Question 1

Which of the following acids or bases is least likely to be encountered in a biochemical setting?
A)nitric acid
B)acetic acid
C)ammonia
D)citric acid
E)phosphoric acid

Accepted Answer

Nitric acid is a strong mineral acid and is unlikely to be encountered in a biochemical setting. The other options are organic acids or bases commonly found in biochemical reactions or processes.

Question 2

A typical amino acid has a carboxylic acid and an amine with pK_a values of 2.3 and 9.6, respectively. In a solution of pH 4.5, which of the following best describes the protonation and charge state of the amino acid? A)carboxylic acid: protonated and neutral; amine: deprotonated and negative B)carboxylic acid: protonated and neutral; amine: protonated and neutral C)carboxylic acid: deprotonated and negative; amine: protonated and neutral D)carboxylic acid: deprotonated and negative; amine: protonated and positive E)carboxylic acid: deprotonated and negative; amine: deprotonated and neutral

Accepted Answer

D

Question 3

Formic acid is the active agent in an ant bite. What is the ratio of base/acid for formic acid (pK_a 3.9)in the blood stream at pH 7.4? A)3.16 * 10^-4 B)3.16 * 10³ C)3.5 D)0.54 E)1.90

Accepted Answer

The ratio of base/acid for formic acid can be calculated using the Henderson-Hasselbalch equation:

pH = pKa + log([A-]/[HA]), where pKa is the acid dissociation constant and [A-]/[HA] is the ratio of the conjugate base to the acid.

Substituting the values gives:

7.4 = 3.9 + log([A-]/[HA])

3.5 = log([A-]/[HA])

[A-]/[HA] = 10^(3.5) = 3.16 * 10^3

Therefore, the ratio of base/acid for formic acid in the blood stream at pH 7.4 is 3.16 * 10^3 : 1, or approximately 3,160 : 1.

Option B is the closest value to the calculated ratio.

Question 4

Which of the following is the most likely noncovalent interaction observed between a carboxylic acid and an alcohol?
A)charge-charge interaction
B)charge-dipole interaction
C)dipole-dipole interaction
D)dipole-induced dipole interaction
E)formation of an ester bond

Accepted Answer

The answer of Which of the following is the most...

Question 5

Which of the following would likely form micelles in an aqueous solution?
A)hexane
B)glucose
C)glutamic acid
D)dodecanoic acid
E)none of the above

Accepted Answer

Dodecanoic acid is a long-chain fatty acid, which tends to be amphipathic (having both hydrophobic and hydrophilic properties). In an aqueous solution, dodecanoic acid molecules can aggregate to form micelles, with the hydrophobic tails oriented inward and the hydrophilic heads oriented outward towards the water. The other molecules listed (hexane, glucose, and glutamic acid) do not possess this amphipathic nature and therefore would not form micelles.

Question 6

Lysine contains two amine groups (pK_a values of 9.0 and 10.0)and a carboxylic acid group (pK_a 2.2). In a solution of pH 9.5, which of the following would best describe the protonation and charge state of lysine? A)carboxylic acid: deprotonated and neutral; both amines: protonated and positive B)carboxylic acid: deprotonated and negative; both amines: deprotonated and neutral C)carboxylic acid: deprotonated and negative; amine (pK_a 9.0): deprotonated and neutral; amine (pK_a 10.0): protonated and positive D)carboxylic acid: deprotonated and negative; amine (pK_a 9.0): protonated and positive; amine (pK_a 10.0): deprotonated and neutral E)carboxylic acid: deprotonated and neutral; amine (pK_a 9.0): deprotonated and neutral; amine (pK_a 10.0): protonated and positive

Accepted Answer

At pH 9.5, which is above the pKa of the carboxylic acid group but below the pKa of the first amine group (9.0), the carboxylic acid group will be deprotonated and negatively charged, while the first amine group will be deprotonated and neutral. However, the second amine group has a pKa of 10.0, which is above the pH of the solution, meaning that it will be protonated and positively charged. Therefore, option C is the best choice.

