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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Problem 39i

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:
i. HON⁺H₃ (pK_a = 6.0)

Verified step by step guidance

Step 1: Understand the problem. The goal is to determine the predominant form of the compound HON+H3 in a solution with a pH of 5.5. This involves comparing the pH of the solution to the pKa of the compound to assess whether it exists primarily in its acidic or basic form.

Step 2: Recall the relationship between pH and pKa. The Henderson-Hasselbalch equation is key here:

pH = pKa + \log^{(} \frac{[A^-]}{[HA]})

. When pH is much higher than pKa, the compound exists predominantly in its deprotonated (basic) form. When pH is much lower than pKa, the compound exists predominantly in its protonated (acidic) form.

Step 3: Compare the pH of the solution (5.5) to the pKa of HON+H3 (−6.0). Since the pH is significantly higher than the pKa, this indicates that the compound will predominantly exist in its deprotonated form.

Step 4: Write the deprotonated form of HON+H3. Deprotonation involves the removal of a proton (H+). The resulting form will be HONH2, as the positively charged proton is removed.

Step 5: Confirm the reasoning. The large difference between the pH and pKa ensures that the equilibrium strongly favors the deprotonated form (HONH2) in a solution with a pH of 5.5.

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Key Concepts

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

pH and pKa Relationship

The pH of a solution indicates its acidity or basicity, with lower values being more acidic. The pKa is a measure of the strength of an acid; it is the pH at which half of the acid is dissociated. When pH is lower than pKa, the protonated form predominates, while at higher pH, the deprotonated form is favored. Understanding this relationship is crucial for predicting the predominant form of an acid in solution.

Acid-Base Equilibrium

Acid-base equilibrium describes the balance between an acid and its conjugate base in solution. The equilibrium can shift depending on the pH of the solution, which affects the concentration of protonated and deprotonated species. For a given acid, if the pH is above its pKa, the deprotonated form is favored, while if it is below, the protonated form predominates. This concept is essential for determining the predominant species at a specific pH.

Strong vs. Weak Acids

Strong acids completely dissociate in solution, while weak acids only partially dissociate. The given compound, HON+H3, has a very low pKa of -6.0, indicating it is a strong acid. In a solution with a pH of 5.5, which is much higher than the pKa, the strong acid will predominantly exist in its protonated form, as it remains largely dissociated even at higher pH levels. Recognizing the strength of the acid is vital for predicting its behavior in solution.

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Related Practice

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.

c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).

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:

a. CH₃COOH(pK_a= 4.76)

b. CH₃CH₂N⁺H₃ (pK_a= 11.0)

Textbook Question

At what pH is the concentration of a compound, with a pK_a = 8.4, 100 times greater in its acidic form than in its basic form? At what pH is 50% of a compound, with a pK_a = 7.3, in its basic form?

Textbook Question

If an acid with a pK_a of 5.3 is in an aqueous solution of pH 5.7, what percentage of the acid is present in its acidic form?

Textbook Question

Calculate K_eq for the following acid–base reaction.

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:

g. HNO₂ (pK_a= 3.4)

h. HNO₃ (pK_a= −1.3)

Textbook Question

As long as the pH is not less than ___________, at least 50% of a protonated amine with a pK_a value of 10.4 will be in its neutral, nonprotonated form.

Textbook Question

Indicate whether a protonated amine (RN⁺H₃) with a pKa value of 9 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

4. pH = 7

5. pH = 10

6. pH = 13

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