Textbook Question
Rank the following species in order of increasing acidity. Explain your reasons for ordering them as you do.
HF NH3 H2SO4 CH3OH CH3COOH H3O+ H2O
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Ch. 2 - Acids and Bases; Functional Groups
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 2, Problem 39a,b
The Ka of phenylacetic acid is 5.2 × 10−5, and the pKa of propionic acid is 4.87.
a. Calculate the pKa of phenylacetic acid and the Ka of propionic acid.
b. Which of these is the stronger acid? Calculate how much stronger an acid it is.
Verified step by step guidance1
To calculate the pKa of phenylacetic acid, use the formula pKa = -log(Ka). Substitute the given Ka value of phenylacetic acid (5.2 × 10^−5) into the formula.
To find the Ka of propionic acid, use the formula Ka = 10^(-pKa). Substitute the given pKa value of propionic acid (4.87) into the formula.
Compare the pKa values of phenylacetic acid and propionic acid. The lower the pKa value, the stronger the acid.
Calculate the difference in pKa values to determine how much stronger one acid is compared to the other. Use the formula: difference = pKa(propionic acid) - pKa(phenylacetic acid).
To express how much stronger one acid is, use the formula: strength factor = 10^(difference in pKa). This will give you the factor by which one acid is stronger than the other.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acid Dissociation Constant (Ka)
The acid dissociation constant (Ka) quantifies the strength of an acid in solution. It measures the extent to which an acid donates protons (H+) to water, forming its conjugate base. A higher Ka value indicates a stronger acid, as it dissociates more completely in solution. In this question, the Ka values for phenylacetic and propionic acids are provided, which are essential for comparing their acid strengths.
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The relationship between equilibrium constant and pKa.
pKa and Its Significance
The pKa is the negative logarithm of the Ka value and provides a more convenient way to express acid strength. It is calculated using the formula pKa = -log(Ka). Lower pKa values correspond to stronger acids, making it easier to compare acid strengths. In this question, the pKa of propionic acid is given, and calculating the pKa of phenylacetic acid from its Ka will help determine which acid is stronger.
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Comparative Acid Strength
To determine which acid is stronger, one can compare their Ka or pKa values. The difference in pKa values can be used to calculate how much stronger one acid is than another. This is done using the formula: difference in pKa = pKa(stronger acid) - pKa(weaker acid). A smaller pKa indicates a stronger acid, and the calculations will reveal the relative strength of phenylacetic acid compared to propionic acid.
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Related Practice
Textbook Question
All of the following compounds can react as acids. Without using a table of acidities, rank them in order of increasing acidity. Explain your ranking.
a. CH3CH2SO3H
b. CH3CH2OH
c. CH3CH2COOH
d. CH3CHClCOOH
e. ClCH2CH2COOH
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2
rank
Textbook Question
The following compound can become protonated on any of the three nitrogen atoms. One of these nitrogens is much more basic than the others, however.
b. Determine which nitrogen atom is the most basic.
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2
rank
Textbook Question
The Ka of phenylacetic acid is 5.2 × 10−5, and the pKa of propionic acid is 4.87.
c. Predict whether the following equilibrium will favor the reactants or the products.
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Textbook Question
Rank the following species in order of increasing basicity. Explain your reasons for ordering them as you do.
NH3 CH3O– H2O CH3COO– NaOH NH2– HSO4–
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Textbook Question
The following compound can become protonated on any of the three nitrogen atoms. One of these nitrogens is much more basic than the others, however.
a. Draw the important resonance forms of the products of protonation on each of the three nitrogen atoms.
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