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

Previous problemNext problem

4:53 minutes

Problem 42a

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

Verified step by step guidance

Determine the relationship between pKa and pH using the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{Base}]}{[\text{Acid}]} \right) \). Here, the base is the neutral amine (RNH2), and the acid is the protonated amine (RN+H3).

Rearrange the equation to solve for the ratio of base to acid: \( \frac{[\text{Base}]}{[\text{Acid}]} = 10^{(\text{pH} - \text{pKa})} \).

Substitute the given pKa value (9) and the pH values from Problem 41 into the equation to calculate the ratio of neutral (base) to charged (acid) molecules for each pH value.

Interpret the ratio: If the ratio \( \frac{[\text{Base}]}{[\text{Acid}]} \) is greater than 1, there are more neutral molecules (RNH2). If the ratio is less than 1, there are more charged molecules (RN+H3).

For each pH value, compare the pH to the pKa: If \( \text{pH} > \text{pKa} \), the solution favors the base (neutral form). If \( \text{pH} < \text{pKa} \), the solution favors the acid (charged form).

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above

Video duration:

Play a video:

Was this helpful?

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 pH at which half of the species are protonated and half are deprotonated. A lower pKa means a stronger acid, while a higher pKa indicates a weaker acid. In this case, the pKa of 9 suggests that at pH values below 9, the amine will predominantly exist in its protonated form (charged), while at pH values above 9, it will be more neutral.

Protonation of Amines

Amines can exist in both protonated (charged) and deprotonated (neutral) forms. The protonated form (RN+H3) occurs when the amine accepts a proton (H+), making it positively charged. The equilibrium between these forms is influenced by the solution's pH relative to the amine's pKa, determining the predominant species in solution.

Acid-Base Equilibrium

Acid-base equilibrium describes the balance between protonated and deprotonated species in a solution. The Henderson-Hasselbalch equation can be used to predict the ratio of these species based on pH and pKa. Understanding this equilibrium is crucial for determining whether more charged or neutral molecules of the amine will be present at specific pH levels.

Watch next

Master Why we use pKa instead of pH. with a bite sized video explanation from Johnny

Start learning

Related Videos

Related Practice

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.

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

Show more practices