Concept explainers
Consider pyridine, C5H5N, a pesticide and deer repellent. Its conjugate acid, C5H5NH+, has
(a) Write a balanced net ionic equation for the reaction that shows the basicity of aqueous solutions of pyridine.
(b) Calculate Kb for the reaction in (a).
(c) Find the pH of a solution prepared by mixing 2.74 g of pyridine in enough water to make 685 mL of solution.
(a)
Interpretation:
The base dissociation reaction of pyridine needs to be determined.
Concept introduction:
The dissociation reaction of a weak base is represented as follows:
The expression for the base dissociation constant will be as follows:
Here,
Answer to Problem 74QAP
Explanation of Solution
The basic nature of pyridine,
Here,
For the given base, the expression for base dissociation constant can be calculated as follows:
(b)
Interpretation:
The base ionization constant for the pyridine needs to be determined.
Concept introduction:
The dissociation reaction of a weak base is represented as follows:
The expression for the base dissociation constant will be as follows:
Here,
The relation between the acid dissociation constant and weak dissociation constant is as follows:
Here,
Answer to Problem 74QAP
Explanation of Solution
The conjugate acid of the weak base
The relation between the acid dissociation constant and weak dissociation constant is as follows:
Here,
On rearranging, the base dissociation constant can be calculated as follows:
Putting the values,
Thus, the value of
(c)
Interpretation:
The pH of the given solution of pyridine needs to be determined.
Concept introduction:
The dissociation reaction of a weak base is represented as follows:
The expression for the base dissociation constant will be as follows:
Here,
The pOH of a solution is calculated as follows:
From pOH, pH of a solution is calculated as follows:
Answer to Problem 74QAP
Explanation of Solution
The mass of the pyridine is 2.74 g and volume of solution is 685 mL.
The molar mass of pyridine is 79.1 g/mol. The number of moles of pyridine can be calculated as follows:
Putting the values,
From the number of moles and volume, molarity of solution can be calculated as follows:
Putting the values,
Thus, the molarity of pyridine is
The base ionization reaction of the
The concentration of all species can be calculated from the ICE table as follows:
Now, putting the values in the base dissociation constant as follows:
Or,
Rearranging,
Or,
This can be related to the quadratic equation as follows:
The value of x can be calculated using the following relation:
Putting the values,
Since, the value of x cannot be negative thus, it is equal to
From the ICE table:
Thus, pOH of solution can be calculated as follows:
Putting the value,
From the pOH of the solution, pH can be calculated as follows:
Putting the values,
Therefore, the pH of the solution is
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Chapter 13 Solutions
Chemistry: Principles and Reactions
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