Question 7

Glutamic acid contains two carboxylic acid groups (pK_a values of 2.2 and 4.2)and an amine group (pK_a 9.7). What is the pI for glutamic acid? A)3.2 B)6.0 C)6.5 D)7.0 E)none of the above

Accepted Answer

The isoelectric point (pI) is calculated by averaging the pKa values of the two acidic groups that lose protons first. For glutamic acid, these are the two carboxylic acid groups with pKa values of 2.2 and 4.2. The pI is (2.2 + 4.2) / 2 = 3.2.

Question 8

Which of the following best explains the hydrogen bonding that occurs in water?
A)the average number of H-bonds formed by one water molecule is the same in liquid and solid water, the only difference is the duration of the H-bond
B)the number of H-bonds formed by one water molecule is greater in liquid water than in solid water
C)the structure of liquid water is best described as flickering clusters of H-bonds due to the relative short duration of individual H-bonds
D)each water molecule is capable of forming 8 H-bonds: 2 from each lone pair of electrons and 2 from each hydrogen
E)none of the above

Accepted Answer

The structure of liquid water is not a regular lattice like in solid water, but rather clusters that form and break apart due to the relative short duration of individual H-bonds. This is because the hydrogen bonding in water is constantly forming and re-forming, which gives water its unique chemical properties.

Question 9

Citric acid is a triprotic acid with three carboxylic acid groups having pK_a values of 3.1, 4.8, and 6.4. If a solution of citric acid has a pH of 5.5, what can be said about the predominant protonation state of the citric acid? A)1 carboxylic acid group is deprotonated, 2 are protonated B)2 carboxylic acid groups are deprotonated, 1 is protonated C)all 3 carboxylic acid groups are deprotonated D)all 3 carboxylic acid groups are protonated E)the protonation state cannot be determined

Accepted Answer

The pH of the solution is 5.5, which is between the second and third pKa values of citric acid (4.8 and 6.4). This means that two of the carboxylic acid groups are likely deprotonated (since the pH is above their pKa values), and one remains protonated (since the pH is below its pKa value).

Question 10

Given the pK_a values for phosphoric acid of 2.14, 6.86 and 12.4, what is the ratio of HPO₄^2-/H₂PO₄^- in a typical muscle cell where the pH is 7.2? A)0.46 B)2.2 C)6.3 * 10^-6 D)1.1 * 10⁵ E)none of the above

Accepted Answer

At pH 7.2, the dominant species of phosphoric acid are HPO4^2- and H2PO4^-. The ratio of these species is given by the Henderson-Hasselbalch equation: ratio = [HPO4^2-]/[H2PO4^-] = 10^(pH-pKa2), where pKa2 = 6.86 for phosphoric acid. Thus, ratio = 10^(7.2-6.86) = 10^0.34 ≈ 2.2.

Question 11

What pH range is generally considered to be the physiological pH range?
A)1.5-3.0
B)4.5-7.0
C)5.5-9.0
D)6.5-8.0
E)7.5-10.0

Accepted Answer

The physiological pH range is generally considered to be between 6.5-8.0, which is slightly acidic to slightly basic. This range allows for optimal function of enzymes and other biochemical processes in the body. A pH lower than 6.5 or higher than 8.0 can lead to health problems and disrupt physiological processes.

Question 12

Which of the following represents the breaking of a noncovalent interaction?
A)hydrolysis of an ester
B)dissolving of salt crystals
C)ionization of water
D)decomposition of hydrogen peroxide
E)none of the above

Accepted Answer

Noncovalent interactions are weaker than covalent bonds, so they can be broken relatively easily. Dissolving salt crystals involves breaking the ionic bonds between the positive and negative ions, which is a noncovalent interaction. Hydrolysis of an ester involves breaking a covalent bond, while ionization of water and decomposition of hydrogen peroxide involve breaking chemical bonds but not necessarily noncovalent interactions.

