak acid, HA, whose pKa is 3.2. Label the axes properly. Indicate with an arrow where on the curve the ratio of salt (A-) to acid (HA) is 3:1. What is the pH at this point?
answer
The plot of pH vs. added base should have the general shape of those shown in Fig. 2-17, p. 59, with the midpoint of the titration (inflection point) at pH 3.2. The ratio of A- to HA is 3 when 0.75 equivalents of base have been added. From the Henderson-Hasselbalch equation, the pH at this point can be calculated: pH = pKa + log = 3.2 + log 3 = 3.2 + 0.48 = 3.68
question
What is the pH of a solution containing 0.2 M acetic acid (pKa = 4.7) and 0.1 M sodium acetate?
answer
pH = pKa + log = 4.7 + log (0.1/0.2) = 4.7 - 0.3 = 4.4
question
You have just made a solution by combining 50 mL of a 0.1 M sodium acetate solution with 150 mL of 1 M acetic acid (pKa = 4.7). What is the pH of the resulting solution?
answer
pH = pKa + log = 4.7 + log (5/150) = 4.7 - 1.48 = 3.22
question
For a weak acid with a pKa of 6.0, show how you would calculate the ratio of acid to salt at pH 5.
answer
10
question
Suppose you have just added 100 mL of a solution containing 0.5 mol of acetic acid per liter to 400 mL of 0.5 M NaOH. What is the final pH? (The pKa of acetic acid is 4.7.)
answer
Addition of 200 mmol of NaOH (400 mL 0.5 M) to 50 mmol of acetic acid (100 mL 0.5 mM) completely titrates the acid so that it can no longer act as a buffer and leaves 150 mmol of NaOH dissolved in 500 mL, an [OH-] of 0.3 M. Given [OH-], [H+] can be calculated from the water constant: [H+][OH-] = 10-14 [H+] = 10-14 M2 / 0.3 M pH is, by definition, log (1/[H+]) pH = log (0.3 M /10-14 M2) = 12.48.
question
A weak acid HA, has a pKa of 5.0. If 1.0 mol of this acid and 0.1 mol of NaOH were dissolved in one liter of water, what would the final pH be?
answer
Combining 1 mol of weak acid with 0.1 mol of NaOH yields 0.9 mol of weak acid and 0.1 mol of salt. pH = pKa + log = 5.0 + log (0.1/0.9) = 4.05
question
Give an example of a biological reaction in which water participates as a reactant and a reaction in which it participates as a product.
answer
See p. 65, for examples such as condensation and hydrolysis reactions.
question
If ice were denser than water, how would that affect life on earth?
answer
Ice formed at the surface of bodies of water would sink, hence streams, ponds, lakes, and so on would freeze from the bottom up. With a reservoir of ice at the bottom they would be perpetually cold, and in the limit they would freeze solid, precluding life as we know it.
