Concept explainers
Determine pH at the equivalence point in the titration of 26.0 mL 1.12 M pyridine with
(a) 7.00
(b) 2.76
(c) 11.24
(d) 1.73
(e) 12.27
Interpretation:
The pH at the equivalence point in the titration of pyridine with hydrochloric acid is to be determined.
Concept introduction:
When a weak base is titrated against a strong acid, the conjugate acid of the weak base is formed in the reaction, as shown:
This conjugate acid now acts as a Bronsted acid and reacts with water to form weak base and hydronium ions according to the reaction:
Here,
The relationship between
Where,
The formula to calculate the pH of the solution from the concentration of hydronium ions is expressed as
Molarity
Rearrange this equation in terms of moles as shown
When volume is given in
Answer to Problem 4KSP
Correct answer: Option (b).
Explanation of Solution
Given information:
The concentration of pyridine
Reason for correct option:
From the given values of concentration and volume, calculate the number of millimoles of pyridineusing equation (4)
Being a strong acid,
At equivalence point, during the titration process, millimoles of weak base must be equal to the millimoles of the strong acid. Thus,
As the concentration of
Thus, the total volume of the solution containing
During titration, the weak base completelyneutralizes. Thus, the moles of weak base reacted is equal to the moles of its conjugate acid formed. Therefore,
Thus, the concentration of the conjugate acid
The anion
From table
Now, prepare an equilibrium table and represent each of the species in terms of
Now, substitute these concentrations in equation (2)
Since the value of
Thus,
Now, substitute the value of
Therefore, the equivalence pH of the solution is
Reason for incorrect options:
Since
Since
Since
Since
Therefore, options (a), (c), (d), and (e) are incorrect.
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