Question 13

In the following titration curve, what does the inflection point represent? A)pH of solution equals pK_a of weak acid B)concentration of weak acid and conjugate base are equal C)the pH where the solution would function most effectively as a buffer D)the weak acid is 50% protonated, 50% deprotonated E)all of the above

Accepted Answer

The answer of In the following titration curve, what does...

Question 14

Which of the following is the conjugate acid of hydrogen phosphate, HPO₄²^-? A)H₂PO₃^- B)H₂PO₄^- C)H₃PO₄ D)H₂PO₄²^- E)none of the above

Accepted Answer

The conjugate acid of hydrogen phosphate (HPO4^2-) is the species that forms when it gains a hydrogen ion (H+). This results in the formation of dihydrogen phosphate (H2PO4^-). The correct formula for dihydrogen phosphate is H₂PO₄^-, which matches option B. Option A has an incorrect charge on the phosphate ion, option C is missing a hydrogen ion, and option D includes additional phosphate ions. None of the options include the correct charge balance for the species.

Question 15

Which of the following atoms could interact through a hydrogen bond?
A)the oxygen of a ketone and the hydrogen of an aldehyde
B)the oxygen of methanol and a hydrogen on the methyl carbon of methanol
C)the hydrogen of an amine and the oxygen of an alcohol
D)the hydrogen on an aromatic ring and the oxygen of carbon dioxide
E)none of the above

Accepted Answer

Hydrogen bonding occurs between a hydrogen atom bonded to an electronegative atom (such as nitrogen, oxygen, or fluorine) and another electronegative atom with a lone pair of electrons. In choice C, the hydrogen on the amine (nitrogen) can form a hydrogen bond with the oxygen of an alcohol. Therefore, choice C is the best choice for atoms that can interact through a hydrogen bond.

Question 16

If a buffer is made with the pH below the pK_a of the weak acid, the ratio of [base]/[acid] will be: A)less than 1 B)greater than 1 C)equal to 1 D)equal to 0 E)cannot be determined

Accepted Answer

If the pH is lower than the pKa of the weak acid, it means that the concentration of H+ ions is higher than the concentration of the weak acid molecules. This results in the partial protonation of the weak acid, reducing the number of free acid molecules. Therefore, the [base]/[acid] ratio is smaller than 1, leading to the best choice being A.

Question 17

Since pK_a = -log K_a, which of the following is a correct statement? A)acetic acid (pK_a = 4.7)is stronger than lactic acid, (pK_a = 3.9) B)lactic acid, (pK_a = 3.9)is weaker than all forms of phosphoric acid, (pK_a = 2.1, 6.9 and 12.4) C)since the pK_a for conversion of the ammonium ion to ammonia is 9.25, ammonia is a weaker base then the acetate ion D)for carbonic acid with pK_a values of 6.3 and 10.3, the bicarbonate ion is a stronger base than the carbonate ion E)none of the above

Accepted Answer

The pK_a value (often referred to as pKa in the context of acids) is a measure of the strength of an acid; the lower the pKa, the stronger the acid. Therefore, the statements given are incorrect based on the pKa values provided. For example, a lower pKa value for lactic acid compared to acetic acid means lactic acid is actually stronger, not weaker. Similarly, the comparisons made in the other options are also based on misunderstandings of how pKa values reflect acid or base strength.

Question 18

Given the structure of a glucose molecule, which of the following explains the hydrogen bonding between glucose and water? 
A)H-bonds will form with glucose always being the H-bond donor
B)H-bonds will form with water always being the H-bond donor
C)each glucose molecule could H-bond with as many as 17 water molecules
D)due to the cyclic structure of glucose, H-bonding with water does not occur
E)none of the above

Accepted Answer

Glucose has multiple hydroxyl (OH) groups that can act as H-bond donors or acceptors. Each glucose molecule can participate in H-bonding with as many as 17 water molecules, leading to its solubility in water.

