Table of contents

1. A Review of General Chemistry(0)
2. Molecular Representations(0)
3. Acids and Bases(0)
4. Alkanes and Cycloalkanes(0)
5. Chirality(0)
6. Thermodynamics and Kinetics(0)
7. Substitution Reactions(0)
8. Elimination Reactions(0)
9. Alkenes and Alkynes(0)
10. Addition Reactions(0)
11. Radical Reactions(0)
12. Alcohols, Ethers, Epoxides and Thiols(0)
13. Alcohols and Carbonyl Compounds(0)
14. Synthetic Techniques(0)
15. Analytical Techniques:IR, NMR, Mass Spect(0)
16. Conjugated Systems(0)
17. Ultraviolet Spectroscopy(0)
18. Aromaticity(0)
19. Reactions of Aromatics: EAS and Beyond(0)
20. Phenols(0)
21. Aldehydes and Ketones: Nucleophilic Addition(0)
22. Carboxylic Acid Derivatives: NAS(0)
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides(0)
24. Enolate Chemistry: Reactions at the Alpha-Carbon(0)
25. Condensation Chemistry(0)
26. Amines(0)
27. Heterocycles(0)
28. Carbohydrates(0)
29. Amino Acids(0)
30. Peptides and Proteins(0)
32. Lipids (0)
34. Nucleic Acids(0)
35. Transition Metals(0)

3. Acids and Bases

pKa

3. Acids and Bases

pKa - Online Tutor, Practice Problems & Exam Prep

pKa Practice Problems

35 problems

1PRACTICE PROBLEM

Rank the following original compounds in order from strongest base to weakest base.

2PRACTICE PROBLEM

Ethaneperoxoic acid, C₂H₄O₃, has a pK_a value of 8.20. It is a weaker acid when compared to ethanoic acid, C₂H₄O₂ (pK_a = 4.74). Why is ethaneperoxoic acid weaker than ethanoic acid in terms of acidity?

3PRACTICE PROBLEM

Estimate the pK_a value of propionic acid, a food preservative.

4PRACTICE PROBLEM

Rank each compound in the following groups in order from weakest base to strongest base. (i) potassium benzoate, potassium ethoxide, and potassium phenoxide; and (ii) 2-methylpyridine, potassium ethoxide, and potassium acetate

5PRACTICE PROBLEM

Arrange the 1.00 M aqueous solutions of the acids listed below in order of increasing acidity.

HF pK_a = 3.17

HCN pK_a = 9.22

CH₄ pK_a = 50.0

6PRACTICE PROBLEM

Arrange the given compounds in increasing order of their basicity. Provide a suitable reason for your answer.

7PRACTICE PROBLEM

Using your knowledge of important pK_a values, estimate the pK_a value of the indicated proton and explain your choice.

8PRACTICE PROBLEM

Provide approximate pK_a values for the species given below.

a. CH₃CH₂NH₃⁺

b. CH₃COOH

9PRACTICE PROBLEM

Given the structure of 2-ethyl-2,5,5-trimethylpyrrolidin-1-ide, show its conjugate acid and choose the pK_a value from the table that could help predict the stability of the base.

10PRACTICE PROBLEM

For the given base, show the conjugate acid and select the pK_a value from the table that could aid in determining the stability of the base.

11PRACTICE PROBLEM

a. Two unknown acids, A and B, have pK_a values of 4.5 and 5.0, respectively. Determine which one is a stronger acid.
b. Similarly, two unknown acids, X and Y, correspond to the acid dissociation constants of 2.9 x 10⁻⁴ and 1.3 x 10⁻⁵, respectively. Determine which one is a stronger acid.

12PRACTICE PROBLEM

The pK_a value of chlorous acid, HClO₂, is 1.94. This makes it almost 290,000 times more acidic than hypochlorous acid, HClO, with a pK_a value of 7.40. Why is HClO₂ stronger than HClO in terms of acidity?

13PRACTICE PROBLEM

Estimate the pK_a value for the indicated proton using the given table. Rationalize your choice.

14PRACTICE PROBLEM

Give the conjugate acid of the following base. Estimate a pK_a value from the table in order to estimate its stability with accuracy.

15PRACTICE PROBLEM

Draw the conjugate acid for the base below and pick a pK_a value from the table below that would best estimate the given base's stability.

16PRACTICE PROBLEM

A student aims to use propyne (CH3C≡CH) for a coupling reaction. The first step is to deprotonate propyne. Which of the following bases can be used for this process?

