Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry

Problem 39g,h

Chapter 3, Problem 39g,h

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)

Verified step by step guidance

Step 1: Understand the problem. The question asks us to determine the predominant form of the given compounds in a solution with a pH of 5.5. To do this, we need to compare the pH of the solution to the pKa of each compound. The relationship between pH and pKa is described by the Henderson-Hasselbalch equation:

pH = pKa + \log (\frac{[A-]}{[HA]})

, where [A⁻] is the concentration of the conjugate base and [HA] is the concentration of the acid.

Step 2: Analyze the first compound, HNO2 (pKa = 3.4). Compare the pH of the solution (5.5) to the pKa of HNO2. Since the pH is greater than the pKa, the solution is more basic relative to the acid's dissociation constant. This means the conjugate base (NO2⁻) will predominate over the acidic form (HNO2).

Step 3: Analyze the second compound, HNO3 (pKa = -1.3). Compare the pH of the solution (5.5) to the pKa of HNO3. The pH is significantly higher than the pKa, indicating that the solution is much more basic relative to the acid's dissociation constant. As a result, the conjugate base (NO3⁻) will predominate over the acidic form (HNO3).

Step 4: Summarize the findings. For HNO2, the predominant form at pH 5.5 is NO2⁻. For HNO3, the predominant form at pH 5.5 is NO3⁻. This conclusion is based on the comparison of pH and pKa values and the application of the Henderson-Hasselbalch equation.

Step 5: Note the general principle. When the pH of a solution is higher than the pKa of an acid, the conjugate base form predominates. Conversely, when the pH is lower than the pKa, the acidic form predominates. This principle can be applied to similar problems in acid-base chemistry.

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Key Concepts

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

pKa and Acid-Base Equilibrium

pKa is a measure of the strength of an acid in solution, indicating the pH at which half of the acid is dissociated. A lower pKa value signifies a stronger acid, meaning it dissociates more readily in solution. Understanding the relationship between pKa and pH is crucial for predicting the predominant form of an acid in a given solution.

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]). This equation helps determine the predominant species in solution by comparing the pH to the pKa of the acid.

Acid-Base Species in Solution

In an aqueous solution, acids can exist in two forms: the protonated (HA) and deprotonated (A-) forms. The predominant form at a specific pH depends on the acid's pKa relative to the solution's pH. For example, if the pH is higher than the pKa, the deprotonated form is favored, while a lower pH favors the protonated form.

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The Lewis definition of acids and bases.

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)

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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:

c. H₃O⁺ (pK_a = −1.7)

d. HBr (pK_a = −9)

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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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)

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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)

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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

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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 (pKa = 3.4)h. HNO3 (pKa = −1.3)

Verified video answer for a similar problem:

Key Concepts

pKa and Acid-Base Equilibrium

Henderson-Hasselbalch Equation

Acid-Base Species in Solution

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)