Question 19

You have been asked to determine the pK_a of an unknown acid. In a solution at pH 7.0, you find that 24% of the acid is in its deprotonated form. What is the pK_a of the acid? A)7.9 B)7.5 C)6.5 D)5.6 E)none of the above

Accepted Answer

To determine the pK_a, we can use the Henderson-Hasselbalch equation: pH = pK_a + log([A-]/[HA]) where [A-] is the concentration of the deprotonated form of the acid, and [HA] is the concentration of the protonated form. We know that at pH 7.0, 24% of the acid is in its deprotonated form, so [A-]/[HA] = 0.24/0.76 = 0.316. Substituting this into the equation and solving for pK_a, we get: 7.0 = pK_a + log(0.316) pK_a = 7.5 Therefore, the answer is B: 7.5.

Question 20

If gastric juice has a pH of about 1.5, which of the following would be predominantly deprotonated in the stomach? A)phenol (pK_a = 9.9) B)acetic acid (pK_a = 4.7) C)lactic acid, (pK_a = 3.9) D)phosphoric acid (pK_a = 2.1) E)hydrochloric acid (pK_a = -6)

Accepted Answer

Hydrochloric acid (HCl) has a pKa of -6, indicating it is a very strong acid and will be completely dissociated (deprotonated) in aqueous solutions, including the highly acidic environment of the stomach with a pH of about 1.5.

Quiz 2: The Matrix of Life: Weak Interactions in an Aqueous Environment

Which of the following acids or bases is least likely to be encountered in a biochemical setting?

A typical amino acid has a carboxylic acid and an amine with pK_a values of 2.3 and 9.6, respectively. In a solution of pH 4.5, which of the following best describes the protonation and charge state of the amino acid?

Formic acid is the active agent in an ant bite. What is the ratio of base/acid for formic acid (pK_a 3.9) in the blood stream at pH 7.4?

Which of the following is the most likely noncovalent interaction observed between a carboxylic acid and an alcohol?

Which of the following would likely form micelles in an aqueous solution?

Lysine contains two amine groups (pK_a values of 9.0 and 10.0) and a carboxylic acid group (pK_a 2.2) . In a solution of pH 9.5, which of the following would best describe the protonation and charge state of lysine?

Glutamic acid contains two carboxylic acid groups (pK_a values of 2.2 and 4.2) and an amine group (pK_a 9.7) . What is the pI for glutamic acid?

Which of the following best explains the hydrogen bonding that occurs in water?

Citric acid is a triprotic acid with three carboxylic acid groups having pK_a values of 3.1, 4.8, and 6.4. If a solution of citric acid has a pH of 5.5, what can be said about the predominant protonation state of the citric acid?

Given the pK_a values for phosphoric acid of 2.14, 6.86 and 12.4, what is the ratio of HPO₄^2-/H₂PO₄^- in a typical muscle cell where the pH is 7.2?

What pH range is generally considered to be the physiological pH range?

Which of the following represents the breaking of a noncovalent interaction?

In the following titration curve, what does the inflection point represent?

Which of the following is the conjugate acid of hydrogen phosphate, HPO₄²^-?

Which of the following atoms could interact through a hydrogen bond?

If a buffer is made with the pH below the pK_a of the weak acid, the ratio of [base]/[acid] will be:

Since pK_a = -log K_a, which of the following is a correct statement?

Given the structure of a glucose molecule, which of the following explains the hydrogen bonding between glucose and water?

You have been asked to determine the pK_a of an unknown acid. In a solution at pH 7.0, you find that 24% of the acid is in its deprotonated form. What is the pK_a of the acid?

If gastric juice has a pH of about 1.5, which of the following would be predominantly deprotonated in the stomach?

Quiz 2: The Matrix of Life: Weak Interactions in an Aqueous Environment

Which of the following acids or bases is least likely to be encountered in a biochemical setting?

A typical amino acid has a carboxylic acid and an amine with pKa values of 2.3 and 9.6, respectively. In a solution of pH 4.5, which of the following best describes the protonation and charge state of the amino acid?

Formic acid is the active agent in an ant bite. What is the ratio of base/acid for formic acid (pKa 3.9) in the blood stream at pH 7.4?