Question 1

Which of the following best explains why water has a much higher boiling point than ammonia ($$NH_3) or $$hydrogen sulfide ($$H_2S$$), despite their similar molecular weights?

Accepted Answer

Water forms extensive hydrogen bonds between molecules, which require more energy to break.

Question 2

At 25°C, the autoionization of water is described by the equation $$2H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + OH^-_{(aq)}. $$What is the concentration of $$H_3O^+ in $$pure water at this temperature?

Accepted Answer

$$1.0 	imes 10$$^{-7}$$ M

Question 3

Which of the following statements correctly distinguishes between pH and pKa?

Accepted Answer

pH describes the solution's proton concentration, while pKa describes a functional group's tendency to donate a proton.

Question 4

A solution contains acetic acid ($$pK_a = 4.76) $$and has a pH of 5.76. Using the Henderson-Hasselbalch equation, what is the ratio of acetate ($$CH_3COO^-) to $$acetic acid ($$CH_3COOH) in $$this solution?

Accepted Answer

10:1

Question 5

Which factor from the CARDIO framework explains why thiols ($$RSH$$, $$pK_a \approx 11) $$are more acidic than alcohols ($$ROH$$, $$pK_a \approx 16$$)?

Accepted Answer

The atom effect: sulfur is larger and less electronegative than oxygen, stabilizing the negative charge better.

Question 6

Citric acid contains three carboxylic acid groups with different $$pK_a$$ values. Why does the third deprotonation (to form the trianion) require a higher pH than the first?

Accepted Answer

Charge-charge repulsion between the accumulating negative charges makes further deprotonation less favorable.

Question 7

Which of the following best describes the structure and function of a phospholipid in biological membranes?

Accepted Answer

A phospholipid contains two fatty acids and a phosphate group attached to glycerol, forming an amphipathic molecule that self-assembles into bilayers.

Question 8

Which of the following statements about the difference between DNA and RNA at the nucleotide level is correct?

Accepted Answer

DNA contains deoxyribose lacking a 2'-OH group, while RNA contains ribose with a 2'-OH group.

Question 9

Which of the following best explains why cellulose is indigestible to humans, while starch is readily digested?

Accepted Answer

Cellulose contains β-1,4-glycosidic bonds, which human enzymes cannot hydrolyze, while starch contains α-1,4-glycosidic bonds, which human enzymes can hydrolyze.

Question 10

Which of the following correctly describes the difference between a nucleoside and a nucleotide?

Accepted Answer

A nucleoside contains a base and a sugar; a nucleotide contains a base, sugar, and one or more phosphate groups.

Question 11

Which of the following best describes the role of a catalyst in a chemical reaction?

Accepted Answer

A catalyst lowers the activation energy ($$\Delta G$$^{\ddagger}$$) of a reaction, increasing the rate without changing the overall free energy change ($$\Delta G$$).

Question 12

Which of the following functional groups is present in all amino acids at physiological pH?

Accepted Answer

A carboxylate anion ($$COO^-) $$and an ammonium cation ($$NH_3$$^+$$)

Question 13

Which of the following best explains why the β-anomer of D-glucopyranose is more abundant than the α-anomer in aqueous solution?

Accepted Answer

The β-anomer places all bulky substituents in equatorial positions in the chair conformation, minimizing steric strain.

Question 14

Which of the following statements about the acid-catalyzed formation of acetals from aldehydes is correct?

Accepted Answer

Acetal formation requires acid catalysis because only acid can protonate the hemiacetal intermediate, making water a good leaving group.

Question 15

Which of the following best describes the difference between pyridine and pyrrole as aromatic heterocycles?

Accepted Answer

Pyridine's nitrogen lone pair is in an sp² orbital and does not participate in aromaticity, making it basic; pyrrole's nitrogen lone pair is in a p orbital and is part of the aromatic π system, making it non-basic.

Question 16

Which of the following best explains why the hydrolysis of ATP to ADP and inorganic phosphate ($$P_i) is $$highly exergonic?

Accepted Answer

Hydrolysis relieves electrostatic repulsion between the negatively charged phosphate groups and increases resonance stabilization in the products.

Question 17

Which of the following best describes the structure of a glycosidic bond in a disaccharide?

Accepted Answer

A glycosidic bond is an ether linkage between the anomeric carbon of one monosaccharide and an oxygen atom of another molecule.

Question 18

Which of the following best explains why the rate of a reaction increases when the activation energy ($$\Delta G$$^{\ddagger}$$) decreases?

Accepted Answer

A lower activation energy means more molecules have sufficient energy to reach the transition state, increasing the rate constant ($$k).$$

Question 19

Which of the following is a correct statement about the isoelectric point ($$pI) of an $$amino acid?

Accepted Answer

The $$pI is $$the pH at which the amino acid has no net electric charge (zwitterion predominates).

Organic Chemistry II: Study Notes on Acidity, Biomolecules, and Core Mechanisms

Guida di studio - Domande di pratica

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