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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6:19 minutes

Problem 76b

Textbook Question

At what pH does 80% of the acid exist in its acidic form?

Verified step by step guidance

Identify the relationship between the acidic form (HA) and its conjugate base (A⁻) using the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{[A^-]}{[HA]} \right) \).

Determine the ratio of \([A^-]\) to \([HA]\) based on the problem statement. If 80% of the acid exists in its acidic form (HA), then \([HA] = 0.8\) and \([A^-] = 0.2\). The ratio \( \frac{[A^-]}{[HA]} \) is \( \frac{0.2}{0.8} = 0.25 \).

Substitute the ratio \( \frac{[A^-]}{[HA]} = 0.25 \) into the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log(0.25) \).

Simplify \( \log(0.25) \) using logarithmic properties: \( \log(0.25) = \log(1/4) = \log(1) - \log(4) = 0 - \log(4) = -\log(4) \).

Combine the simplified logarithmic term with the \( \text{pKa} \) to express the pH: \( \text{pH} = \text{pKa} - \log(4) \). This is the pH at which 80% of the acid exists in its acidic form.

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Key Concepts

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

Acid-Base Equilibrium

Acid-base equilibrium refers to the balance between the concentrations of an acid and its conjugate base in a solution. This equilibrium is influenced by the pH of the solution, which determines the degree of ionization of the acid. Understanding this concept is crucial for determining the proportions of acidic and basic forms of a substance at a given pH.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a mathematical formula that relates the pH of a solution to the pKa of an acid and the ratio of the concentrations of its deprotonated (base) and protonated (acid) forms. It is expressed as pH = pKa + log([A-]/[HA]). This equation is essential for calculating the pH at which a specific percentage of an acid exists in its acidic form.

Percent Ionization

Percent ionization is a measure of the extent to which an acid dissociates into its ions in solution. It is calculated as the ratio of the concentration of ionized acid to the initial concentration of the acid, multiplied by 100. Knowing the percent ionization helps in understanding how much of the acid exists in its acidic form versus its ionized form at a specific pH.

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Textbook Question

Given that the indicated pK_a values correspond to the acid dissociation reactions shown, calculate the ratio of acid to conjugate base for the reactions shown.

(a)

Textbook Question

Using pK_a values, calculate an approximate K_eq value for the following substitution reaction.

Textbook Question

Amino acids exist predominantly in one of the following forms. Which is it? Explain your answer.

Textbook Question

At what pH is the concentration of a compound, with a pK_a = 4.6, 10 times greater in its basic form than in its acidic form?

Textbook Question

The pK_a of a protonated alcohol is about -2.5, and the pK_a of an alcohol is about 15. Therefore, as long as the pH of the solution is greater than _______ and less than _______, more than 50% of 2-propanol (the product of the reaction on p. 244) will be in its neutral, nonprotonated form.

Textbook Question

The pK_a of protonated acetone is about –7.5 and the pK_a of protonated hydroxylamine is 6.0.

a. In a reaction with hydroxylamine at pH 4.5 (Figure 16.2), what fraction of acetone is present in its acidic, protonated form?

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Textbook Question

The pK_a of acetic acid in water is 4.76. What effect will a decrease in the polarity of the solvent have on the pK_a? Why?

Textbook Question

Calculate the equilibrium constant for the acid–base reaction between the reactants in each of the following pairs:

c. CH₃NH₂+ H₂O

d. CH₃N⁺H₃ + H2O

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