17PRACTICE PROBLEM

Consider HCOOH and CH₃CH₂NH₂, placed in a water and diethyl ether mixture. Identify the pH of the water layer for the carboxylic acid to dissolve in the water layer and the amine in the diethyl ether layer. pK_a(HCOOH) = 3.8, pK_a(CH₃CH₂NH₃⁺) = 11.0.

18PRACTICE PROBLEM

Propionic acid, an agent that inhibits mold in baked products, has a pK_a value of 4.874. Calculate its K_a value. Is it a weaker acid than sorbic acid (K_a = 1.58×10⁻⁵)?

19PRACTICE PROBLEM

Identify the stronger base in each of the following pairs:

(i) C₆H₅COO^– or C₆H₁₁COO^– (pK_a C₆H₅COOH = 4.2; pK_a C₆H₁₁COOH = 4.9)

(ii) CH₃O^– or ^–NHCH₃ (pK_a CH₃OH = 15.5; pK_a CH₃NH₂ = 40)

(iii) CH₃CH₂OH or H₂O (pK_a CH₃CH₂O⁺H₂ = –2.4; pK_a H₃O⁺ = –1.7)

20PRACTICE PROBLEM

Arrange the following bases in order of decreasing strength:

Br^–, CH₃CH₂C≡C^–, ^–NHCH₃, CH₃CH₂O^–

21PRACTICE PROBLEM

For each of the following compounds, predict which atom will be protonated:

22PRACTICE PROBLEM

Refer to the organic compounds below and their pK_a values:

Identify the following:

a. The most acidic carboxylic acid.

b. The most acidic organic compound.

c. The least acidic organic compound.

23PRACTICE PROBLEM

Maleic acid contains two carboxyl (–COOH) groups. Its structure is shown below. Explain why the value of the second pK_a for maleic acid (6.2) is greater than the pK_a of acetic acid (4.7).

24PRACTICE PROBLEM

The compound methyl propionate can react as a very weak base (pK_b = 20.5). Taking into consideration the two possible protonation sites on methyl propionate to give the conjugate acid, answer the following:

i) Provide the structures of the possible conjugate acids.

ii) Label the most stable conjugate acid.

iii) Calculate the pK_a of the more stable conjugate acid.

25PRACTICE PROBLEM

Oxygen shows the same hybridization effect on acidity as carbon and nitrogen.

Each structure below loses an H⁺ from its positively charged oxygen, resulting in a neutral oxygen. One of the structures has a pK_a of −3.2, while the other has a pK_a of −7.3.

(i) Show how each reacts with water by writing the chemical reaction.

(ii) Match each structure with its pK_a. Justify.

26PRACTICE PROBLEM

Determine which is the stronger base between methoxide ion and formate ion by using pK_b values (without looking up such values).

27PRACTICE PROBLEM

The following is the structure of citric acid:

(i) Determine if the given structure is a carboxylic acid.

(ii) Compare the acidic strength of citric acid with lactic acid. Citric acid and lactic acid have pK_a values of 3.13 and 3.86, respectively.

28PRACTICE PROBLEM

The following is the structure of citric acid:

(i) Determine which is the most acidic proton in citric acid.

(ii) Which form of citric acid would exist in lemon juice? (Note: pH of lemon juice is 2.0.)

29PRACTICE PROBLEM

Arrange the compounds below in terms of decreasing acidity. (pK_a values are given.)

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30PRACTICE PROBLEM

Explain why triazole (pK_a = 10.2) is a stronger acid compared to pyrazole (p K_a = 14.2).

31PRACTICE PROBLEM

Benzenesulfonic acid (C₆H₅SO₃H) has a pK_a value of −0.60 while acetic acid (CH₃COOH) has a K_a value of 1.8×10⁻⁵. Determine the K_a value of benzenesulfonic acid and the pK_a value of acetic acid. Which one of the two acids is the stronger acid? Additionally, calculate how many times stronger it is.

32PRACTICE PROBLEM

Does ethanol behave as an acid or a base in the following chemical reaction?
(pKa of ethanol and ethylamine are 15 and 40 respectively)
CH₃CH₂OH + CH₃CH₂NH₂ →

33PRACTICE PROBLEM

Identify which of the following bases can deprotonate a terminal alkyne.

34PRACTICE PROBLEM

A chemist conducts a reaction that produces protons buffered at pH = 9.8. Determine if it is better to use a protonated triethylamine/triethylamine buffer (pKa of protonated triethylamine = 10.8) or would be better to use an ammonium/ammonia buffer (pKa of ammonium = 9.3) instead.

35PRACTICE PROBLEM

True or False: A base with a conjugate acid that has a pK_a less than 25 can deprotonate a terminal alkyne to form an alkynide ion.