Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

3. Acids and Bases

Equilibrium Constant

Organic Chemistry

3. Acids and Bases

Equilibrium Constant

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5:09 minutes

Problem 42b(3,4,5)

Textbook Question

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.
3. pH = 7
4. pH = 10
5. pH = 13

Verified step by step guidance

Step 1: Recall the relationship between pKa, pH, and the ratio of charged to neutral species. The Henderson-Hasselbalch equation is used:

pH = pKa + \log (\frac{[A⁻]}{[HA]})

, where [A⁻] is the concentration of the deprotonated (charged) form and [HA] is the concentration of the neutral form.

Step 2: For each pH value, calculate the ratio of [A⁻] to [HA] using the equation:

\frac{[A⁻]}{[HA]} = 10^{pH - pKa}

. Substitute the given pKa value of 15 and the respective pH values (7, 10.5, and 13) into the equation.

Step 3: Analyze the ratio for each pH value. If the ratio

\frac{[A⁻]}{[HA]}

is greater than 1, the solution has more charged molecules ([A⁻]). If the ratio is less than 1, the solution has more neutral molecules ([HA]). If the ratio equals 1, the concentrations of charged and neutral molecules are equal.

Step 4: For pH = 7, calculate

= 10^{7 - 15}

. Since the pH is much lower than the pKa, expect more neutral molecules ([HA]).

Step 5: For pH = 10.5 and pH = 13, repeat the calculation using

= 10^{10.5 - 15}

and

= 10^{13 - 15}

. Compare the results to determine whether the solution has more charged or neutral molecules at these pH values.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pKa and pH Relationship

The pKa value of a compound indicates the acidity of that compound, with lower pKa values corresponding to stronger acids. In a solution, the pH reflects the concentration of hydrogen ions (H+). When the pH is lower than the pKa, the compound is more likely to be protonated (neutral), while at a higher pH, it tends to lose a proton and become deprotonated (charged).

Protonation and Deprotonation

Protonation refers to the addition of a proton (H+) to a molecule, which can change its charge state. For alcohols, when the pH is lower than the pKa, the alcohol remains mostly in its neutral form (ROH). Conversely, at higher pH levels, the alcohol can lose a proton, resulting in a negatively charged alkoxide ion (RO-), thus affecting the balance between charged and neutral species in solution.

Equilibrium in Acid-Base Chemistry

In acid-base chemistry, equilibrium describes the balance between protonated and deprotonated forms of a compound in solution. The Henderson-Hasselbalch equation can be used to predict the ratio of charged to neutral species based on pH and pKa. Understanding this equilibrium is crucial for determining the predominant form of the alcohol at different pH levels, which directly influences its behavior in chemical reactions.

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Related Practice

Textbook Question

For each of the following compounds, indicate the pH at which

a. 50% of the compound is in a form that possesses a charge.

1. CH₃CH₂COOH (pK_a= 4.9)

2. CH₃N⁺H₃ (pK_a= 10.7)

Textbook Question

For each of the following compounds, draw the form that predominates at pH = 3, pH = 6, pH = 10, and pH = 14:

a. CH₃COOH (pK_a= 4.8)

Textbook Question

For each of the following compounds (here shown in their acidic forms), write the form that predominates in a solution with a pH = 5.5:

e. ⁺NH4 (pK_a= 9.4)

f. HC≡N (pK_a= 9.1)

Textbook Question

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

1. pH = 1

2. pH = 3

3. pH = 5

Textbook Question

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pK_a values of the two groups are shown.

d. Draw the structure of alanine in a solution at pH = 12.

Textbook Question

For the following acid–base reaction, (c) calculate the ratio of butan-2-ol to 2-butoxide.

Textbook Question

What percent of imidazole is protonated at physiological pH (7.4)?

Textbook Question

For the following acid–base pairs, (iii) predict the favored side of equilibrium; (iv) calculate K_eq;

(h)

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Indicate whether an alcohol (ROH) with a pKa value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.3. pH = 74. pH = 105. pH = 13

Key Concepts

pKa and pH Relationship

Protonation and Deprotonation

Equilibrium in Acid-Base Chemistry

Watch next

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.
3. pH = 7
4. pH = 10
5. pH = 13