Concept explainers
Caffeine, the stimulant in coffee and tea, is a weak base that ionizes in water according to the equation
A 0.15-M solution of caffeine at 25°C has a pH of 8.45. Determine the Kb of caffeine.
Interpretation:
The
Concept Information:
Base ionization constant:
The ionization of a weak base
The equilibrium expression for the ionization of weak base
Where,
pOH definition:
The
Relationship between pH and pOH
The relationship between the hydronium ion concentration and the hydroxide ion concentration is given by the equation,
As
To Calculate: The
Answer to Problem 16.15WE
Answer
The
Explanation of Solution
Given data:
Caffeine is the stimulant in tea and coffee.
Caffeine is a weak base which ionizes in water as follows,
The pH of the given 0.15 M solution of caffeine is 8.45
Calculation strategy:
Using the given pH, the pOH can be found, then from pOH obtained, the hydroxide ion concentration is calculated.
From the obtained hydroxide ion concentration, use reaction stoichiometry to determine the other equilibrium concentrations, then evaluate
Calculation of pOH:
The pOH can be calculated using the following formula as follows,
Therefore, the pOH of the given caffeine solution is 5.55
Calculation of hydroxide ion:
The hydroxide ion can be calculated as follows,
Therefore, the concentration of
Calculation of
The concentration of hydroxide ion for the given weak base can be found out from its equilibrium reaction with water.
Based on the stoichiometry,
If the concentration of
We summarize the changes as follows,
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Initial
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0.15
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Change
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Equilibrium
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Therefore, the base ionization constant
The
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