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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8:03 minutes

Problem 76a

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?

Verified step by step guidance

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

Rearrange the Henderson-Hasselbalch equation to solve for the ratio \( \frac{[A^-]}{[HA]} \): \( \frac{[A^-]}{[HA]} = 10^{\text{pH} - \text{pKa}} \).

Substitute the given values for pH (5.7) and pKa (5.3) into the equation: \( \frac{[A^-]}{[HA]} = 10^{5.7 - 5.3} \).

Once the ratio \( \frac{[A^-]}{[HA]} \) is determined, calculate the fraction of the acid in its acidic form (HA) using the formula: \( \text{Fraction of HA} = \frac{1}{1 + \frac{[A^-]}{[HA]}} \).

Convert the fraction of HA into a percentage by multiplying by 100: \( \text{Percentage of HA} = \text{Fraction of HA} \times 100 \).

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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 of an acid is a measure of its strength, indicating the pH at which half of the acid is dissociated. A lower pKa value means a stronger acid. The pH of a solution reflects the concentration of hydrogen ions; when pH is higher than pKa, the acid is more likely to be in its deprotonated (conjugate base) form.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates pH, pKa, and the ratio of the concentrations of the deprotonated and protonated forms of an acid. It is expressed as pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the acid. This equation is essential for calculating the proportions of the acid in different forms at a given pH.

Percentage of Acidic Form

To determine the percentage of an acid present in its acidic form, one can use the ratio derived from the Henderson-Hasselbalch equation. By calculating the ratio of [HA] to the total concentration of acid ([HA] + [A-]), the percentage of the acid in its protonated form can be expressed as ( [HA] / ([HA] + [A-]) ) × 100%. This calculation is crucial for understanding the acid's behavior in solution.

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Related Practice

Textbook Question

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

b. CH₃CH₂N⁺H₃ (pK_a= 11.0)

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

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)

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.

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

Key Concepts

pKa and pH Relationship

Henderson-Hasselbalch Equation

Percentage of Acidic Form

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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?